Chapter XIV: Act 1890: , No. 1108, ss. 45-48: Tasmania, Landlord and Tenant Act 1874, (2)

To the unaided eye the main difference between the newly formed
zygotes of different species of animals is that of bulk, and this is
due to the amount of food-yolk held in suspension in the protoplasm.
The ovum of the fowl is 30 mm. in diameter, that of the frog 1.75 mm.,
while the ova of the rabbit and _Amphioxus_ have a diameter of .l mm.
The food-yolk is deposited in the ovum as a result of the vital
activity of its protoplasm, while the ovum is still a part of the
ovary of the parent. It is an inert substance which is used as food
later on by the developing embryo, and it acts as a dilutant of the
living matter of the ovum. It has a profound influence on the
subsequent developmental process. The newly formed zygotes of
different species of animals have undoubtedly, as staved above, a
certain family resemblance to one another; but however great this
superficial resemblance may be, the differences must be most profound,
and this fact becomes at once obvious when the properties of these
remarkable masses of matter are closely investigated.

Causes of development.

As in the case of so many other forms of matter, the more important properties of the zygote do not become apparent until it is submitted to the action of external forces. These forces constitute the external conditions of existence, and the properties which are called forth by their action are called the acquired characters of the organism. The investigation of these properties, particularly of those which are called forth in the early stages of the process, constitutes the science of Embryology. With regard to the manifestation of these properties, certain points must be clearly understood at the outset:--(1) If the zygote is withheld from the appropriate external influences, e.g. if a plant-seed be kept in a box free from moisture or at a low temperature, no properties are evolved, and the zygote remains apparently unchanged; (2) the acquisition of the properties which constitutes the growth and development of the organism proceeds in a perfectly definite sequence, which, so far as is known, cannot be altered; (3) just as the features of the growing organism change under the continued action of the external conditions, so the external conditions themselves must change as the organism is progressively evolved. With regard to this last change, it may be said generally that it is usually, if not always, effected by the organism itself, making use of the properties which it has acquired at earlier stages of its growth, and acting in response to the external conditions. There is, to use a phrase of Mr Herbert Spencer, a continuous adjustment between the external and internal relations. For every organism a certain succession of conditions is necessary if the complete and normal evolution of properties is to take place. Within certain limits, these conditions may vary without interfering with the normal evolution of the properties, though such variations are generally responded to by slight but unimportant variation of the properties (variation of acquired characters). But if the variation of the conditions is too great, the evolved properties become abnormal, and are of such a nature as to preclude the normal evolution of the organism; in other words, the action of the conditions upon the organism is injurious, causing abortions and, ultimately, death. For many organisms the conditions of existence are well known for all stages of life, and can be easily imitated, so that they can be reared artificially and kept alive and made to breed in confinement--e.g. the common fowl. But in a large number of cases it is not possible, through ignorance of the proper conditions, or on account of the difficulty of imitating them, to make the organism evolve all its properties. For instance, there are many marine larvae which have never been reared beyond a certain point, and there are some organisms which, even when nearly full-grown--a stage of life at which it is generally most easy to ascertain and imitate the natural conditions--will not live, or at any rate will not breed, in captivity. Of late years some naturalists have largely occupied themselves with experimental observation of the effects on certain organisms of marked and definite changes of the conditions, and the name of Developmental Mechanics (or _Physiology of Development_) has been applied to this branch of study (see below).

Gametogeny.

In normal fertilization, as a rule, only one spermatozoon fuses with
the ovum. It has been observed in some eggs that a membrane, formed
round the ovum immediately after the entrance of the spermatozoon,
prevents the entrance of others. If than one spermatozoon enters, a
corresponding number of male pronuclei are formed, and the subsequent
development, if it takes place at all, is abnormal and soon ceases. An
egg by ill-treatment (influence of chloroform, carbonic acid, &c.) can
be made to take more than one spermatozoon. In some animals it appears
that several spermatozoa may normally enter the ovum (some Arthropoda,
Selachians, Amphibians and Mammals), but of these only one forms a
male pronucleus (see below), the rest being absorbed. Gametogeny is
the name applied to the formation of the gametes, i.e. of the ova and
spermatozoa. The cells of the reproductive glands are the germ cells
(_oogonia_, _spermatogonia_). They undergo division and give rise to
the progametes, which in the case of the female are sometimes called
_oocytes_, in the case of the male _spermatocytes_. The oocytes are
more familiarly called the ovarian ova. The nucleus of the oocyte is
called the germinal vesicle. The oocyte (progamete) gives rise by
division to the ovum or true gamete, the nucleus of which is called
the _female pronucleus_. As a general rule the oocyte divides
unequally twice, giving rise to two small cells called polar bodies,
and to the ovum. The first formed polar body frequently divides when
the oocyte undergoes its second and final division, so that there are
three polar bodies as well as the ovum resulting from the division of
the oocyte or progamete. Sometimes the ovum arises from the oocyte by
one division only, and there is only one polar body (e.g. mouse,
Sobotta, _Arch. f. mikr. Anat., 1895_, p. 15). The polar bodies are
oval, but as a rule they are so small as to be incapable of
fertilization. They may therefore be regarded as abortive ova. In one
case, however (see Francotte, _Bull. Acad. Belg._ (3), xxxiii., 1897,
p. 278), the first formed polar body is nearly as large as the ovum,
and is sometimes fertilized and develops. The spermatogonia are the
cells of the testis; these produce by division the spermatocytes
(progametes), which divide and give rise to the spermatids. In most
cases which have been investigated the divisions by which the
spermatids arise from the spermatocytes are two in number, so that
each spermatocyte gives origin to four spermatids. Each spermatid
becomes a functional spermatozoon or male gamete. The gametogeny of
the male therefore closely resembles that of the female, differing
from it only in the fact that all the four products of the progamete
become functional gametes, whereas in the female only one, the ovum,
becomes functional, the other three (polar bodies) being abortive. In
the spermatogenesis of the bee, however, the spermatocyte only divides
once, giving rise to a small polar-body-like structure and one
spermatid (Meves, _Anat. Anzeiger_, 24, 1904, pp. 29-32). The nucleus
of the male gamete is not called the male pronucleus, as would be
expected, that term being reserved for the second nucleus which
appears in the ovum after fertilization. As this is in all probability
derived entirely from the nucleus of the spermatozoon, we should be
almost justified in calling the nucleus of the spermatozoon the male
pronucleus. In most forms in which the formation of the gametes from
the progamete has been accurately followed, and in which the progamete
of both sexes divides twice in forming the gametes, the division of
the nucleus presents certain peculiarities. In the first place,
between the first division and the second it does not enter into the
resting state, but immediately proceeds to the second division. In the
second place, the number of chromosomes which appear in the final
divisions of the progametes and assist in constituting the nuclei of
the gametes is half the number which go to constitute the new nuclei
in the ordinary nuclear divisions of the animal. The number of
chromosomes of the nucleus of the gamete is therefore reduced, and the
divisions by which the gametes arise from the progametes are called
reducing (_maiotic_) divisions. It is not certain, however, that this
phenomenon is of universal occurrence, or has the significance which
is ordinarily attributed to it. In the parthenogenetic ova of certain
insects, e.g. _Rhodites rosae_ (Henking), _Nematus lacteus_
(Doncaster, _Quart. Journal Mic. Science_, 49, 1906, pp. 561-589),
reduction does not occur, though two polar bodies are formed.

Fertilization.

As soon as the spermatozoon has conjugated with the ovum, a second
nucleus appears in the ovum. This is undoubtedly derived from the
spermatozoon, possibly from its nucleus only, and is called the male
pronucleus. It possesses in the adjacent protoplasm a well-marked
centrosome. The general rule appears to be that the female pronucleus
is without a centrosome, and that no centrosome appears in the female
in the divisions by which the gamete arises from the progamete. If
this is true, the centrosome of the zygote nucleus must be entirely
derived from that of the male pronucleus. This accounts for the fact,
which has been often observed, that the female pronucleus is not
surrounded by protoplasmic radiations, whereas such radiations are
present round the male pronucleus in its approach to the female. In
the mouse the subsequent events are as follow:--Both pronuclei assume
the resting form, the chromatin being distributed over the nuclear
network, and the nuclei come to lie side by side in the centre of the
egg. A long loop of chromatin then appears in each nucleus and divides
up into twelve pieces, the chromosomes. The centrosome now divides,
the membranes of both nuclei disappear, and a spindle is formed. The
twenty-four chromosomes arrange themselves at the centre of this
spindle and split longitudinally, so that forty-eight chromosomes are
formed. Twenty-four of these, twelve male and twelve female, as it is
supposed, travel to each pole of the spindle and assist in giving rise
to the two nuclei. At the next nuclear division twenty-four
chromosomes appear in each nucleus, each of which divides
longitudinally; and so in all subsequent divisions. The fusion of the
two pronuclei is sometimes effected in a manner slightly different
from that described for the mouse. In _Echinus_, for instance, the two
pronuclei fuse, and the spindle and chromosomes are formed from the
zygote nucleus, whereas in the mouse the two pronuclei retain their
distinctness during the formation of the chromosomes. There appears,
however, to be some variation in this respect: cases have been
observed in the mouse in which fusion of the pronuclei occurs before
the separation of the chromosomes.

Parthenogenesis.

Parthenogenesis, or development of the female gamete without
fertilization, is known to occur in many groups of the animal kingdom.
Attempts have been made to connect this phenomenon with peculiarities
in the gametogeny. For instance, it has been said that parthenogenetic
ova form only one polar body. But, as we have seen, this is sometimes
the case in eggs which are fertilized, and parthenogenetic ova are
known which form two polar bodies, e.g. ova of the honey-bee which
produce drones (_Morph. Jahrb._ xv., 1889, p. 85). ova of Rotifera
which produce males (_Zool. Anzeiger_, xx., 1897, p. 455), ova of some
saw-flies and gall flies which produce females (L. Doncaster, _Quart.
Journ. Mic. Sc._, 49, 1906, pp. 561-589). Again it has been asserted
that in parthenogenetic eggs the polar bodies are not extruded from
the ovum; in such cases, though the nucleus divides, those of its
products which would in other cases be extruded in polar bodies remain
in the protoplasm of the ovum. But this is not a universal rule, for
in some cases of parthenogenesis polar bodies are extruded in the
usual way (_Aphis_, some Lepidoptera), and in some fertilized eggs the
polar bodies are retained in the ovum.

It is quite probable that parthenogenesis is more common than has been
supposed, and it appears that there is some evidence to show that ova,
which in normal conditions are incapable of developing without
fertilization, may yet develop if subjected to an altered
environment. For instance, it has been asserted that the addition of a
certain quantity of chloride of magnesium and other substances to
sea-water will cause the unfertilized ova of certain marine animals
(_Arbacia_, _Chaetopterus_) to develop (J. Loeb, _American Journal of
Physiology_, ix., 1901, p. 423); and according to M.Y. Delage
(_Comptes rendus_, 135, 1902. Nos. 15 and 16) such development may
occur after the formation of polar bodies, the chromosomes undergoing
reduction and the full number being regained in the segmenting stage.
These experiments, if authenticated, suggest that ova have the power
of development, but are not able to exercise it in their normal
surroundings. There is reason to believe that the same assertion may
be made of spermatozoa. Phenomena of the nature of parthenogenesis
have never been observed in the male gamete, but it has been suggested
by A. Giard (_Cinquantenaire de la Soc. de Biol._, 1900) that the
phenomenon of the so-called fertilization of an enucleated ovum which
has been described by T. Boveri and Delage in various eggs, and which
results in development up to the larval form (_merogony_), is in
reality a case in which the male gamete, unable to undergo development
in ordinary circumstances on account of its small size and
specialization of structure has obtained a nutritive environment which
enables it to display its latent power of development. Moreover, A.M.
Giard suggests that in some cases of apparently normal fertilization
one of the pronuclei may degenerate, the resultant embryo being the
product of one pronucleus only. In this way he explains certain cases
of hybridization in which the paternal (rarely the maternal) type is
exclusively reproduced. For instance, in the batrachiate Amphibia,
Heron Royer succeeded in 1883 in rearing, out of a vast number of
attempts, a few hybrids between a female _Pelobates fuscus_ and a male
_Rana fusca_; the product was a _Rana fusca_. He also crossed a female
_Bufo vulgaris_ with a male _Bufo calamita_; in the few cases which
reached maturity the product was obviously a _Bufo calamita_. Finally,
H.E. Ziegler (_Arch. f. Ent.-Mech._, 1898, p. 249) divided the
just-fertilized ovum of a sea-urchin in such a way that each half had
one pronucleus; the half with the male pronucleus segmented and formed
a blastula, the other degenerated. It is said that in a few species of
animals males do not occur, and that parthenogenesis is the sole means
of reproduction (a species of Ostracoda among Crustacea; species of
Tenthredinidae, Cynipidae and Coccidae among Insecta); this is the
thelytoky of K.T.E. von Siebold. The number of species in which males
are unknown is constantly decreasing, and it is quite possible that
the phenomenon does not exist. Parthenogenesis, however, is
undoubtedly of frequent occurrence, and is of four kinds, namely, (1)
that in which males alone are produced, e.g. honey-bees
(_arrhenotoky_); (2) that in which females only are produced
(_thelytoky_), as in some saw-flies; (3) that in which both sexes are
produced (_deuterotoky_), as in some saw-flies; (4) that in which
there is an alternation of sexual and parthenogenetic generations, as
in Aphidae, many Cynipidae, &c. It would appear that "parthenogenesis
does not favour the production of one sex more than another, but it is
clear that it decidedly favours the production of a brood that is
entirely of one sex, but which sex that is differs according to
circumstances" (D. Sharp, _Cambridge Natural History_, "Insects," pt.
i. p. 498). In some Insecta and Crustacea exceptional parthenogenesis
occurs: a certain proportion of the eggs laid are capable of
undergoing either the whole or a part of development
parthenogenetically, e.g. _Bombyx mori_, &c. (A. Brauer, _Arch. f.
mikr. Anat._, 1893; consult also E. Maupas on parthenogenesis of
Rotifera, _Comp. rend._, 1889-1891, and R. Lauterborn, _Biol.
Centralblatt_, xviii., 1898, p. 173).

Determination of sex.

The question of the determination of sex may be alluded to here. Is
sex determined at the act of conjugation of the two gametes? Is it, in
other words, an unalterable property of the zygote, a genetic
character? Or does it depend upon the conditions to which the zygote
is subjected in its development? In other words, is it an acquired
character? It is impossible in the present state of knowledge to
answer these questions satisfactorily, but the balance of evidence
appears to favour the view that sex is an unalterable, inborn
character. Thus those twins which are believed to come from a split
zygote are always of the same sex, members of the same litter which
have been submitted to exactly similar conditions are of different
sexes, and all attempts to determine the sex of offspring in the
higher animals by treatment have failed. On the other hand, the male
bee is a portion of a female zygote--the queen-bee. The same remark
applies to the male Rotifer, in which the zygote always gives rise to
a female, from which the male arises parthenogenetically, but in these
cases it does not appear that the production of males is in any way
affected by external conditions (see R.C. Punnett, _Proc. Royal Soc._,
78 B, 1906, p. 223). It is said that in human societies the number of
males born increases after wars and famines, but this, if true, is
probably due to an affection of the gametes and not of the young
zygote. For a review of the whole subject see L. Cuenot, _Bull. sci.
France et Belgique_, xxxii., 1899, pp. 462-535.

Cleavage.

The first change the zygote undergoes in all animals is what is generally called the segmentation or cleavage of the ovum. This consists essentially of the division of the nucleus into a number of nuclei, around which the protoplasm sooner or later becomes arranged in the manner ordinarily spoken of as cellular. This division of the nucleus is effected by the process called binary fission; that is to say, it first divides into two, then each of these divides simultaneously again into two, giving four nuclei; each of these after a pause again simultaneously divides into two. So the process continues for some time until the ovum becomes possessed of a large number of nuclei, all of which have proceeded from the original nucleus by a series of binary fissions. This division of the nucleus, which constitutes the essential part of the cleavage of the ovum, continues through the whole of life, but it is only in the earliest period that it is distinguished by a distinct name and used to characterize a stage of development. The nuclear division of cleavage is usually at first a rhythmical process; all the nuclei divide simultaneously, and periods of nuclear activity alternate with periods of rest. Nuclear divisions may be said to be of three kinds, according to the accompanying changes in the surrounding protoplasm: (1) accompanied by no visible change, e.g. the multinucleated Protozoon _Actinosphaerium_; (2) accompanied by a rearrangement of the protoplasm around each nucleus, but not by its division into two separate masses, e.g. the division which results in the formation of a colony of Protozoa; (3) accompanied by the division of the protoplasm into two parts, so that two distinct cells result, e.g. the divisions by which the free wandering leucocytes are produced, the reproduction of uninuclear Protozoa, &c. In the cleavage of the ovum the first two of these methods of division are found, but probably not the third. At one time it was thought that the nuclear divisions of cleavage were always of the third kind, and the result of cleavage was supposed to be a mass of isolated cells, which became reunited in the subsequent development to give rise to the later connexion between the tissues which were known to exist. But in 1885 it was noticed that in the ovum of _Peripatus capensis_ (A. Sedgwick, _Quart. Journ. Mic. Science_, xxv., 1885, p. 449) the extra-nuclear protoplasm did not divide in the cleavage of the ovum, but merely became rearranged round the increasing nuclei; the continuity of the protoplasm was not broken, but persisted into the later stages of growth, and gave rise to the tissue-connexions which undoubtedly exist in the adult. This discovery was of some importance, because it rendered intelligible the unity of the embryo so far as its developmental processes are concerned, the maintenance of this unity being somewhat surprising on the previous view. On further inquiry and examination it was found that the ova of many other animals presented a cleavage essentially similar to that of _Peripatus_. Indeed, it was found that the nuclear divisions of cleavage were of the first two kinds just described. In some eggs, e.g. the Alcyonaria, the first nuclear divisions are effected on the first plan, i.e. they take place without at first producing any visible effect upon the protoplasm of the egg. But in the later stages of cleavage the protoplasm becomes arranged around each nucleus and related to it as to a centre. In the majority of eggs, however, the protoplasm, though not undergoing complete cleavage, becomes rearranged round each nucleus as these are formed. The best and clearest instance of this is afforded by many Arthropodan eggs, in which the nucleus of the just-formed zygote takes up a central position, where it undergoes its first division, subsequent divisions taking place entirely within the egg and not in any way affecting its exterior. The result is to give rise to a nucleated network or foam-work of protoplasm, ramifying through the yolk-particles and containing these in its meshes.

In other Arthropodan eggs the cleavage is on the so-called centrolecithal type, in which the dividing nuclei pass to the cortex of the ovum, and the surface of the ovum becomes indented with grooves corresponding to each nucleus. In this kind of cleavage all the so-called segments are continuous with the central undivided yolk-mass. It sometimes happens that in Arthropods the egg breaks up into masses, which cannot be said to have the value of cells, as they are frequently without nuclei. In other eggs, characterized by a considerable amount of yolk, e.g. the ova of Cephalopoda, and of the Vertebrata with much yolk, the first nucleus takes up an eccentric position in a small patch of protoplasm which is comparatively free from yolk-particles. This patch is the germinal disc, and the nuclear divisions are confined to it and to the transitional region, where it merges into the denser yolk which makes up the bulk of the egg. At the close of segmentation the germinal disc consists of a number of nuclei, each surrounded by its own mass of protoplasm, which is, however, not separated from the protoplasm round the neighbouring nuclei, as was formerly supposed, but is continuous at the points of contact. In this manner the germinal disc has become converted into the blastoderm, which consists of a small watch-glass-shaped mass of so-called cells resting on, but continuous with, the large yolk-mass. It is characteristic of this kind of ovum that there is always a row of nuclei, called the yolk-nuclei, placed in the denser yolk immediately adjacent to the blastoderm. These nuclei are continually undergoing division, one of the products of division, together with a little of the sparse yolk protoplasm, passing into the blastoderm to reinforce it (so-called formative cells). The other product of the dividing yolk-nuclei remains in the yolk, in readiness for the next division. In this manner nucleated masses of protoplasm are continually being added to the periphery of the blastoderm and assisting in its growth. But it must be borne in mind that all the nucleated masses of which the blastoderm consists are in continuity with each other and with the sparse protoplasmic reticulum of the subjacent yolk.

In the great majority of eggs, then, the nuclear division of cleavage is not accompanied by a complete division of the ovum into separate cells, but only by a rearrangement of the protoplasm, which produces, indeed, the so-called cellular arrangement, and an appearance only of separate cells. But there still remain to be mentioned those small eggs in which the amount of yolk is inconsiderable, and in which division of the nuclei does appear to be accompanied by a complete division of the surrounding protoplasm into separate unconnected cells--ova of many Annelida, Mollusca, Echinoderma, &c., and of Mammalia amongst Vertebrata. In the case of these also (G.F. Andrews, _Zool. Bulletin_, ii., 1898) it has been shown that the apparently separate spheres are connected by a number of fine anastomosing threads of a hyaline protoplasm, which are not easy to detect and are readily destroyed by the action of reagents. It is therefore probable that the divisions of the nuclei in cleavage are in no case accompanied by complete division of the surrounding protoplasm, and the organism in the cleavage stage is a continuous whole, as it is in all the other stages of its existence.

Division of embryo.

Of late years a great number of experiments have been made to discover the effects of dividing the embryo during its cleavage, and of destroying certain portions of it. These experiments have been made with the object of testing the view, held by some authorities, that certain segments are already set apart in cleavage to give rise to certain adult organs, so that if they were destroyed the organs in question could not be developed. The results obtained have not borne out this view. Speaking generally, it may be said that they have been different according to the stage at which the separation was effected and the conditions under which the experiment was carried out. If the experiment be made at a sufficiently early stage, each part, if not too small, will develop into a normal, though small, embryo. In some cases the embryo remained imperfect for a certain time after the experiment, but the loss is eventually made good by regeneration. (For a summary of the work done on this subject see R.S. Bergh, _Zool. Centralblatt_, vii., 1900, p. 1.)

The layer theory.

The end of cleavage is marked by the commencement of the differentiation of the organs. The first differentiation is the formation of the layers. These are three in number, being called respectively the ectoderm, endoderm and mesoderm, or, in embryos in which at their first appearance they lie like sheets one above the other, the epiblast, hypoblast and mesoblast. The layers are sometimes spoken of as the primary organs, and their importance lies in the fact that they are supposed to be generally homologous throughout the series of the Metazoa. This view, which is based partly on their origin and partly on their fate, had great influence on the science of comparative anatomy during the last thirty years of the 19th century, for the homology of the layers being admitted, they afforded a kind of final court of appeal in determining questions of doubtful homologies between adult organs. Great importance was therefore attached to them by embryologists, and both their mode of development and the part which they play in forming the adult organs were examined with the greatest care. It is very unusual for all the layers to be established at the same time. As a general rule the ectoderm and endoderm, which may be called the primary layers, come first, and later the mesoderm is developed from one or other of them. There are two main methods in which the first two are differentiated--invagination and delamination. The former is generally found in small eggs, in which the embryo at the close of cleavage assumes the form of a sphere, having a fluid or gelatinous material in its centre, and bounded externally by a thin layer of protoplasm, in which all the nuclei are contained. Such a sphere is called a blastosphere, and may be regarded as a spherical mass of protoplasm, of which the central portion is so much vacuolated that it seems to consist entirely of fluid. The central part of the blastosphere is called the segmentation cavity or blastocoel. The blastosphere soon gives rise, by the invagination of one part of its wall upon the other, and a consequent obliteration of the segmentation cavity, to a double-walled cup with a wide opening, which, however, soon becomes narrowed to a small pore. This cup-stage is called the gastrula stage; the outer wall of the gastrula is the ectoderm, and its inner the endoderm; while its cavity is the enteron, and the opening to the exterior the blastopore. Origin of the primary layers by delamination occurs universally in eggs with large yolks (Cephalopoda and many Vertebrata), and occasionally in others. In it cleavage gives rise to a solid mass, which divides by delamination into two layers, the ectoderm and endoderm. The main difference between the two methods of development lies in the fact that in the first of them the endoderm at its first origin shows the relations which it possesses in the adult, namely, of forming the epithelial wall of the enteric space, whereas in the second method the endoderm is at first a solid mass, in which the enteric space makes its appearance later by excavation. In the delaminate method the enteric space is at first without a blastopore, and sometimes it never acquires this opening, but a blastopore is frequently formed, and the two-layered gastrula stage is reached, though by a very different route from that taken in the formation of the invaginate gastrula. According to the layer-theory, these two layers are homologous throughout the series of Metazoa; their limits can always be accurately defined, they give rise to the same organs in all cases, and the adult organs (excluding the mesodermal organs) can be traced back to one or other of them with absolute precision. Thus the ectoderm gives rise to the epidermis, to the nervous system, and to the lining of the stomodaeum and proctodaeum, if such parts of the alimentary canal are present. The endoderm, on the other hand, gives rise to the lining of the enteron, and of the glands which open into it.

Mesoderm.

So far as these two layers are concerned, and excluding the mesoderm, it would appear that the layer-theory does apply in a very remarkable manner to the whole of the Metazoa. But even here, when the actual facts are closely scanned, there are found to be difficulties, which appear to indicate that the theory may not perhaps be such an infallible guide as it seems at first sight. Leaving out of consideration the case of the Mammalia, in which the differentiation of the segmented ovum is not into ectoderm and endoderm, and the case of the sponges, the most important of these difficulties concern the stomodaeum and proctodaeum. The best case to examine is that of _Peripatus capensis_, in which the blastopore is at first a long slit, and gives rise to both the mouth and the anus of the adult. Here there is always found at the lips of the blastopore, and extending for a short distance inwards as enteric lining, a certain amount of tissue, which by its characters must be regarded as ectoderm. Now, in the closure of the blastopore between the mouth and anus, this tissue, which at the mouth and anus develops into the lining of the stomodaeum and proctodaeum, is left inside, and actually gives rise to the median ventral epithelium of the alimentary canal. Hence the development of _Peripatus capensis_ suggests the conclusion, if we strictly apply the layer-theory, that a considerable portion of the true mesenteron is lined by ectoderm, and is not homologous with the corresponding portion of the mesenteron of other animals--a conclusion which will on all hands be admitted to be absurd. The difficulties in the application of the layer-theory become vastly greater when the origin and fate of the mesoderm is considered. The mesoderm is, if we may judge from the number of organs which are derived from it, much the most important of the three layers. It generally arises later than the others, and in its very origin presents difficulties to the theory, which are much increased when we consider its history. It is generally, though not always, developed from the endoderm, either as hollow outgrowths containing prolongations of the enteric cavity, which become the coelom, or as solid proliferations. But in some groups the mesoderm is actually laid down in cleavage, and is present at the end of that process. In others it is entirely derived from the ectoderm (_Peripatus capensis_). In yet others it is partly derived from endoderm and partly from ectoderm (primitive streak of amniotic Vertebrates). Finally, in whatever manner the first rudiments are developed, it frequently receives considerable reinforcements from one of the primary layers. For instance, the structure known as the nerve crest of the vertebrate embryo is not, as was formerly supposed, exclusively concerned with the formation of the spinal nerves and ganglia, but contributes largely to the mesoderm of the axial region of the body. This is particularly clearly seen in the case of the anterior part of the head of Elasmobranch and probably of other vertebrate embryos, where all the mesoderm present is derived from the anterior part of the neural crest (_Quart. Journ. Mic. Science_, xxxvii. p. 92).

The layer-theory, then, will not bear critical examination. It is clear, both from their origin and history, that the layers or masses of cells called ectoderm, endoderm and mesoderm have not the same value in different animals; indeed, it is misleading to speak of three layers. At the most we can only speak of two, for the mesoderm is formed after the others, has a composite origin, and has no more claim to be considered an embryonic layer than has the rudiment of the central nervous system, which in some animals, indeed, appears as soon as the mesoderm. Arguments as to homology, based on derivation or non-derivation from the same embryonic layer, have therefore in themselves but little value.

It has frequently been asserted that the reproductive cells are marked
off at a very early stage of the development (_Sagitta_, certain
Crustacea, _Scorpio_). Recently it has been asserted that in _Ascaris_
(T. Boveri, _Kuppfer's Festschrift_, 1899, p. 383) the reproductive
cells are set apart after the first cleavage, and that they can be
traced by certain peculiarities of their nuclei into the adult
reproductive glands.

Mesenchyme.

It has been already stated that the mesoderm is a composite tissue.
This fact is frequently conspicuous at its first establishment. In
many Coelomata it is present under two forms from the beginning. One
of these is epithelial in character, while the other has the form of a
network of protoplasm, with nuclei at the nodes. The former is called
simply epithelial mesoderm, the latter mesenchyme. Sometimes the
epithelial mesoderm is the first formed, and what little mesenchyme
there is is developed from it (_Amphioxus, Balanoglossus_, &c.)
Sometimes the mesenchyme is the first to arise, the epithelial
mesoderm developing from it (most, if not all, Vertebrates). Finally,
it sometimes happens that these two kinds of tissue arise separately
from one or other of the primary layers (Echinodermata). As already
hinted, in _Balanoglossus_ and _Amphioxus_ the whole of the mesoderm
of the body is at first in an epithelial condition, being developed as
an outgrowth of the gut-wall. In _Peripatus capensis_ also, and
possibly in other Arthropods, it has at first an intermediate form,
being derived from a primitive streak and not from the gut-wall, but
it rapidly assumes an epithelial structure, from which all the
mesodermal tissues are developed. In Annelids the bulk of the mesoderm
has at first a modified epithelial form similar to that of Arthropods,
but it is formed, not from a primitive streak, but from some peculiar
cells produced in cleavage, called pole-cells. In Annelids with
trochosphere larvae a certain amount of mesenchyme is formed at an
earlier stage and gives rise to the muscular bands of the young
larva. In Echinodermata a certain amount of mesenchyme appears before
the epithelial mesoderm, which is formed later as gut-diverticula. In
these forms the mesenchyme is said to arise as wandering amoeboid
cells, which are budded into the blastocoel by the endoderm just
before and during its invagination, but the writer has reason to
believe that this account of it does not quite describe what happens.
It would seem to be more probable that the mesenchyme arises in these
forms, as it certainly does in the case of the later-formed mesenchyme
of the Vertebrate embryo, as a protoplasmic outflow from its tissue of
origin, passing at first along the line of pre-existent protoplasmic
strands which traverse the blastocoel, and sending out at the same
time processes which branch and anastomose with neighbouring processes
(see E.W. MacBride, _Proc. Camb. Phil. Soc._, 1896, p. 153). In the
Vertebrata the whole of the mesoderm has at first the mesenchyme form.
Afterwards, when the body-cavity split appears, the bulk of it assumes
a kind of modified epithelial condition, which later on yields, by a
process of outflow very similar in its character to what has been
supposed to occur in the Echinoderm blastula, a considerable
mesenchyme of the reticulate character. Mesenchyme is the tissue which
in Vertebrate embryology has frequently been called embryonic
connective tissue. This name is no doubt due to the fact that it was
supposed to consist of isolated stellate cells. It is, however, in no
sense of the word connective tissue, because it gives rise to many
organs having nothing whatever to do with connective tissue. For
instance, in Vertebrata this tissue gives rise to nervous tissue,
blood-vessels, renal tubules, smooth muscular fibres, and other
structures, as well as to connective and skeletal tissues. The
Vertebrata, indeed, are remarkable for the fact that the epithelial
tissues of the so-called mesoderm, e.g. the epithelial lining of the
body-cavity, and of the renal tubules and urogenital tracts, all pass
through the mesenchymatous condition, whereas in _Amphioxus_,
_Balanoglossus_ and presumably _Sagitta_ and the Brachiopoda, all the
mesodermal tissues pass through the epithelial condition, most of the
mesodermal tissues of the adult retaining this condition permanently.
As has been implied in the above account, mesenchyme is usually formed
from epithelial mesoderm or from endoderm, or from tissue destined to
form endoderm. It is also sometimes formed from ectoderm, as in the
Vertebrata at the nerve crest and other places. In some Coelenterata
also it appears certain that the ectoderm does furnish tissue of a
mesenchymatous nature which passes into the jelly, but this phenomenon
takes place comparatively late in life, at any rate after the
embryonic period. In this connexion it may be interesting to point out
that in many Coelenterates all the tissues of the body retain
throughout life the epithelial condition, nothing comparable to
mesenchyme ever being formed.

Continuity of the layers.

Finally, before leaving this branch of the subject, the fact that the three germinal layers are continuous with one another, and not isolated masses of tissue, may be emphasized. Indeed, an embryo may be defined as a multinucleated protoplasmic mass, in which the protoplasm at any surface--whether internal or external--is in the form of a relatively dense layer, while that in the interior is much vacuolated and reduced to a more or less sparse reticulum, the nuclei either being exclusively found in the surface protoplasm, or if the embryo has any bulk and the internal reticulum is at all well developed, at the nodes of the internal reticulum as well.

Mouth and anus.

The origin of some of the more important organs may now be considered. It is a remarkable fact that the mouth and anus develop in the most diverse ways in different groups, but as a rule either one or both of them can be traced into relation with the blastopore, the history of which must therefore be examined. In most, if not all, the great groups of the animal kingdom, e.g. in Coelenterata, Annelida, Mollusca, Vertebrata, and in Arthropoda, the blastopore or its representative is placed on the neural surface of the body, and, as will be shown later on, within the limits of the central nerve rudiment. Here it undergoes the most diverse fate, even in members of the same group. For instance, in _Peripatus capensis_ it extends as a slit along the ventral surface, which closes up in the middle, but remains open at the two ends as the permanent mouth and anus. In other Arthropods, though full details have not yet in all cases been worked out, the following general statement may be made:--A blastopore (certain Crustacea) or its representative is formed on the neural surface of the embryo and always becomes closed, the mouth and anus arising as independent perforations later. Here no one would doubt the homology of the mouth and anus throughout the group; yet within the limits of a single genus--_Peripatus_--they show the most diverse modes of development. In Annelids the blastopore sometimes becomes the mouth (most Chaetopoda); sometimes it becomes the anus (_Serpula_); sometimes it closes up, giving rise to neither, though in this case it may assume the form of a long slit along the ventral surface before disappearing. In Mollusca its fate presents the same variations as in Annelida. Now in these groups no zoologist would deny the homology of the mouth and anus in the different forms, and yet how very different is their history even in closely allied animals. How are these apparently diverse facts to be reconciled? The only satisfactory explanation which has been offered (Sedgwick, _Quart. J. Mic. Science_, xxiv., 1884, p. 43) is that the blastopore is homologous in all the groups mentioned, and is the representative of the original single opening into the enteric cavity, such as at present characterizes the Coelenterata. From it the mouth and anus have been derived, as is indicated by its history in _Peripatus capensis_, and by the variability in its behaviour in closely allied forms; such variability in its subsequent history is due to its specialization as a larval organ, as a result of which it has lost its capacity to give rise to both mouth and anus, and sometimes to either.

That the blastopore does become specialized as a larval organ is
obvious in those cases in which it becomes transformed into the single
opening with which some larvae are, for a time at least, alone
provided, e.g. _Pilidium_, Echinoderm larvae, &c., and that larval
characters have been the principal causes of the form of embryonic
characters, strong reason to believe will be adduced later on. In the
Vertebrata the behaviour of the blastopore (anus of Rusconi) is also
variable in a very remarkable manner. As a rule it is slit-like in
form and closes completely, but in most cases one portion of it
remains open longer than the rest, as the neurenteric canal. In a few
forms (e.g. Newt, _Lepidosiren_, &c.) the very hindermost portion of
the slit-like blastopore remains permanently open as the anus, and
from such cases it can be shown that the neurenteric aperture (when
present) is derived from a portion of the blastopore just anterior to
its hindermost end. The words "hindermost" and "anterior" are used on
the assumption that the whole blastopore has retained its dorsal
position; as a matter of fact the hindermost part of it--the part
which persists or reopens as the anus--loses this position in the
course of development and becomes shifted on to the ventral surface.
This is clearly seen in _Lepidosiren_ (Kerr, _Phil. Trans._ cxcii.,
1900), in Elasmobranchii, and in Amniota (primitive streak). Moreover,
in _Lepidosiren_, and possibly in some other forms, the anus, i.e. the
hind end of the blastopore, is at first contained within the medullary
plate and bounded behind by the medullary folds. Later the portions of
the medullary plate in the neighbourhood of the anus completely
atrophy, and this relation is lost. This extension of the hind end of
the blastopore on to the ventral surface, and atrophy of the portion
of the medullary plate in relation with it, is a highly important
phenomenon, and one to which attention will be again called when the
relation of the mouth to the blastopore is being considered. The
remarkable fact about the Vertebrata, a feature which that group
shares in common with all other Chordata (_Amphioxus_, Tunicata,
Enteropneusta) and with the Echinodermata, is that the mouth has never
been traced into relation with the blastopore. For this reason, among
others, it has been held by some zoologists that the mouth of the
Vertebrata is not homologous with the mouth of such groups as the
Annelida, Arthropoda and Mollusca. But, as has been explained above,
in face of the extraordinary variability in the history of the mouth
and anus in these groups, this view cannot be regarded as in any way
established. On the contrary, there are distinct reasons for thinking
that the Vertebrate mouth is a derivate of the blastopore. In the
first place, in Elasmobranchii (Sedgwick, _Quart. Journ. Mic. Sci._
xxxiii., 1892, p. 559), and in a less conspicuous form in other
vertebrate groups, the mouth has at first a slit-like form, extending
from the anterior end of the central nerve-tube backwards along the
ventral surface of the anterior part of the embryo. This slit-like
rudiment, recalling as it does the form which the blastopore assumes
in so many groups and in many Vertebrata, does suggest the view that
possibly the mouth of the Vertebrata may in reality be derived from a
portion of an originally long slit-like neural blastopore, which has
become extended anteriorly on to the ventral surface and has lost its
original relation to the nerve rudiment, as has undoubtedly happened
with the posterior part, which persists as the anus.

Central nervous system.

Of the other organs which develop from the two primary layers it is only possible to notice here the central nervous system. This in almost all animals develops from the ectoderm. In Cephalopods among Mollusca--the development of which is remarkable from the almost complete absence of features which are supposed to have an ancestral significance--and in one or two other forms, it has been said to develop from the mesoderm; but apart from these exceptional and perhaps doubtful cases, the central nervous system of all embryos arises as thickenings of the ectoderm, and in the groups above mentioned, namely, Annelida, Mollusca, Arthropoda and Vertebrata, and probably others, from the ectoderm of the blastoporal surface of the body. This surface generally becomes the ventral surface, but in Vertebrata it becomes the dorsal. These thickened tracts of ectoderm in _Peripatus_ and a few other forms can be clearly seen to surround the blastopore. This relation is retained in the adult in _Peripatus_, some Mollusca and some Nemertines, in which the main lateral nerve cords are united behind the anus as well as in front of the mouth; in other forms it cannot always be demonstrated, but it can, as in the case of the Vertebrata just referred to, always be inferred; only, in the Invertebrate groups the part of the nerve rudiment which has to be inferred is the posterior part behind the blastopore, whereas in Vertebrata it is the anterior part, namely, that in front of the blastopore, assuming that the mouth is a blastoporal derivate.

In the Echinodermata, Enteropneusta and one or two other groups, it is
not possible, in the present state of knowledge, to bring the mouth
into relation with the blastopore, nor can the blastopore be shown to
be a perforation of the neural surface. For the Echinoderms, at any
rate, this fact loses some of the importance which might at first
sight be attributed to it when the remarkable organization of the
adult and the sharp contrast which exists between it and the larva is
remembered. In some Annelids the central nervous system remains
throughout life as part of the outer epidermis, but as a general rule
it becomes separated from the epidermis and embedded in the mesodermal
tissues. The mode in which this separation is effected varies
according to the form and structure of the central nervous system. In
the Vertebrata, in which this organ has the form of a tube extending
along the dorsal surface of the body, it arises as a groove of the
medullary plate, which becomes constricted into a canal. The wall of
this canal consists of ectoderm, which at an earlier stage formed part
of the outer surface of the body, but which after invagination
thickens, to give rise to the epithelial lining of the canal and to
the nervous tissue which forms the bulk of the canal wall. The fact
that the blastopore remains open at the hind end of the medullary
plate explains to a certain extent the peculiar relation which always
exists in the embryo between the hind end of the neural and alimentary
canals. This communication between the hind end of the neural tube and
the gut is one of the most remarkable and constant features of the
Vertebrate embryo. As has been pointed out, it is not altogether
unintelligible when we remember the relation of the blastopore to the
medullary plate of the earlier stage, but to give a complete
explanation of it is, and probably always will be, impossible. It is
no doubt the impress of some remarkable larval condition of the
blastopore of a stage of evolution now long past.

In _Ceratodus_ the open part of the blastopore is enclosed by the
medullary folds, as in _Lepidosiren_, and probably persists as the
anus, the portion of the folds around the anus undergoing atrophy
(Semon, _Zool. Forschungsreisen in Australien_, 1893, Bd. i. p. 39).
In Urodeles the blastopore persists as anus, so far as is known, but
the relation to the medullary folds has not been noticed. The same may
be said of _Petromyzon_ (A.E. Shipley, _Quart. Journ. Mic. Sci._
xxviii., 1887).

Cranial flexure.

The nerve tube of the Vertebrata at a certain early stage of the
embryo becomes bent ventralwards in its anterior portion, in such a
manner that the anterior end, which is represented in the adult by the
infundibulum, comes to project backwards beneath the mid-brain. This
bend, which is called the cranial flexure, takes place through the
mid-brain, so that the hind-brain is unaffected by it. The cranial
flexure is not, however, confined to the brain: the anterior end of
the notochord, which at first extends almost to the front end of the
nerve tube (this extension, which is quite obvious in the young embryo
of Elasmobranchs, becomes masked in the later stages by the
extraordinary modifications which the parts undergo), is also affected
by it. Moreover, it affects even other parts, as may be seen by the
oblique, almost antero-posterior, direction of the anterior gill slits
as compared with the transverse direction of those behind. No
satisfactory explanation has ever been offered of the cranial flexure.
It is found in all Vertebrates, and is effected at an early stage of
the development. In the later stages and in the adult it ceases to be
noticeable, on account of an alteration of the relative sizes of parts
of the brain. This is due almost entirely to the enormous growth of
the cerebral vesicle, which is an outgrowth of the dorsal wall of the
fore-brain just short of its anterior end. The anterior end of the
fore-brain remains relatively small throughout life as the
infundibulum, and the junction of this part of the fore-brain with the
part which is so largely developed, as the rudiment of the cerebrum,
is marked by the attachment of the optic chiasma. The optic nerve,
indeed, is morphologically the first cranial nerve, the olfactory
being the second; both are attached to what is morphologically the
dorsal side of the nerve tube. The morphological anterior end of the
central nerve tube is the point of the infundibulum which is in
contact with the pituitary body. While on the subject of the cranial
flexure, it may be pointed out that there is a similar downward curve
of the hind end of the nervous axis, which leads into the hind end of
the enteron. If it be supposed that originally there was a
communication between the infundibulum and pituitary body, then the
ventral flexure found at both ends of the nerve axis would originally
have had the same result, namely, of placing the neural and alimentary
canals in communication. Moreover, the mouth would have had much the
same relation to this imaginary anterior neurenteric canal that the
anus has to the actual posterior one.