Chapter XIII: General Conclusions (1)

I. THE MODE OF ORIGIN AND HOMOLOGIES OF THE GERMINAL LAYERS.

It has already been shewn in the earlier chapters of the work that during the first phases of development the history of all the Metazoa is the same. They all originate from the coalescence of two cells, the ovum and spermatozoon. The coalesced product of these cells--the fertilized ovum--then undergoes a process known as the segmentation, in the course of which it becomes divided in typical cases into a number of uniform cells. An attempt was made from the point of view of evolution to explain these processes. The ovum and spermatozoon were regarded as representing phylogenetically two physiologically differentiated forms of a Protozoon; their coalescence was equivalent to conjugation: the subsequent segmentation of the fertilized ovum was the multiplication by division of the organism resulting from the conjugation; the resulting organisms, remaining, however, united to form a fresh organism in a higher state of aggregation.

In the systematic section of this work the embryological history of the Metazoa has been treated. The present chapter contains a review of the cardinal features of the various histories, together with an attempt to determine how far there are any points common to the whole of these histories; and the phylogenetic interpretation to be given to such points.

Some years ago it appeared probable that a definite answer would be given to the questions which must necessarily be raised in the present chapter; but the results of the extended investigations made during the last few years have shewn that these expectations were premature, and in spite of the numerous recent valuable contributions to this branch of Embryology, amongst which special attention may be called to those of Kowalevsky (No. 277), Lankester (Nos. 278 and 279), and Haeckel (No. 266), there are few embryologists who would venture to assert that any answers which can be given are more than tentative gropings towards the truth.

In the following pages I aim more at summarising the facts, and critically examining the different theories which can be held, than at dogmatically supporting any definite views of my own.

In all the Metazoa, the development of which has been investigated, the first process of differentiation, which follows upon the segmentation, consists in the cells of the organism becoming divided into two groups or layers, known respectively as epiblast and hypoblast.

These two layers were first discovered in the young embryos of vertebrated animals by Pander and Von Baer, and have been since known as the germinal layers, though their cellular nature was not at first recognised. They were shewn, together with a third layer, or mesoblast, which subsequently appears between them, to bear throughout the Vertebrata constant relations to the organs which became developed from them. A very great step was subsequently made by Remak (No. 287), who successfully worked out the problem of vertebrate embryology on the cellular theory.

Rathke in his memoir on the development of Astacus (No. 286) attempted at a very early period to extend the doctrine of the derivation of the organs from the germinal layers to the Invertebrata. In 1859 Huxley made an important step towards the explanation of the nature of these layers by comparing them with the ectoderm and endoderm of the Hydrozoa; while the brilliant researches of Kowalevsky on the development of a great variety of invertebrate forms formed the starting point of the current views on this subject.

The differentiation of the epiblast and hypoblast may commence during the later phases of the segmentation, but is generally not completed till after its termination. Not only do the cells of the blastoderm become differentiated into two layers, but these two layers, in the case of a very large number of ova with but little food-yolk, constitute a double-walled sack--the gastrula (fig. 198)--the characters of which are too well known to require further description. Following the lines of phylogenetic speculation above indicated, it may be concluded that the two-layered condition of the organism represents in a general way the passage from the protozoon to the metazoon condition. It is probable that we may safely go further, and assert that the gastrula reproduces, with more or less fidelity, a stage in the evolution of the Metazoa, permanent in the simpler Hydrozoa, during which the organism was provided with (1) a fully developed digestive cavity (fig. 198 _b_) lined by the hypoblast with digestive and assimilative functions, (2) an oral opening (_a_), and (3) a superficial epiblast (_d_). These generalisations, which are now widely accepted, are no doubt very valuable, but they leave unanswered the following important questions:

(1) By what steps did the compound Protozoon become differentiated
into a Metazoon?
(2) Are there any grounds for thinking that there is more than one
line along which the Metazoa have become independently evolved
from the Protozoa?
(3) To what extent is there a complete homology between the two
primary germinal layers throughout the Metazoa?

[FIG. 198. DIAGRAM OF A GASTRULA. (From Gegenbaur.)

_a._ mouth; _b._ archenteron; _c._ hypoblast; _d._ epiblast.]

Ontogenetically there is a great variety of processes by which the passage from the segmented ovum to the two-layered or diploblastic condition is arrived at.

These processes may be grouped under the following heads:

1. Invagination. Under this term a considerable number of closely connected processes are included. When the segmentation results in the formation of a blastosphere, one half of the blastosphere may be pushed in towards the opposite half, and a gastrula be thus produced (fig. 199, A and B). This process is known as embolic invagination. Another process, known as epibolic invagination, consists in epiblast cells growing round and enclosing the hypoblast (fig. 200). This process replaces the former process when the hypoblast cells are so bulky from being distended by food-yolk that their invagination is mechanically impossible.

[FIG. 199. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA,
VIEWED IN OPTICAL SECTION. (After Selenka.)

A. Stage at the close of segmentation. B. Gastrula stage.

_mr._ micropyle; _fl._ chorion; _s.c._ segmentation cavity; _bl._
blastoderm; _ep._ epiblast; _hy._ hypoblast; _ms._ amoeboid cells
derived from hypoblast; _a.e._ archenteron.]

There are various peculiar modifications of invagination which cannot be dealt with in detail.

Invagination in one form or other occurs in some or all the members of the following groups:

[FIG. 200. TRANSVERSE SECTION THROUGH THE OVUM OF EUAXES DURING AN
EARLY STAGE OF DEVELOPMENT, TO SHEW THE NATURE OF EPIBOLIC
INVAGINATION. (After Kowalevsky.)

_ep._ epiblast; _ms._ mesoblastic band; _hy._ hypoblast.]

The Dicyemidæ, Calcispongiæ (after the amphiblastula stage) and Silicispongiæ, Coelenterata, Turbellaria, Nemertea, Rotifera, Mollusca, Polyzoa, Brachiopoda, Chætopoda, Discophora, Gephyrea, Chætognatha, Nematelminthes, Crustacea, Echinodermata, and Chordata.

The gastrula of the Crustacea is peculiar, as is also that of many of the Chordata (Reptilia, Aves, Mammalia), but there is every reason to suppose that the gastrulæ of these groups are simply modifications of the normal type.

2. Delamination. Three types of delamination may be distinguished:

_a._ Delamination where the cells of a solid morula become divided
into a superficial epiblast, and a central solid mass in which the
digestive cavity is subsequently hollowed out (fig. 201).

[FIG. 201. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM, TO
ILLUSTRATE THE FORMATION OF THE LAYERS BY DELAMINATION. (After
Metschnikoff.)

A. Stage after the delamination; _ep._ epiblastic invagination to
form pneumatocyst.

B. Later stage after the formation of the gastric cavity in the
solid hypoblast. _po._ polypite; _t._ tentacle; _pp._ pneumatocyst;
_ep._ epiblast of pneumatocyst; _hy._ hypoblast surrounding
pneumatocyst.]

_b._ Delamination where the segmented ovum has the form of a
blastosphere, the cells of which give rise by budding to scattered
cells in the interior of the vesicle, which, though they may at
first form a solid mass, finally arrange themselves in the form of a
definite layer around a central digestive cavity (fig. 202).

_c._ Delamination where the segmented ovum has the form of a
blastosphere in the cells of which the protoplasm is differentiated
into an inner and an outer part. By a subsequent process the inner
parts of the cells become separated from the outer, and the walls of
the blastosphere are so divided into two distinct layers (fig. 205).

Although the third of these processes is usually regarded as the type of delamination, it does not, so far as I know, occur in nature, but is most nearly approached in Geryonia (fig. 203).

The first type of delamination is found in the Ceratospongiæ, some Silicispongiæ (?), and in many Hydrozoa and Actinozoa, and in Nemertea and Nematelminthes (_Gordioidea?_). The second type occurs in many Porifera [_Calcispongiæ_ (_Ascetta_), _Myxospongiæ_], and in some Coelenterata, and Brachiopoda (_Thecidium_).

[FIG. 202. THREE LARVAL STAGES OF EUCOPE POLYSTYLA. (After
Kowalevsky.)

A. Blastosphere stage with hypoblast spheres becoming budded off
into central cavity. B. Planula stage with solid hypoblast. C.
Planula stage with a gastric cavity. _ep._ epiblast; _hy._
hypoblast; _al._ gastric cavity.]

Delamination and invagination are undoubtedly the two most frequent modes in which the layers are differentiated, but there are in addition several others. In the first place the whole of the Tracheata (with the apparent exception of the Scorpion) develop, so far as is known, on a plan peculiar to them, which approaches delamination. This consists in the appearance of a superficial layer of cells enclosing a central yolk mass, which corresponds to the hypoblast (figs. 204 and 214). This mode of development might be classed under delamination, were it not for the fact that the early development of many Crustacea is almost the same, but is subsequently followed by an invagination (fig. 208), which apparently corresponds to the normal invagination of other types. There are strong grounds for thinking that the tracheate type of formation of the epiblast and hypoblast is a _secondary modification of an invaginate type_ (_vide_ Vol. II. p. 457).

[FIG. 203. DIAGRAMMATIC FIGURES SHEWING THE DELAMINATION OF THE
EMBRYO OF GERYONIA. (After Fol.)

A. Stage at the commencement of the delamination; the dotted lines
_x_ shew the course of the next planes of division. B. Stage at the
close of the delamination. _cs._ segmentation cavity; _a._
endoplasm; _b._ ectoplasm; _ep._ epiblast; _hy._ hypoblast.]

[FIG. 204. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER.
(After Metschnikoff.)

In A the ovum is divided into a number of separate segments. In B a
number of small cells have appeared (_bl_) which form a blastoderm
enveloping the large yolk-spheres. In C the blastoderm has become
divided into two layers.]

The type of some Turbellaria (_Stylochopsis ponticus_) and that of Nephelis amongst the Discophora is not capable of being reduced to the invaginate type.

The development of almost all the parasitic groups, _i.e._ the Trematoda, the Cestoda, the Acanthocephala, and the Linguatulida, and also of the Tardigrada, Pycnogonida, and other minor groups, is too imperfectly known to be classed with either the delaminate or invaginate types.

It will, I think, be conceded on all sides that, if any of the ontogenetic processes by which a gastrula form is reached are repetitions of the process by which a simple two-layered gastrula was actually evolved from a compound Protozoon, these processes are most probably of the nature either of invagination or of delamination.

The much disputed questions which have been raised about the gastrula and planula theories, originally put forward by Haeckel and Lankester, resolve themselves then into the simple question, whether any, and if so which, of the ontogenetic processes by which the gastrula is formed are repetitions of the phylogenetic origin of the gastrula.

It is very difficult to bring forward arguments of a conclusive kind in favour of either of these processes. The fact that delaminate and invaginate gastrulæ are in several instances found coexisting in the same group renders it certain that there are not two independent phyla of the Metazoa, derived respectively from an invaginate and a delaminate gastrula[119].

[119] It is not difficult to picture a possible derivation of
delamination from invagination; while a comparison of the
formation of the inner layers (mesoblast and hypoblast) in
Ascetta (amongst the Sponges), and in the Echinodermata, shews a
very simple way in which it is possible to conceive of a passage
of delamination into invagination. In Ascetta the cells, which
give rise to the mesoblast and hypoblast, are budded off from the
inner wall of the blastosphere, especially at one point; while in
Echinodermata (fig. 199) there is a small invaginated sack which
gives rise to the hypoblast, while from the walls of this sack
amoeboid cells are budded off which give rise to a large part of
the mesoblast. If we suppose the hypoblast cells budded off at
one point in Ascetta gradually to form an invaginated sack, while
the mesoblast cells continued to be budded off as before, we
should pass from the delaminate type of Ascetta to the invaginate
type of an Echinoderm.

The four most important cases in which the two processes coexist are the Porifera, the Coelenterata, the Nemertea, and the Brachiopoda. In the cases of the Porifera and Coelenterata, there do not appear to me to be any means of deciding which of these processes is derived from the other; but in the Nemertea and the Brachiopoda the case is different. In all the types of Nemertea in which the development is relatively not abbreviated there is an invaginate gastrula, while in the types with a greatly abbreviated development there is a delaminate gastrula. It would seem to follow from this that a delaminate gastrula has here been a secondary result of an abbreviation in the development. In the Brachiopoda, again, the majority of types develop by a process of invagination, while Thecidium appears to develop by delamination; here also the delaminate type would appear to be secondarily derived from the invaginate.

If these considerations are justified, delamination must be in some instances secondarily derived from invagination; and this fact is so far an argument in favour of the more primitive nature of invagination; though it by no means follows that in the invaginate process the steps by which the Metazoa were derived from the Protozoa are preserved.

It does not, therefore, seem possible to decide conclusively in favour of either of these processes by a comparison of the cases where they occur in the same groups.

The relative frequency of the two processes supplies us with another possible means for deciding between them; and there is no doubt that here again the scale inclines towards invagination. It must, however, be borne in mind that the frequency of the process of invagination admits of another possible explanation. There is a continual tendency for the processes of development to be abbreviated and simplified, and it is quite possible that the frequent occurrence of invagination is due to the fact of its being, in most cases, the simplest means by which the two-layered condition can be reached. But this argument can have but little weight until it can be shewn in each case that invagination is a simpler process than delamination; and it is rendered improbable by the cases already mentioned in which delamination has been secondarily derived from invagination.

If it were the case that the blastopore had _in all types_ the same relation to the adult mouth, there would be strong grounds for regarding the invaginate gastrula as an ancestral form; but the fact that this is by no means so is an argument of great weight in favour of some other explanation of the frequency of invagination.

The force of this consideration can best be displayed by a short summary of the fate of the blastopore in different forms.

The fate of the blastopore is so variable that it is difficult even to classify the cases which have been described.

(1) It becomes the permanent mouth in the following forms[120]:
_Coelenterata._--Pelagia, Cereanthus.
_Turbellaria._--Leptoplana (?), Thysanozoon.
_Nemertea._--Pilidium, larvæ of the type of Desor.
_Mollusca._--In numerous examples of most Molluscan groups, except
the Cephalopoda.
_Chætopoda._--Most Oligochæta, and probably many Polychæta.
_Gephyrea._--Phascolosoma, Phoronis.
_Nematelminthes._--Cucullanus.
(2) It closes in the position where the mouth is subsequently
formed.
_Coelenterata._--Ctenophora (?).
_Mollusca._--In numerous examples of most Molluscan groups, except
the Cephalopoda.
_Crustacea._--Cirripedia (?), some Cladocera (Moina) (?).
(3) It becomes the permanent anus.
_Mollusca._--Paludina.
_Chætopoda._--Serpula and some other types.
_Echinodermata._--Almost universally, except amongst the Crinoidea.
(4) It closes in the position where the anus is subsequently formed.
_Echinodermata._--Crinoidea.
(5) It closes in a position which does not correspond or is not
known to correspond[121] either with the future mouth or
anus.--_Porifera_--Sycandra. _Coelenterata_--Chrysaora*,
Aurelia*. _Nemertea*_--Some larvæ which develop without a
metamorphosis. _Rotifera*. Mollusca_--Cephalopoda. _Polyzoa*._
_Brachiopoda_--Argiope, Terebratula, Terebratulina.
_Chætopoda_--Euaxes. _Discophora_--Clepsine.
_Gephyrea_--Bonellia*. _Chætognatha. Crustacea_--Decapoda.
_Chordata._

[120] The above list is somewhat tentative; and future
investigations will probably shew that many of the statements at
present current about the position of the blastopore are
inaccurate.

[121] The forms in which the position of the blastopore in
relation to the mouth or anus is not known are marked with an
asterisk.

The forms which have been classed together under the last heading vary considerably in the character of the blastopore. In some cases the fact of its not coinciding either with the mouth or anus appears to be due simply to the presence of a large amount of food-yolk. The cases of the Cephalopoda, of Euaxes, and perhaps of Clepsine and Bonellia, are to be explained in this way: in the case of all these forms, except Bonellia, the blastopore has the form of an elongated slit along the ventral surface. This type of blastopore is characteristic of the Mollusca generally, of the Polyzoa, of the Nematelminthes, and very possibly of the Chætopoda and Discophora. In the Chætognatha (fig. 209 B) the blastopore is situated, so far as can be determined, behind the future anus. In many Decapoda the blastopore is placed behind, but not far from, the anus. In the Chordata it is also placed posteriorly to the anus, and, remarkably enough, remains, in a large number of forms, for some time in connection with the neural tube by a neurenteric canal.

The great variations in the character of the gastrula, indicated in the above summary, go far to shew that if the gastrulæ, as we find them in most types, have any ancestral characters, these characters can only be of the most general kind. This may best be shewn by the consideration of a few striking instances. The blastopore in Mollusca has an elongated slit-like form, extending along the ventral surface from the mouth to the anus. In Echinodermata it is a narrow pore, remaining as the anus. In most Chætopoda it is a pore remaining as the mouth, but in some as the anus. In Chordata it is a posteriorly-placed pore, opening into both the archenteron and the neural canal.

It is clearly out of the question to explain all these differences as having connection with the characters of ancestral forms. Many of them can only be accounted for as secondary adaptations for the convenience of development.

The epibolic gastrula of Mammalia (_vide_ pp. 215 and 291) is a still more striking case of a secondary embryonic process, and is not directly derived from the gastrula of the lower Chordata. It probably originated in connection with the loss of food-yolk which took place on the establishment of a placental nutrition for the foetus. The epibolic gastrula of the Scorpion, of Isopods, and of other Arthropoda, seems also to be a derived gastrula. These instances of secondary gastrulæ are very probably by no means isolated, and should serve as a warning against laying too much stress upon the frequency of the occurrence of invagination. The great influence of the food-yolk upon the early development might be illustrated by numerous examples, especially amongst the Chordata (_vide_ Chapter XI.).

If the descendants of a form with a large amount of food-yolk in its ova were to produce ova with but little food-yolk, the type of formation of the germinal layers which would thereby result would be by no means the same as that of the ancestors of the forms with much food-yolk, but would probably be something very different, as in the case of Mammalia. Yet amongst the countless generations of ancestors of most existing forms, such oscillations in the amount of the food-yolk must have occurred in a large number of instances.

The whole of the above considerations point towards the view that the formation of the hypoblast by invagination, as it occurs in most forms at the present day, can have in many instances no special phylogenetic significance, and that the argument from frequency, in favour of invagination as opposed to delamination, is not of prime importance.

A third possible method of deciding between delamination and invagination is to be found in the consideration as to which of these processes occurs in the most primitive forms. If there were any agreement amongst primitive forms as to the type of their development this argument might have some weight. On the whole, delamination is, no doubt, characteristic of many primitive types, but the not infrequent occurrence of invagination in both the Coelenterata and the Porifera--the two groups which would on all hands be admitted to be amongst the most primitive--deprives this argument of much of the value it might otherwise have.

To sum up--considering the almost indisputable fact that both the processes above dealt with have in many instances had a purely secondary origin, no valid arguments can be produced to shew that either of them reproduces the mode of passage between the Protozoa and the ancestral two-layered Metazoa. These conclusions do not, however, throw any doubt upon the fact that the gastrula, however evolved, was a primitive form of the Metazoa; since this conclusion is founded upon the actual existence of adult gastrula forms independently of their occurrence in development.

[FIG. 205. DIAGRAM SHEWING THE FORMATION OF A GASTRULA BY
DELAMINATION. (From Lankester.)

Fig. 1, ovum; fig. 2, stage in segmentation; fig. 3, commencement of
delamination after the appearance of a central cavity; fig. 4,
delamination completed, mouth forming at _M._ In figs. 1, 2, and 3,
_Ec._ is ectoplasm, and _En._ is endoplasm. In fig. 4, _Ec._ is
epiblast, and _En._ hypoblast. _E._ and _F._ food particles.]

Though embryology does not at present furnish us with a definite answer to the question how the Metazoa became developed from the Protozoa, it is nevertheless worth while reviewing some of the processes by which this can be conceived to have occurred.

On purely _à priori_ grounds there is in my opinion more to be said for invagination than for any other view.

On this view we may suppose that the colony of Protozoa in the course of conversion into Metazoa had the form of a blastosphere; and that at one pole of this a depression appeared. The cells lining this depression we may suppose to have been amoeboid, and to have carried on the work of digestion; while the remaining cells were probably ciliated. The digestion may be supposed to have been at first carried on in the interior of the cells, as in the Protozoa; but, as the depression became deeper (in order to increase the area of nutritive cells and to retain the food) a digestive secretion probably became poured out from the cells lining it, and the mode of digestion generally characteristic of the Metazoa was thereby inaugurated. It may be noted that an intracellular protozoon type of digestion persists in the Porifera, and appears also to occur in many Coelenterata, Turbellaria, &c., though in most of these cases both kinds of digestion probably go on simultaneously[122].

[122] J. Parker, "On the Histology of _Hydra fusca_," _Quart.
Journ. Micr. Science_, vol. XX. 1880; and El. Metschnikoff, "Ueb.
die intracelluläre Verdauung bei Coelenteraten," _Zoologischer
Anzeiger_, No. 56, vol. III. 1880 and Lankester, "On the
intracellular digestion and endoderm of Limnocodium," _Quart.
Journ. Micr. Science_, vol. XXI. 1881.

Another hypothetical mode of passage, which fits in with delamination, has been put forward by Lankester, and is illustrated by fig. 205. He supposes that at the blastosphere stage the fluid in the centre of the colony acquired special digestive properties; the inner ends of the cells having at this stage somewhat different properties from the outer, and the food being still incepted by the surface of the cells (fig. 205, 3). In a later stage of the process the inner portions of the cells became separated off as the hypoblast; while the food, though still ingested in the form of solid particles by the superficial cells, was carried through the protoplasm into the central digestive cavity. Later (fig. 205, 4), the point where the food entered became localised, and eventually a mouth became formed at this point.

The main objection which can be raised against Lankester's view is that it presupposes a type of delamination which does not occur in nature except in Geryonia.

Metschnikoff has propounded a third view with reference to delamination. He starts as before with a ciliated blastosphere. He next supposes the cells from the walls of this to become budded off into the central cavity, as in Eucope (fig. 202), and to lose their cilia. These cells give rise to an internal parenchyma, which carries on an intracellular digestion. At a later stage a central digestive cavity is supposed to be formed. This view of the passage from the protozoon to the metazoon state, though to my mind improbable in itself, fits in very well with the ontogeny of the lower Hydrozoa.

Another view has been put forward by myself in the chapter on the Porifera[123], to the effect that the amphiblastula larva of Calcispongiæ may be a transitional form between the Protozoa and the Metazoa, composed of a hemisphere of nutritive amoeboid cells, and a hemisphere of ciliated cells. The absence of such a larval form in the Coelenterata and higher Metazoa is opposed, however, to this larva being regarded as a transitional form, except for the Porifera.

[123] Vol. II. p. 149.

It is obvious that so long as there is complete uncertainty as to the value to be attached to the early developmental processes, it is not possible to decide from these processes whether there is only a single metazoon phylum or whether there may not be two or more such phyla. At the same time there appear to be strong arguments for regarding the Porifera as a phylum of the Metazoa derived independently from the Protozoa. This seems to me to be shewn (1) by the striking larval peculiarities of the Porifera; (2) by the early development of the mesoblast in the Porifera, which stands in strong contrast to the absence of this layer in the embryos of most Coelenterata; and above all, (3) by the remarkable characters of the system of digestive channels. A further argument in the same direction is supplied by the fact that the germinal layers of the Sponges very probably do not correspond physiologically to the germinal layers of other types. The embryological evidence is insufficient to decide whether the amphiblastula larva is, as suggested above, to be regarded as the larval ancestor of the Porifera.

Homologies of the germinal layers. The question as to how far there is a complete homology between the two primary germinal layers throughout the Metazoa was the third of the questions proposed to be discussed here.

Since there are some Metazoa with only two germinal layers, and other Metazoa with three, and since, as is shewn in the following section, the third layer or mesoblast can only be regarded as a derivative of one or both the primary layers, it is clear that a complete homology between the two primary germinal layers does not exist.

That there is a general homology appears on the other hand hardly open to doubt.

The primary layers are usually continuous with each other, near one or both (when both are present) the openings of the alimentary tract.

As a rule an oral and anal section of the alimentary tract--the stomodæum and proctodæum--are derived from the epiblast; but the limits of both these sections are so variable, sometimes even in closely allied forms, that it is difficult to avoid the conclusion that there is a border-land between the epiblast and hypoblast, which appears by its development to belong in some forms to the epiblast and in other forms to the hypoblast. If this is not the case it is necessary to admit that there are instances in which a very large portion of the alimentary canal is phylogenetically an epiblastic structure. In some of the Isopods, for example, the stomodæum and proctodæum give rise to almost the whole of the alimentary canal with its appendages, except the liver.

The origin of the Mesoblast. A diploblastic condition of the organism preceded, as we have seen, the triploblastic. The epiblast during the diploblastic condition was, as appears from such forms as Hydra, especially the sensory and protective layer, while the hypoblast was the secretory and assimilating layer; both layers giving rise to muscular elements. It must not, however, be supposed that in the early diploblastic ancestors there was a complete differentiation of function, but there is reason to think that both the primary layers retained an indefinite capacity for developing into any form of tissue[124]. The fact of the triploblastic condition being later than the diploblastic proves in a conclusive way that the mesoblast is a derivative of one or both the primary layers. In the Coelenterata we can study the actual origin from the two primary layers of various forms of tissue which in the higher types are derived from the mesoblast[125]. This fact, as well as general _à priori_ considerations, conclusively prove that the mesoblast did not at first originate as a mass of independent cells between the two primary layers, but that in the first instance it gradually arose as differentiations of the two layers, and that its condition in the embryo as an independent layer of undifferentiated cells is a secondary condition, brought about by the general tendency towards a simplification of development, and a retardation of histological differentiation[126].

[124] The Hertwigs (No. 270) have for instance shewn that nervous
structures are developed in the hypoblast in the Actinozoa and
other Coelenterata.

[125] There is considerable confusion in the use of the names for
the embryonic layers. In some cases various tissues formed by
differentiations of the primary layers have been called
mesoblast. Schultze, and more recently the Hertwigs, have pointed
out the inconvenience of this nomenclature. In the case of the
Coelenterata it is difficult to decide in certain instances
(_e.g._ Sympodium) whether the cells which give rise to a
particular tissue of the adult are to be regarded as forming a
mesoblast, _i.e._ a middle undifferentiated layer of cells, or
whether they arise as already histologically differentiated
elements from one of the primary layers. The attempt to
distinguish by a special nomenclature the epiblast and hypoblast
after and before the separation of the mesoblast, which has been
made by Allen Thomson (No. 1), appears incapable of being
consistently applied, though it is convenient to distinguish a
primary and a secondary hypoblast. A proposal of the Hertwigs to
adopt special names for the outer and inner limiting membranes of
the adult, and for the interposed mass of organs, appears to me
unnecessary.

[126] The causes which give rise to a retardation of histological
differentiation will be dealt with in the second part of this
chapter which deals with larval characters and larval forms.

The Hertwigs have recently attempted (No. 271) to distinguish two types of differentiation of the mesoblast, viz. (1) a direct differentiation from the primitive epithelial cells; (2) a differentiation from primitively indifferent cells budded off into the gelatinous matter between the two primary layers.

It is quite possible that this distinction may be well founded, but no conclusive evidence of the occurrence of the second process has yet been adduced. The Ctenophora are the type upon which special stress is laid, but the early passage of amoeboid cells into the gelatinous tissue, which subsequently become muscular, is very probably an embryonic abbreviation; and it is quite possible that these cells may phylogenetically have originated from epithelial cells provided with contractile processes passing through the gelatinous tissue.

The conversion of non-embryonic connective-tissue cells into muscle cells in the higher types has been described, but very much more evidence is required before it can be accepted as a common occurrence.

In addition to the probably degraded Dicyemidæ and Orthonectidæ, the Coelenterata are the only group in which a true mesoblast is not always present. In other words, the Coelenterata are the only group in which there is not found in the embryo an undifferentiated group of cells from which the majority of the organs situated between the epidermis and the alimentary epithelium are developed.

The organs invariably derived, in the triploblastic forms, from the mesoblast, are the vascular and lymphatic systems, the muscular system, and the greater part of the connective tissue and the excretory and generative (?) systems. On the other hand, the nervous systems (with a few possible exceptions) and organs of sense, the epithelium of most glands, and a few exceptional connective-tissue organs, as for example the notochord, are developed from the two primary layers.

The fact of the first-named set of organs being invariably derived from the mesoblast points to the establishment of the two following propositions:--(1) That with the differentiation of the mesoblast as a distinct layer by the process already explained, the two primary layers lost for the most part the capacity they primitively possessed of giving rise to muscular and connective-tissue differentiations[127], to the epithelium of the excretory organs, and to generative cells. (2) That the mesoblast throughout the triploblastic Metazoa, in so far as these forms have sprung from a common triploblastic ancestor, is an homologous structure.

[127] The connective-tissue test of the Tunicata, though derived
from the epiblast, is not really an example of such a
differentiation.

The second proposition follows from the first. The mesoblast can only have ceased to be homologous throughout the triploblastica by additions from the two primary layers, and the existence of such additions is negatived by the first proposition.

These two propositions, which hang together, are possibly only approximately true, since it is quite possible that future investigations may shew that differentiations of the two primary layers are not so rare as has been hitherto imagined.

Ranvier[128] finds that the muscles of the sweat-glands are developed from the inner part of the layer of epiblast cells, invaginated to form these glands.

[128] M. L. Ranvier. "Sur la structure des glandes sudoripares."
_Comptes Rendus_, Dec. 29, 1879.

Götte[129] describes the epiblast cells of the larva of Comatula as being at a certain stage contractile and compares them with the epithelio-muscular cells of Hydra. These cells would appear subsequently to be converted into a simple cuticular structure.

[129] A. Götte, "Vergleich. Entwick. d. Comatula mediterranea."
_Archiv f. mikr. Anat._ vol. XII. p. 597.

It is moreover quite possible that fresh differentiations from the two primary layers may have arisen after the triploblastic condition had been established, and by the process of simplification of development and precocious segregation, as Lankester calls it, have become indistinguishable from the normal mesoblast. In spite of these exceptions it is probable that the major part of the muscular system of all existing triploblastic forms has been differentiated from the muscular system of the ancestor or ancestors (if there is more than one phylum) of the triploblastica. In the case of other tissues there are a few instances which might be regarded as examples of an organ primitively developed in one of the two primary layers having become secondarily carried into the mesoblast. The notochord has sometimes been cited as such an organ, but, as indicated in a previous chapter, it is probable that its hypoblastic origin can always be demonstrated.

[FIG. 206. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.)

A. Stage when the four hypoblast cells are nearly enclosed.
B. Stage after the formation of the mesoblast has commenced by an
infolding of the lips of the blastopore.

_ep._ epiblast; _me._ mesoblast; _bl._ blastopore.]

The nervous system, although imbedded in mesoblastic derivates in the adults of all the higher triploblastica, retains with marvellous constancy its epiblastic origin (though it is usually separated from the epiblast prior to its histogenic differentiation); yet in the Cephalopoda, and some other Mollusca, the evidence is in favour of its developing in the mesoblast. Should future investigations confirm these conclusions, a good example will be afforded of an organ changing the layer from which it usually develops[130]. The explanation of such a change would be precisely the same as that already given for the mesoblast as a whole.

[130] The Hertwigs hold that there is a distinct part of the
nervous system which was at first differentiated in the mesoblast
in many types, amongst others the Mollusca. The evidence in
favour of this view is extremely scanty and the view itself
appears to me highly improbable.

The actual mode of origin of various tissues, which in the true triploblastic forms arise in mesoblast, can be traced in the Coelenterata[131]. In this group the epiblast and hypoblast both give rise to muscular and connective-tissue elements; and although the main part of the nervous system is formed in the epiblast, it seems certain that in some types nerves may be derived from the hypoblast[132]. These facts are extremely interesting, but it is by no means certain that any conclusions can be directly drawn from them as to the actual origin of the mesoblast in the triploblastic forms, till we know from what diploblastic forms the triploblastica originated. All that they shew is that any of the constituents of the mesoblast may have originated from either of the primitive layers.

[131] The reader is referred for this subject to the valuable
memoirs which have been recently published by the Hertwigs,
especially to No. 270. He will find a general account of the
subject written before the appearance of the Hertwigs' memoir in
pp. 180-182 of Volume II. of this treatise.

[132] It would be interesting to know the history of the various
nervous structures found in the walls of the alimentary tract in
the higher forms. I have shewn (_Development of Elasmobranch
Fishes_, p. 172) that the central part of the sympathetic system
is derived from the epiblast. It would however be well to work
over the development of Auerbach's plexus.

[FIG. 207. TWO TRANSVERSE SECTIONS THROUGH EMBRYOS OF HYDROPHILUS
PICEUS. (After Kowalevsky.)

A. Section through an embryo at the point where the two germinal
folds most approximate.
B. Section through an embryo, in the anterior region where the folds
of the amnion have not united.

_gg._ germinal groove; _me._ mesoblast; _am._ amnion; _yk._ yolk.]

For further light as to the origin of the mesoblast, it is necessary to turn to its actual development.

The following summary illustrates the more important modes in which the mesoblast originates.

[FIG. 208. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS. (From
Parker; after Reichenbach.)

A. Section through part of the ovum during segmentation. _n._
nuclei; _w.y._ white yolk; _y.p._ yolk pyramids; _c._ central
yolk mass.
B. and C. Longitudinal sections of the gastrula stage. _a._
_en._ archenteron; _b._ blastopore; _ms._ mesoblast; _ec._ epiblast;
hypoblast, distinguished from epiblast by shading.
D. Highly magnified view of anterior lip of blastopore, to shew the
origin of the primary mesoblast from the wall of the archenteron.
_p.ms._ primary mesoblast; _ec._ epiblast; _en._ hypoblast.
E. Two hypoblast cells to shew the amoeba-like absorption of yolk
spheres. _y._ yolk; _n._ nucleus; _p._ pseudopodial process.
F. Hypoblast cells giving rise endogenously to the secondary
mesoblast (_s.ms._); _n._ nucleus.]

1. It grows inwards from the lips of the blastopore as a pair of bands. In these cases it may originate (_a_) from cells which are clearly hypoblastic, (_b_) from cells which are clearly epiblastic, (_c_) from cells which cannot be regarded as belonging to either layer.

Mollusca.--Gasteropoda, Cephalopoda, and Lamellibranchiata. In Gasteropoda and Lamellibranchiata the mesoblast sometimes originates from a pair of cells at the lips of the blastopore, though very probably some of the elements subsequently come from the epiblast; and in Cephalopoda it begins as a ring of cells round the edge of the blastoderm.

Polyzoa Entoprocta.--It originates from a pair of cells at the lips of the blastopore.

Chætopoda.--Euaxes. It arises as a ridge of cells at the lips of the blastopore (fig. 200).

Gephyrea.--Bonellia. It arises (fig. 206) as an infolding of the epiblastic lips of the blastopore.

Nematelminthes.--Cucullanus. It grows backwards from the hypoblast cells at the persistent oral opening of the blastopore.

Tracheata.--Insecta. It grows inwards from the lips of the germinal groove (fig. 207), which probably represent the remains of a blastopore. Part of the mesoblast is probably also derived from the yolk-cells. A similar though more modified development of the mesoblast occurs in the Araneina (fig. 214).

Crustacea.--Decapoda. It partly grows in from the hypoblastic lips of the blastopore, and is partly derived from the yolk-cells (fig. 208).

[FIG. 209. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A. and C.
after Bütschli, and B. after Kowalevsky.)

The three embryos are represented in the same positions.
A. Represents the gastrula stage.
B. Represents a succeeding stage, in which the primitive archenteron
is commencing to be divided into three.
C. Represents a later stage, in which the mouth involution (_m_) has
become continuous with the alimentary tract, and the blastopore has
become closed.

_m._ mouth; _al._ alimentary canal; _ae._ archenteron; _bl.p._
blastopore; _pv._ perivisceral cavity; _sp._ splanchnic mesoblast;
_so._ somatic mesoblast; _ge._ generative organs.]

2. The mesoblast is developed from the walls of hollow outgrowths of the archenteron, the cavities of which become the body cavity.

Brachiopoda.--The walls of a pair of outgrowths form the whole of the mesoblast.

Chætognatha.--The mesoblast arises in the same manner as in the Brachiopoda (fig. 209).

Echinodermata.--The lining of the peritoneal cavity is developed from the walls of outgrowths of the archenteron, but the greater part of the mesoblast is derived from the amoeboid cells budded off from the walls of the archenteron (fig. 210).

[FIG. 210. LONGITUDINAL SECTION THROUGH AN EMBRYO OF CUCUMARIA
DOLIOLUM AT THE END OF THE FOURTH DAY.

_Vpv._ vaso-peritoneal vesicle; _ME._ mesenteron; _Blp._, _Ptd._
blastopore, proctodæum.]

Enteropneusta (Balanoglossus).--The body cavity is derived from two pairs of alimentary diverticula, the walls of which give rise to the greater part of the mesoblast.

Chordata.--Paired archenteric outgrowths give rise to the whole mesoblast in Amphioxus (fig. 211), and the mode of formation of the mesoblast in other Chordata is probably secondarily derived from this.

3. The cells which will form the mesoblast become marked out very early, and cannot be regarded as definitely springing from either of the primary layers.

Turbellaria.--Leptoplana (fig. 212), Planaria polychroa (?).

Chætopoda.--Lumbricus, &c.

Discophora.

It is very possible that the cases quoted under this head ought more properly to belong to group 1.

4. The mesoblast cells are split off from the epiblast.

Nemertea.--Larva of Desor. The mesoblast is stated to be split off from the four invaginated discs.

5. The mesoblast is split off from the hypoblast.

Nemertea.--Some of the types without a metamorphosis.

Mollusca.--Scaphopoda. It is derived from the lateral and ventral cells of the hypoblast.

Gephyrea.--Phascolosoma.

Vertebrata.--In most of the Ichthyopsida the mesoblast is derived from the hypoblast (fig. 213). In some types (_i.e._ most of the Amniota) the mesoblast might be described as originating at the lips of the blastopore (primitive streak).

6. The mesoblast is derived from both germinal layers.

Tracheata.--Araneina (fig. 214). It is derived partly from cells split off from the epiblast and partly from the yolk-cells; but it is probable that the statement that the mesoblast is derived from both the germinal layers is only formally accurate; and that the derivation of part of the mesoblast from the yolk-cells is not to be interpreted as a derivation from the hypoblast.

Amniota.--The derivation of the mesoblast of the Amniota from both the primary germinal layers is without doubt a secondary process.

The conclusions to be drawn from the above summary are by no means such as might have been anticipated. The analogy of the Coelenterata would lead us to expect that the mesoblast would be derived partly from the epiblast and partly from the hypoblast. Such, however, is not for the most part the case, though more complete investigations may shew that there are a greater number of instances in which the mesoblast has a mixed origin than might be supposed from the above summary.

[FIG. 211. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. (After
Kowalevsky.)

A. Section at gastrula stage.
B. Section of a somewhat older embryo.
C. Section through the anterior part of still older embryo.

_np._ neural plate; _nc._ neural canal; _mes._ archenteron in A, and
mesenteron in B and C; _ch._ notochord; _so._ mesoblastic somite.]

I have attempted to reduce the types of development of the mesoblast to six; but owing to the nature of the case it is not always easy to distinguish the first of these from the last four. Of the six types the second will on most hands be admitted to be the most remarkable. The formation of hollow outgrowths of the archenteron, the cavities of which give rise to the body cavity, can only be explained on the supposition that the body cavity of the types in which such outgrowths occur is derived from diverticula cut off from the alimentary tract. The lining epithelium of the diverticula--the peritoneal epithelium--is clearly part of the primitive hypoblast, and this part of the mesoblast is clearly hypoblastic in origin.

[FIG. 212. SECTIONS THROUGH THE OVUM OF LEPTOPLANA TREMELLARIS IN
THREE STAGES OF DEVELOPMENT. (After Hallez.)

_ep._ epiblast; _m._ mesoblast; _hy._ yolk-cells (hypoblast); _bl._
blastopore.]

In the case of the Chætognatha (Sagitta), Brachiopoda, and Amphioxus, the whole of the mesoblast originates from the walls of the diverticula; while in the Echinodermata the walls of the diverticula only give rise to the vaso-peritoneal epithelium, the remainder of the mesoblast being derived from amoeboid cells which spring from the walls of the archenteron before the origin of the vaso-peritoneal outgrowths (figs. 199 and 210).

Reserving for the moment the question as to what conclusions can be deduced from the above facts as to the origin of the mesoblast, it is important to determine how far the facts of embryology warrant us in supposing that in the whole of the triploblastic forms the body cavity originated from the alimentary diverticula. There can be but little doubt that the mode of origin of the mesoblast in many Vertebrata, as two solid plates split off from the hypoblast, in which a cavity is secondarily developed, is an abbreviation of the process observable in Amphioxus; but this process approaches in some forms of Vertebrata to the ingrowth of the mesoblast from the lips of the blastopore.

It is, therefore, highly probable that the paired ingrowths of the mesoblast from the lips of the blastopore may have been in the first instance derived from a pair of archenteric diverticula. This process of formation of the mesoblast is, as may be seen by reference to the summary, the most frequent, including as it does the Chætopoda, the Mollusca, the Arthropoda, &c.[133]

[133] The wide occurrence of this process was first pointed out
by Rabl. He holds, however, a peculiar modification of the
gastræa theory, for which I must refer the reader to his paper
(No. 284); according to this theory the mesoblast has sprung from
a zone of cells of the blastosphere, at the junction between the
cells which will be invaginated and the epiblast cells. In the
bilateral blastosphere, from which he holds that all the higher
forms (Bilateralia) have originated, these cells had a bilateral
arrangement, and thus the bilateral origin of the mesoblast is
explained. The origin of the mesoblast from the lips of the
blastopore is explained by the position of its mother-cells in
the blastosphere. It need scarcely be said that the views already
put forward as to the probable mode of origin of the mesoblast,
founded on the analogy of the Coelenterata, are quite
incompatible with Rabl's theories.

[FIG. 213. TWO SECTIONS OF A YOUNG ELASMOBRANCH EMBRYO, TO SHEW THE
MESOBLAST SPLIT OFF AS TWO LATERAL MASSES FROM THE HYPOBLAST.

_mg._ medullary groove; _ep._ epiblast; _m._ mesoblast; _hy._
hypoblast; _n.al._ cells formed around the nuclei of the yolk which
have entered the hypoblast.]

While there is no difficulty in the view that the body cavity may have originated from a pair of enteric diverticula in the case of the forms where a body cavity is present, there is a considerable difficulty in holding this view, for forms in which there is no body cavity distinct from the alimentary diverticula.

Of these types the Platyelminthes are the most striking. It is, no doubt, possible that a body cavity may have existed in the Platyelminthes, and become lost; and the case of the Discophora, which in their muscular and connective tissue systems as well as in the absence of a body cavity resemble the Platyelminthes, may be cited in favour of this view, in that, being closely related to the Chætopoda, they are almost certainly descended from ancestors with a true body cavity. The usual view of the primitive character of the Platyelminthes, which has much to support it, is, however, opposed to the idea that the body cavity has disappeared.

If Kowalevsky[134] is right in stating that he has found a form intermediate between the Coelenterata and the Platyelminthes, there will be strong grounds for holding that the Platyelminthes are, like the Coelenterata, forms the ancestors of which were not provided with a body cavity.

[134] _Zoologischer Anzeiger_, No. 52, p. 140. This form has been
named by Kowalevsky _Coeloplana Metschnikowii_. Kowalevsky's
description appears, however, to be quite compatible with the
view that this form is a creeping Ctenophor, in no way related to
the Turbellarians.

Perhaps the triploblastica are composed of two groups, viz. (1) a more ancestral group (the Platyelminthes), in which there is no body cavity as distinct from the alimentary, and (2) a group descended from these, in which two of the alimentary diverticula have become separated from the alimentary tract to form a body cavity (remaining triploblastica). However this may be, the above considerations are sufficient to shew how much there is that is still obscure with reference even to the body cavity.

[FIG. 214. SECTION THROUGH AN EMBRYO OF AGELENA LABYRINTHICA.

The section is represented with the ventral plate upwards. In the
ventral plate is seen a keel-like thickening, which gives rise to
the main mass of the mesoblast.

_yk._ yolk divided into large polygonal cells, in several of which
are nuclei.]

If embryology gives no certain sound as to the questions just raised with reference to the body cavity, still less is it to be hoped that the remaining questions with reference to the origin of the mesoblast can be satisfactorily answered. It is clear, in the first place, from an inspection of the summary given above, that the process of development of the mesoblast is, in all the higher forms, very much abbreviated and modified. Not only is its differentiation relatively deferred, but it does not in most cases originate, as it must have done to start with, as a more or less continuous sheet, split off from parts of one or both the primary layers. It originates in most cases from the hypoblast, and although the considerations already urged preclude us from laying very great stress on this mode of origin, yet the derivation of the mesoblast from the walls of archenteric outgrowths suggests the view that the whole, or at any rate the greater part, of the mesoblast primitively arose by a process of histogenic differentiation from the walls of the archenteron or rather from diverticula of these walls. This view, which was originally put forward by myself (No. 260), appears at first sight very improbable, but if the statement of the Hertwigs (No. 270), that there is a large development of a hypoblastic muscular system in the Actinozoa, is well founded, it cannot be rejected as impossible. Lankester (No. 279), on the other hand, has urged that the mode of origin of the mesoblast in the Echinodermata is more primitive; and that the amoeboid cells which here give rise to the muscular and connective tissues represent cells which originally arose from the whole inner surface of the epiblast. It is, however, to be noted that even in the Echinodermata the amoeboid cells actually arise from the _hypoblast_, and their mode of origin may, therefore, be used to support the view that the main part of the muscular system of higher types is derived from the primitive hypoblast.

The great changes which have taken place in the development of the mesoblast would be more intelligible on this view than on the view that the major part of the mesoblast primitively originated from the epiblast. The presence of food-yolk is much more frequent in the hypoblast than in the epiblast; and it is well known that a large number of the changes in early development are caused by food-yolk. If, therefore, the mesoblast has been derived from the hypoblast, many more changes might be expected to have been introduced into its early development than if it had been derived from the epiblast. At the same time the hypoblastic origin of the mesoblast would assist in explaining how it has come about that the development of the nervous system is almost always much less modified than that of the mesoblast, and that the nervous system is not, as might, on the grounds of analogy, have been anticipated, as a rule secondarily developed in the mesoblast.