Chapter XVII: Tracheata (1)

PROTOTRACHEATA.

The remarkable researches of Moseley (No. 396) on Peripatus capensis have brought clearly to light the affinities of this form with the tracheate Arthropoda; and its numerous primitive characters, such as the generally distributed tracheal apertures, the imperfectly segmented limbs, the diverging ventral nerve cords with imperfectly marked ganglia, and the nephridia (segmental organs[160]), would render its embryology of peculiar interest. Unfortunately Moseley was unable, from want of material, to make so complete a study of its development as of its anatomy. The youngest embryo observed was in part distinctly segmented, and coiled up within the egg (fig. 168 A). The procephalic lobes resemble those of the Arthropoda generally, and are unlike the præ-oral lobe of Chætopods or Discophora. They are not marked off by a transverse constriction from the succeeding segments. The three embryonic layers are differentiated, and the interior is filled with a brownish mass--the remnant of the yolk--which is probably enclosed in a distinct intestinal wall, and is lobed in correspondence with the segmentation of the body. The mouth invagination is not present, and but two pairs of slight prominences mark the rudiments of the two anterior post-oral appendages.

[160] F. M. Balfour, "On certain points in the Anatomy of
Peripatus capensis." _Quart. Journ. of Micros. Science_, Vol.
XIX. 1879.

[FIG. 167. ADULT EXAMPLE OF PERIPATUS CAPENSIS, natural size.
(From Moseley.)]

[FIG. 168. TWO STAGES IN THE DEVELOPMENT OF PERIPATUS CAPENSIS.
(After Moseley.)

A. Youngest stage hitherto observed before the appearance of the
legs.
B. Later stage after the legs and antennæ have become developed.
Both figures represent the larva as it appears within the egg.
1 and 2. First and second post-oral appendages.]

The single pair of antennæ is formed in the next stage, and is followed by the remaining post-oral appendages, which arise in succession from before backwards somewhat later than the segments to which they appertain.

The posterior part of the embryo becomes uncoiled, and the whole embryo bent double in the egg (fig. 168 B).

[FIG. 169. EMBRYO OF PERIPATUS CAPENSIS. Slightly older than A in fig. 168; unrolled. (After Moseley.)

_a._ antennæ; _o._ mouth; _i._ intestine; _c._ procephalic lobe. 1, 2, 3, etc., post-oral appendages.]

The mouth appears as a slit-like opening between and below the procephalic lobes. On each side and somewhat behind it there grows out an appendage--the first post-oral pair (fig. 169, 1)--while in front and behind it are formed the upper and lower lips. These two appendages next turn inwards towards the mouth, and their bases become gradually closed over by two processes of the procephalic region (fig. 170, _m_). The whole of these structures assist in forming a kind of secondary mouth cavity, which is at a later period further completed by the processes of the procephalic region meeting above the mouth, covering over the labrum, and growing backwards to near the origin of the second pair of post-oral appendages.

[FIG. 170. VENTRAL VIEW OF THE HEAD OF AN EMBRYO OF PERIPATUS
CAPENSIS AT A LATE STAGE OF DEVELOPMENT.

_l._ thickening of epiblast of procephalic lobe to form
supra-oesophageal ganglion; _m._ process from procephalic lobe
growing over the first post-oral appendage; _o._ mouth; _e._ eye; 1
and 2, first and second pair of post-oral appendages.]

The antennæ early become jointed, and fresh joints continue to be added throughout embryonic life; in the adult there are at present fully thirty joints. It appears to me probable (though Mr Moseley takes the contrary view) from the late development of the paired processes of the procephalic lobes, which give rise to the circular lip of the adult, that they are not true appendages. The next pair therefore to the antennæ is the first post-oral pair. It is the only pair connected with the mouth. At their extremities there is formed a pair of claws similar to those of the ambulatory legs (fig. 171). The next and largest pair of appendages in the embryo are the oral papillæ. They are chiefly remarkable for containing the ducts of the slime glands which open at their bases. They are without claws. The succeeding appendages become eventually imperfectly five-jointed; two claws are formed as cuticular investments of papillæ in pockets of the skin at the ends of their terminal joints.

[FIG. 171. HEAD OF AN EMBRYO PERIPATUS. (From Moseley.)

The figure shews the jaws (mandibles), and close to them epiblastic
involutions, which grow into the supra-oesophageal ganglia. The
antennæ, oral cavity, and oral papillæ are also shewn.]

I have been able to make a few observations on the internal structure of the embryos from specimens supplied to me by Moseley. These are so far confined to a few stages, one slightly earlier, the others slightly later, than the embryo represented in fig. 168 B. The epiblast is formed of a layer of columnar cells, two deep on the ventral surface, except along the median line where there is a well-marked groove and the epiblast is much thinner (fig. 172).

The ventral cords of the trunk are formed as two independent epiblastic thickenings. In my earlier stage these are barely separated from the epiblast, but in the later ones are quite independent (fig. 172, _v.n_), and partly surrounded by mesoblast.

The supra-oesophageal ganglia are formed as thickenings of the epiblast of the ventral side of the procephalic lobes in front of the stomodæum. They are shewn at _l_ in fig. 170. The thickenings of the two sides are at first independent. At a somewhat later period an invagination of the epiblast grows into each of these lobes. The openings of these invaginations extend from the oral cavity forwards; and they are shewn in fig. 171[161]. Their openings become closed, and the walls of the invaginations constitute a large part of the embryonic supra-oesophageal ganglia.

[161] This figure is taken from Moseley. The epiblastic
invaginations are represented in it very accurately, and though
not mentioned in the text of the paper, Moseley informs me that
he has long been aware of the homology of these folds with those
in various other Tracheata.

Similar epiblastic invaginations assist in forming the supra-oesophageal ganglia of other Tracheata. They are described in the sequel for Insects, Spiders and Scorpions. The position of the supra-oesophageal ganglia on the ventral side of the procephalic lobes is the same as that in other Tracheata.

[FIG. 172. SECTION THROUGH THE TRUNK OF AN EMBRYO OF PERIPATUS. The
embryo from which the section is taken was somewhat younger than
fig. 171.

_sp.m._ splanchnic mesoblast. _s.m._ somatic mesoblast. _mc._ median
section of body cavity. _lc._ lateral section of body cavity. _v.n._
ventral nerve cord. _me._ mesenteron.]

The mesoblast is formed, in the earliest of my embryos, of scattered cells in the fairly wide space between the mesenteron and the epiblast. There are two distinct bands of mesoblast on the outer sides of the nervous cords. In the later stage the mesoblast is divided into distinct somatic and splanchnic layers, both very thin; but the two layers are connected by transverse strands (fig. 172). There are two special longitudinal septa dividing the body cavity into three compartments, a median (mc), containing the mesenteron, and two lateral (_lc_) containing the nerve cords. This division of the body cavity persists, as I have elsewhere shewn, in the adult. A similar division is found in some Chætopoda, _e.g._ Polygordius.

I failed to make out that the mesoblast was divided into somites, and feel fairly confident that it is not so in the stages I have investigated.

There is a section of the body cavity in the limbs as in embryo Myriapods, Spiders, etc.

In the procephalic lobe there is a well-developed section of the body cavity, which lies dorsal to and in front of the rudiment of the supra-oesophageal ganglia.

The alimentary tract is formed of a mesenteron (fig. 172), a stomodæum, and proctodæum. The wall of the mesenteron is formed, in the stages investigated by me, of a single layer of cells with yolk particles, and encloses a lumen free from yolk. The forward extension of the mesenteron is remarkable.

The stomodæum in the earlier stage is a simple pit, which meets but does not open into the mesenteron. In the later stage the external opening of the pit is complicated by the structures already described. The proctodæum is a moderately deep pit near the hinder end of the body.

The existence of a tracheal system[162] is in itself almost sufficient to demonstrate the affinities of Peripatus with the Tracheata, in spite of the presence of nephridia. The embryological characters of the procephalic lobes, of the limbs and claws, place however this conclusion beyond the reach of scepticism. If the reader will compare the figure of Peripatus with that of an embryo Scorpion (fig. 196 A) or Spider (fig. 200 C) or better still with Metschnikoff's figure of Geophilus (No. 399) Pl. XXI. fig. II, he will be satisfied on this point.

[162] The specimens shewing tracheæ which Moseley has placed in
my hands are quite sufficient to leave no doubt whatever in my
mind as to the general accuracy of his description of the
tracheal system.

The homologies of the anterior appendages are not very easy to determine; but since there does not appear to me to be sufficient evidence to shew that any of the anterior appendages have become aborted, the first post-oral appendages embedded in the lips may provisionally be regarded as equivalent to the mandibles, and the oral papillæ to the first pair of maxillæ, etc. Moseley is somewhat doubtful about the homologies of the appendages, and hesitates between considering the oral papillæ as equivalent to the second pair of maxillæ (on account of their containing the openings of the mucous glands, which he compares with the spinning glands of caterpillars), or to the poison claws (fourth post-oral appendages) of the Chilopoda (on account of the poison glands which he thinks may be homologous with the mucous glands).

The arguments for either of these views do not appear to me conclusive. There are glands opening into various anterior appendages in the Tracheata, such as the poison glands in the Cheliceræ (mandibles) of Spiders, and there is some evidence in Insects for the existence of a gland belonging to the first pair of maxillæ, which might be compared with the mucous gland of Peripatus. For reasons already stated I do not regard the processes of the cephalic lobes, which form the lips, as a pair of true appendages.

BIBLIOGRAPHY.

(396) H. N. Moseley. "On the Structure and Development of Peripatus capensis." _Phil. Trans._ Vol. 164, 1874.

MYRIAPODA[163].

[163] The classification of the Myriapoda employed in the present
section is:
I. Chilognatha. (Millipedes.)
II. Chilopoda. (Centipedes.)

Chilognatha. The first stages in the development of the Chilognatha have been investigated by Metschnikoff and Stecker, but their accounts are so contradictory as hardly to admit of reconciliation.

According to Metschnikoff, by whom the following four species have been investigated, viz., Strongylosoma Guerinii, Polydesmus complanatus, Polyxenus lagurus, and Julus Moneletei, the segmentation is at first regular and complete, but, when the segments are still fairly large, the regular segmentation is supplemented by the appearance of a number of small cells at various points on the surface, which in time give rise to a continuous blastoderm.

The blastoderm becomes thickened on the ventral surface, and so forms a ventral plate[164].

[164] Stecker's (No. 400) observations were made on the eggs of
Julus fasciatus, Julus foetidus, Craspedosoma marmoratum,
Polydesmus complanatus, and Strongylosoma pallipes, and though
carried on by means of sections, still leave some points very
obscure, and do not appear to me deserving of much confidence.
The two species of Julus and Craspedosoma undergo, according to
Stecker, a nearly identical development. The egg before
segmentation is constituted of two substances, a central
protoplasmic, and a peripheral deutoplastic. It first divides
into two equal segments, and coincidentally with their formation
part of the central protoplasm travels to the surface as two
clear fluid segments. The ovum is thus composed of two yolk
segments to two protoplasmic segments. The two former next divide
into four, with the production of two fresh protoplasmic
segments. The four protoplasmic segments now constitute the upper
or animal pole of the egg, and occupy the position of the future
ventral plate. The yolk segments form the lower pole, which is
however _dorsal_ in relation to the future animal. The
protoplasmic segments increase in number by a regular division,
and arrange themselves in three rows, of which the two outermost
rapidly grow over the yolk segments. A large segmentation cavity
is stated to be present in the interior of the ovum.

It would appear from Stecker's description that the yolk segments
(hypoblast) next become regularly invaginated, so as to enclose a
gastric cavity, opening externally by a blastopore; but it is
difficult to believe that a typical gastrula, such as that
represented by Stecker, really comes into the cycle of
development of the Chilognatha.

The mesoblast is stated to be derived mainly from the epiblast.
This layer in the region of the future ventral plate becomes
reduced to two rows of cells, and the inner of these by the
division of its constituent elements gives rise to the mesoblast.
The development of Polydesmus and Strongylosoma is not very
different from that of Julus. The protoplasm at the upper pole
occupies from the first a superficial position. Segmentation
commences at the lower pole, where the food-yolk is mainly
present! The gastrula is stated to be similar to that of Julus.
The mesoblast is formed in Polydesmus as a layer of cells split
off from the epiblast, but in Strongylosoma as an outgrowth from
the lips of the blastopore. Stecker, in spite of the statements
in his paper as to the origin of the mesoblast from the epiblast,
sums up at the end to the effect that both the primary layers
have a share in the formation of the mesoblast, which originates
by a process of endogenous cell division!

It may be noted that the closure of the blastopore takes place,
according to Stecker, on the dorsal side of the embryo.

The most important sources of information for the general embryology of the Chilognatha are the papers of Newport (No. 397) and Metschnikoff (No. 398). The development of Strongylosoma may be taken as fairly typical for the group; and the subsequent statements, unless the reverse is stated, apply to the species of Strongylosoma investigated by Metschnikoff.

[FIG. 173. THREE STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA
GUERINII. (After Metschnikoff.)

A. Embryo on eleventh day with commencing ventral flexure (_x_)
B. Embryo with three pairs of post-oral appendages.
C. Embryo with five pairs of post-oral appendages.
_gs._ ventral plate; _at._ antennæ; 1-5 post-oral appendages; _x._
point of flexure of the ventral plate.]

After the segmentation and formation of the layers the first observable structure is a transverse furrow in the thickening of the epiblast on the ventral surface of the embryo. This furrow rapidly deepens, and gives rise to a ventral flexure of the embryo (fig. 173 A, _x_), which is much later in making its appearance in Julus than in Strongylosoma and Polyxenus. A pair of appendages, which become the antennæ, makes its appearance shortly after the formation of the transverse furrow, and there soon follow in order the next three pairs of appendages. All these parts are formed in the infolded portion of the ventral thickening of the blastoderm (fig. 173 B). The ventral thickening has in the meantime become marked by a longitudinal furrow, but whether this is connected with the formation of the nervous system, or is equivalent to the mesoblastic furrow in Insects, and connected with the formation of the mesoblast, has not been made out. Shortly after the appearance of the three pairs of appendages behind the antennæ two further pairs become added, and at the same time oral and anal invaginations become formed (fig. 173 C). In front of the oral opening an unpaired upper lip is developed. The præ-oral part of the ventral plate develops into the bilobed procephalic lobes, the epiblast of which is mainly employed in the formation of the supra-oesophageal ganglia. The next important change which takes place is the segmentation of the body of the embryo (fig. 174 A), the most essential feature in which is the division of the mesoblast into somites. Segments are formed in order from before backwards, and soon extend to the region behind the appendages. On the appearance of segmentation the appendages commence to assume their permanent form. The two anterior pairs of post-oral appendages become jaws; and the part of the embryo which carries them and the antennæ is marked off from the trunk as the head. The three following pairs of appendages grow in length and assume a form suited for locomotion. Behind the three existing pairs of limbs there are developed three fresh pairs, of _which the two anterior belong to a single primitive segment_. While the above changes take place in the appendages the embryo undergoes an ecdysis, which gives rise to a cuticular membrane within the single egg membrane (chorion, _Metschnikoff_). On this cuticle a tooth-like process is developed, the function of which is to assist in the hatching of the embryo (fig. 174 A).

In Polyxenus a cuticular membrane is present as in Strongylosoma, but it is not provided with a tooth-like process. In the same form amoeboid cells separate themselves from the blastoderm at an early period. These cells have been compared to the embryonic envelopes of Insects described below.

In Julus _two_ cuticular membranes are present at the time of hatching: the inner one is very strongly developed and encloses the embryo after hatching. After leaving the chorion the embryo Julus remains connected with it by a structureless membrane which is probably the outer of the two cuticular membranes.

[FIG. 174. TWO STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUERINII.
(After Metschnikoff.)

A. A seventeen days' embryo, already segmented.
B. A just hatched larva.]

At the time when the embryo of Strongylosoma is hatched (fig. 174 B) nine post-cephalic segments appear to be present. Of these segments the second is apparently (from Metschnikoff's figure, 174 B) without a pair of appendages; the third and fourth are each provided with a single functional pair of limbs; the fifth segment is provided with two pairs of rudimentary limbs, which are involuted in a single sack and not visible without preparation, and therefore not shewn in the figure. The sixth segment is provided with but a single pair of appendages, though a second pair is subsequently developed on it[165].

[165] Though the superficially hexapodous larva of Strongylosoma
and other Chilognatha has a striking resemblance to some larval
Insects, no real comparison is possible between them, even on the
assumption that the three functional appendages of both are
homologous, because Embryology clearly proves that the hexapodous
Insect type has originated from an ancestor with numerous
appendages by the atrophy of those appendages, and not from an
hexapodous larval form prior to the development of the full
number of adult appendages.

Julus, at the time it leaves the chorion, is imperfectly segmented, but is provided with antennæ, mandibles, and maxillæ, and seven pairs of limbs, of which the first three are much more developed than the remainder. Segmentation soon makes its appearance, and the head becomes distinct from the trunk, and on each of the three anterior trunk segments a single pair of limbs is very conspicuous (Metschnikoff)[166]. Each of the succeeding segments bears eventually two pairs of appendages. At the time when the inner embryonic cuticle is cast off, the larva appears to be hexapodous, like the young Strongylosoma, but there are in reality four pairs of rudimentary appendages behind the three functional pairs. The latter only appear on the surface after the first post-embryonic ecdysis. Pauropus (Lubbock) is hexapodous in a young stage. At the next moult two pairs of appendages are added, and subsequently one pair at each moult.

[166] Newport states however that a pair of limbs is present on
the first, second, and fourth post-oral segments, but that the
third segment is apodous; and this is undoubtedly the case in the
adult.

There appear to be eight post-oral segments in Julus at the time of hatching. According to Newport fresh segments are added in post-embryonic life by successive budding from a blastema between the penultimate segment and that in front of it. They arise in batches of six at the successive ecdyses, till the full number is completed. A functional, though not a real hexapodous condition, appears to be characteristic of Chilognatha generally at the time of hatching.

The most interesting anatomical feature of the Chilognatha is the double character of their segments, the feet (except the first three or four, or more), the circulatory, the respiratory, and the nervous systems shewing this peculiarity. Newport's and Metschnikoff's observations have not thrown as much light on the nature of the double segments as might have been hoped, but it appears probable that they have _not_ originated from a fusion of two primitively distinct segments, but from a later imperfect division of each of the primitive segments into two, and the supply to each of the divisions of a primitive segment of a complete set of organs.

[FIG. 175. TWO STAGES IN THE DEVELOPMENT OF GEOPHILUS. (After
Metschnikoff.)

A. Side view of embryo at the stage when the segments are beginning
to be formed.
B. Later stage after the appendages have become established.

_at._ antennæ; _an.i._ proctodæum.]

Chilopoda. Up to the present time the development of only one type of Chilopoda, viz. that of Geophilus, has been worked out. Most forms lay their eggs, but Scolopendra is viviparous. The segmentation appears to resemble that in the Chilognatha, and at its close there is present a blastoderm surrounding a central mass of yolk cells. A ventral thickening of the blastoderm is soon formed. It becomes divided into numerous segments, which continue to be formed successively from the posterior unsegmented part. The antennæ are the first appendages to appear, and are well developed when eighteen segments have become visible (fig. 175 A). The post-oral appendages are formed slightly later, and in order from before backwards. As the embryo grows in length, and fresh segments continue to be formed, the posterior part of it becomes bent over so as to face the ventral surface of the anterior, and it acquires an appearance something like that of many embryo Crustaceans (fig. 175 B). Between forty and fifty segments are formed while the embryo is still in the egg. The appendages long remain unjointed. The fourth post-oral appendage, which becomes the poison claw, is early marked out by its greater size: on the third post-oral there is formed a temporary spine to open the egg membrane.

It does not appear, from Metschnikoff's figures of Geophilus, that any of the anterior segments are without appendages, and it is very probable that Newport is mistaken in supposing that the embryo has a segment without appendages behind that with the poison claws, which coalesces with the segment of the latter. It also appears to me rather doubtful whether the third pair of post-oral appendages, _i.e._ those in front of the poison claws, can fairly be considered as forming part of the basilar plate. The basilar plate is really the segment of the poison claws, and may fuse more or less completely with the segment in front and behind it, and the latter is sometimes without a pair of appendages (Lithobius, Scutigera).

Geophilus, at the time of birth, has a rounded form like that of the Chilognatha.

The young of Lithobius is born with only six pairs of limbs.

_General observation on the homologies of the appendages of
Myriapoda._

The chief difficulty in this connection is the homology of the third pair of post-oral appendages.

In adult Chilognatha there is present behind the mandibles a four-lobed plate, which is usually regarded as representing two pairs of appendages, viz. the first and second pairs of maxillæ of Insects. Metschnikoff's observations seem however to shew that this plate represents but a single pair of appendages, which clearly corresponds with the first pair of maxillæ in Insects. The pair of appendages behind this plate is ambulatory, but turned towards the head; it is in the embryo the foremost of the three functional pairs of legs with which the larva is born. Is it equivalent to the second pair of maxillæ of Insects or to the first pair of limbs of Insects? In favour of the former view is the fact (1) that in embryo Insects the second pair of maxillæ sometimes resembles the limbs rather than the jaws, so that it might be supposed that in Chilognatha a primitive ambulatory condition of the third pair of appendages has been retained; (2) that the disappearance of a pair of appendages would have to be postulated if the second alternative is adopted, and that if Insects are descended from forms related to the Myriapods it is surprising to find a pair of appendages always present in the former, absent in the latter. The arguments which can be urged for the opposite view do not appear to me to have much weight, so that the homology of the appendages in question with the second pair of maxillæ may be provisionally assumed.

The third pair of post-oral appendages of the Chilopoda may probably also be assumed to be equivalent to the second pair of maxillæ; though they are limb-like and not connected with the head. The subjoined table shews the probable homologies of the appendages.

+------------------------+---------------------+----------------------+
| | CHILOGNATHA | CHILOPODA |
| | (Strongylosoma | (Scolopendra adult). |
| | at time of birth). | |
+------------------------+---------------------+----------------------+
| Pre-oral region. | Antennæ. | Antennæ. |
+------------------------+---------------------+----------------------+
| 1st Post-oral segment. | Mandibles. | Mandibles. |
+------------------------+------------------- -+----------------------+
| 2nd " " | Maxillæ 1. (Four- | Maxillæ 1. |
| | lobed plate in | (Palp and bilobed |
| | adult, but a simple| median process). |
| | pair of appendages | |
| | in embryo). | |
+------------------------+---------------------+----------------------+
| 3rd " " | 1st pair of | Limb-like appendages |
| (probably equivalent | ambulatory limbs | with basal parts in |
| to segment bearing | | contact. |
| 2nd pair of maxillæ | | |
| in Insects). | | |
+------------------------+---------------------+----------------------+
| 4th " " | (?) Apodous. | Poison claws. |
+------------------------+---------------------+----------------------+
| 5th " " | 2nd pair of | 1st pair of |
| | ambulatory limbs. | ambulatory limbs. |
+------------------------+---------------------+----------------------+
| 6th " " | 3rd " " " | 2nd " " " |
+------------------------+---------------------+----------------------+
| 7th " " | 4th and 5th " " | 3rd " " " |
| | (rudimentary.) | |
+------------------------+---------------------+----------------------+
| 8th " " | 6th " " " | 4th " " " |
| | (the 7th pair is | |
| | developed in this | |
| | segment later). | |
+------------------------+---------------------+----------------------+
| 9th " " | Apodous. | 5th " " " |
+------------------------+---------------------+----------------------+
| 10th " " | " (last segment | 6th " " " |
| | in embryo). | |
+------------------------+---------------------+----------------------+

_The germinal layers and formation of organs._

The development of the organs of the Myriapoda, and the origin of the germinal layers, are very imperfectly known: Myriapoda appear however to be closely similar to Insects in this part of their development, and the general question of the layers will be treated more fully in connection with that group.

The greater part of the blastoderm gives rise to the epiblast, which furnishes the skin, nervous system, tracheal system, and the stomodæum and proctodæum.

The mesoblast arises in connection with the ventral thickening of the blastoderm, but the details of its formation are not known. Metschnikoff describes a longitudinal furrow which appears very early in Strongylosoma, which is perhaps equivalent to the mesoblastic furrows of Insects, and so connected with the formation of the mesoblast.

The mesoblast is divided up into a series of protovertebra-like bodies--the mesoblastic somites--the cavities of which become the body cavity and the walls the muscles and probably the heart. They are (Metschnikoff) prolonged into the legs, though the prolongations become subsequently segmented off from the main masses. The splanchnic mesoblast is, according to Metschnikoff, formed independently of the somites, but this point requires further observation.

The origin of the hypoblast remains uncertain, but it appears probable that it originates, in a large measure at least, from the yolk segments. In the Chilognatha the mesenteron is formed in the interior of the yolk segments, so that those yolk segments which are not employed in the formation of the alimentary canal lie freely in the body cavity. In the relation of the yolk segments to the alimentary canal the Chilopoda present a strong contrast to the Chilognatha, in that the greater part of the yolk lies within their mesenteron. The mesenteron is at first a closed sack, but is eventually placed in communication with the stomodæum and the proctodæum. The Malpighian bodies arise as outgrowths from the blind extremity of the latter.

BIBLIOGRAPHY.

(397) G. Newport. "On the Organs of Reproduction and Development of the Myriapoda." _Philosophical Transactions_, 1841.

(398) E. Metschnikoff. "Embryologie der doppeltfüssigen Myriapoden (Chilognatha)." _Zeit. f. wiss. Zool._, Vol. XXIV. 1874.

(399) ---- "Embryologisches über Geophilus." _Zeit. f. wiss. Zool._, Vol. XXV. 1875.

(400) Anton Stecker. "Die Anlage d. Keimblatter bei den Diplopoden." _Archiv f. mik. Anatomie,_ Bd. XIV. 1877.

INSECTA[167].

[167] The following classification of the Insecta is employed in
this chapter:
I. Aptera. {(1) Collembola.
{(2) Thysanura.
II. Orthoptera. {(1) Orthoptera genuina
(_Blatta_, _Locusta_, etc.).
{(2) " pseudoneuroptera
(_Termes_, _Ephemera_, _Libellula_).
III. Hemiptera. {(1) Hemiptera heteroptera
(_Cimex_, _Notonecta_, etc.).
{(2) " homoptera (_Aphis_, _Cicada_, etc.).
{(3) " parasita (_Pediculus_, etc.).
IV. Diptera. {(1) Diptera genuina (_Musca_, _Tipula_, etc.).
{(2) " aphaniptera (_Pulex_, etc.).
{(3) " pupipara (_Braula_, etc.).
V. Neuroptera. {(1) Neuroptera planipennia (_Myrmeleon_, etc.).
{(2) " trichoptera (_Phryganea_, etc.).
VI. Coleoptera.
VII. Lepidoptera.
VIII. Hymenoptera. {(1) Hymenoptera aculeata
(_Apis_, _Formica_, etc.).
{(2) " entomophaga
(_Ichneumon_, _Platygaster_, etc.).
{(3) " phytophaga
(_Tenthredo_, _Sirex_, etc.).

The formation of the embryonic layers in Insects has not been followed out in detail in a large number of types; but, as in so many other instances, some of the most complete histories we have are due to Kowalevsky (No. 416). The development of Hydrophilus has been worked out by him more fully than that of any other form, and will serve as a type for comparison with other forms.

[FIG. 176. FOUR EMBRYOS OF HYDROPHILUS PICEUS VIEWED FROM THE
VENTRAL SURFACE. (After Kowalevsky.)

The upper end is the anterior. _gg._ germinal groove; _am._ amnion.]

The segmentation has not been studied, but no doubt belongs to the centrolecithal type (_vide_ pp. 110-120). At its close there is an uniform layer of cells enclosing a central mass of yolk. These cells, in the earliest observed stage, were flat on the dorsal, but columnar on part of the ventral surface of the egg, where they form a thickening which will be called the ventral plate. At the posterior part of the ventral plate two folds, with a furrow between them, make their appearance. They form a structure which may be spoken of as the germinal groove (fig. 176 A, _gg_). The cells which form the floor of the groove are far more columnar than those of other parts of the blastoderm (fig. 177 A). The two folds on each side of it gradually approach each other. They do so at first behind, and then in the middle; from the latter point the approximation gradually extends backwards and forwards (fig. 176 B and C). In the middle and hinder parts of the ventral plate the groove becomes, by the coalescence of the folds, converted into a canal (fig. 178 A, _gg_), the central cavity of which soon disappears, while at the same time the cells of the wall undergo division, become more rounded, and form a definite layer (_me_)--the mesoblast--beneath the columnar cells of the surface. Anteriorly the process is slightly different, though it leads to the similar formation of mesoblast (fig. 177 B). The flat floor of the groove becomes in front bodily converted into the mesoblast, but the groove itself is never converted into a canal. The two folds simply meet above, and form a continuous superficial layer.

[FIG. 177. TWO TRANSVERSE SECTIONS THROUGH EMBRYOS OF HYDROPHILUS
PICEUS. (After Kowalevsky.)

A. Section through an embryo of the stage represented in fig. 176 B,
at the point where the two germinal folds most approximate.
B. Section through an embryo somewhat later than the stage fig. 176
D, through the anterior region where the amnion has not completely
closed over the embryo.
_gg._ germinal groove; _me._ mesoblast; _am._ amnion; _yk._ yolk.]

[FIG. 178. SECTIONS THROUGH TWO EMBRYOS OF HYDROPHILUS PICEUS.
(After Kowalevsky.)

A. Section through the posterior part of the embryo fig. 176 D,
shewing the completely closed amnion and the germinal groove.
B. Section through an older embryo in which the mesoblast has grown
out into a continuous plate beneath the epiblast.
_gg._ germinal groove; _am._ amnion; _yk._ yolk; _ep._ epiblast.]

During the later stages of the process last described remarkable structures, eminently characteristic of the Insecta, have made their first appearance. These structures are certain embryonic membranes or coverings, which present in their mode of formation and arrangement a startling similarity to the true and false amnion of the Vertebrata. They appear as a double fold of the blastoderm round the edge of the germinal area, which spreads over the ventral plate, from behind forwards, in a general way in the same manner as the amnion in, for instance, the chick. The folds at their origin are shewn in surface view in fig. 176 D, am, and in section in fig. 177 B, _am_. The folds eventually meet, coalesce (fig. 178, am) and give rise to two membranes covering the ventral plate, viz. an inner one, which is continuous with the edge of the ventral plate; and an outer, continuous with the remainder of the blastoderm. The vertebrate nomenclature may be conveniently employed for these membranes. The inner limb of the fold will therefore be spoken of as the amnion, and the outer one, including the dorsal part of the blastoderm, as the serous envelope[168]. A slight consideration of the mode of formation of the membranes, or an inspection of the figures illustrating their formation, makes it at once clear that the yolk can pass in freely between the amnion and serous envelope (_vide_ fig. 181). At the hind end of the embryo this actually takes place, so that the ventral plate covered by the amnion appears to become completely imbedded in the yolk: elsewhere the two membranes are in contact. At first (fig. 176) the ventral plate occupies but a small portion of the ventral surface of the egg, but during the changes above described it extends over the whole ventral surface, and even slightly on the dorsal surface both in front and behind. It becomes at the same time (fig. 179) divided by a series of transverse lines into segments, which increase in number and finally amount in all to seventeen, not including the most anterior section, which gives off as lateral outgrowths the two procephalic lobes (_pc.l_). The changes so far described are included within what Kowalevsky calls his first embryonic period; at its close the parts contained within the chorion have the arrangement shewn in fig. 178 B. The whole of the body of the embryo is formed from the ventral plate, and no part from the amnion or serous envelope.

[168] The reverse nomenclature to this is rather inconveniently
employed by Metschnikoff.

[FIG. 179. EMBRYO OF HYDROPHILUS PICEUS VIEWED FROM THE VENTRAL
SURFACE. (After Kowalevsky.)

_pc.l._ procephalic lobe.]

The general history of the succeeding stages may be briefly told.

[FIG. 180. TWO STAGES IN THE DEVELOPMENT OF HYDROPHILUS PICEUS.
(From Gegenbaur, after Kowalevsky.)

_ls._ labrum; _at._ antenna; _md._ mandible; _mx._ maxilla I.; _li._
maxilla II.; _p´ p´´ p´´´_. feet; _a._ anus.]

The appendages appear as very small rudiments at the close of the last stage, but soon become much more prominent (fig. 180 A). They are formed as outgrowths of both layers, and arise nearly simultaneously. There are in all eight pairs of appendages. The anterior or antennæ (_at_) spring from the procephalic lobes, and the succeeding appendages from the segments following. The last pair of embryonic appendages, which disappears very early, is formed behind the third pair of the future thoracic limbs. Paired epiblastic involutions, shewn as pits in the posterior segments in fig. 180 A, give rise to the tracheæ; and the nervous system is formed as two lateral epiblastic thickenings, one on each side of the mid-ventral line. These eventually become split off from the skin; while between them there passes in a median invagination of the skin (fig. 189 C). The two nervous strands are continuous in front with the supra-oesophageal ganglia, which are formed of the epiblast of the procephalic lobes. These plates gradually grow round the dorsal side of the embryo, and there is formed immediately behind them an oral invagination, in front of which there appears an upper lip (fig. 180, _ls_). A proctodæum is formed at the hind end of the body slightly later than the stomodæum. The mesoblast cells become divided into two bands, one on each side of the middle line (fig. 189 A), and split into splanchnic and somatic layers. The central yolk mass at about the stage represented in fig. 179 begins to break up into yolk spheres. The hypoblast is formed first on the ventral side at the junction of the mesoblast and the yolk, and gradually extends and forms a complete sack-like mesenteron, enveloping the yolk (fig. 185 _al_). The amnion and serous membrane retain their primitive constitution for some time, but gradually become thinner on the ventral surface, where a rupture appears eventually to take place. The greater part of them disappears, but in the closure of the dorsal parietes the serous envelope plays a peculiar part, which is not yet understood. It is described on p. 404. The heart is formed from the mesoblastic layers, where they meet in the middle dorsal line (fig. 185 C, _ht_). The somatic mesoblast gives rise to the muscles and connective tissue, and the splanchnic mesoblast to the muscular part of the wall of the alimentary tract, which accompanies the hypoblast in its growth round the yolk. The proctodæum forms the rectum and Malpighian bodies[169], and the stomodæum the oesophagus and proventriculus. The two epiblastic sections of the alimentary tract are eventually placed in communication with the mesenteron.

[169] This has not been shewn in the case of Hydrophilus.

The development of Hydrophilus is a fair type of that of Insects generally, but it is necessary to follow with somewhat greater detail the comparative history of the various parts which have been briefly described for this type.

_The embryonic membranes and the formation of the layers._

All Insects have at the close of segmentation a blastoderm formed of a single row of cells enclosing a central yolk mass, which usually contains nuclei, and in the Poduridæ is divided up in the ordinary segmentation into distinct yolk cells. The first definite structure formed is a thickening of the blastoderm, which forms a ventral plate.

The ventral plate is very differently situated in relation to the yolk in different types. In most Diptera, Hymenoptera and (?) Neuroptera (Phryganea) it forms from the first a thickening extending over nearly the whole ventral surface of the ovum, and in many cases extends in its subsequent growth not only over the whole ventral surface, but over a considerable part of the apparent dorsal surface as well (Chironomus, Simulia, Gryllotalpa, etc.). In Coleoptera, so far as is known, it commences as a less extended thickening either of the central part (Donacia) or posterior part (Hydrophilus) of the ventral surface, and gradually grows in both directions, passing over to the dorsal surface behind.

Embryonic membranes. In the majority of Insects there are developed enveloping membranes like those of Hydrophilus.

[FIG. 181. DIAGRAMMATIC LONGITUDINAL SECTIONS OF AN INSECT EMBRYO AT
TWO STAGES TO SHEW THE DEVELOPMENT OF THE EMBRYONIC ENVELOPES.

In A the amniotic folds have not quite met so as to cover the
ventral plate. The yolk is represented as divided into yolk cells.
In B the sides of the ventral plate have extended so as nearly to
complete the dorsal integument. The mesenteron is represented as a
closed sack filled with yolk cells. _am._ amnion; _se._ serous
envelope; _v.p._ ventral plate; _d.i._ dorsal integument; _me._
mesenteron; _st._ stomodæum; _an i._ proctodæum.]

The typical mode of formation of these membranes is represented diagrammatically in fig. 181 A and B. A fold of the blastoderm arises round the edge of the ventral plate. This fold, like the amniotic fold of the higher Vertebrata, is formed of two limbs, an outer, the serous membrane (_se_), and an inner, the true amnion (_am_). Both limbs extend so as to cover over the ventral plate, and finally meet and coalesce, so that a double membrane is present over the ventral plate. At the same time (fig. 181 B) the point where the fold originates is carried dorsalwards by the dorsal extension of the edges of the ventral plate, which give rise to the dorsal integument (_d.i_). This process continues till the whole dorsal surface is covered by the integument. The amnion then separates from the dorsal integument, and the embryo becomes enveloped in two membranes--an inner, the amnion, and an outer, the serous membrane. In fig. 181 B the embryo is represented at the stage immediately preceding the closure of the dorsal surface.

By the time that these changes are effected, the serous membrane and amnion are both very thin and not easily separable. The amnion appears to be usually absorbed before hatching; but in hatching both membranes, if present, are either absorbed, or else ruptured and thrown off.

The above mode of development of the embryonic membranes has been especially established by the researches of Kowalevsky (No. 416) and Graber (No. 412) for various Hymenoptera (_Apis_), Diptera (_Chironomus_), Lepidoptera and Coleoptera (_Melolontha_, _Lina_).

Considerable variations in the development of the enveloping membranes are known.

When the fold which gives rise to the membranes is first formed, there is, as is obvious in fig. 181 A, a perfectly free passage by which the yolk can pass in between the amnion and serous membrane. Such a passage of the yolk between the two membranes takes place posteriorly in Hydrophilus and Donacia: in Lepidoptera the yolk passes in everywhere, so that in this form the ventral plate becomes first of all imbedded in the yolk, and finally, on the completion of the dorsal integument, the embryo is enclosed in a complete envelope of yolk contained between the amnion and the serous membrane. During the formation of the dorsal integument the external yolk-sack communicates by a dorsally situated umbilical canal with the yolk cavity within the body. On the rupture of the amnion the embryo is nourished at the expense of the yolk contained in the external yolk-sack.

In the Hemiptera and the Libellulidæ the ventral plate also becomes imbedded in the yolk, but in a somewhat different fashion to the Lepidoptera, which more resembles on an exaggerated scale what takes place in Hydrophilus.

[FIG. 182. THREE STAGES IN THE DEVELOPMENT OF THE EMBRYO OF
CALOPTERYX. (After Brandt.)

The embryo is represented in the egg-shell.
A. Embryo with ventral plate.
B. Commencing involution of ventral plate.
C. Involution of ventral plate completed.
_ps._ ventral plate; _g._ edge of ventral plate; _am._ amnion; _se._
serous envelope.]

In the Libellulidæ (_Calopteryx_) there is first of all formed (Brandt, No. 403) a small ventral and posterior thickening of the blastoderm (fig. 182 A). The hinder part of this becomes infolded into the yolk as a projection (fig. 182 B), which consists of two laminæ, an anterior and a posterior, continuous at the apex of the invagination. The whole structure, which is completely imbedded within the yolk, rapidly grows in length, and turns towards the front end of the egg (fig. 182 C). Its anterior lamina remains thick and gives rise to the ventral plate (_ps_), the posterior (_am_) on the other hand becomes very thin, and forms a covering corresponding with the amnion of the more ordinary types. The remainder of the blastoderm covering the yolk (_se_) forms the homologue of the serous membrane of other types. The ventral surface of the ventral plate is turned towards the dorsal side (retaining the same nomenclature as in ordinary cases) of the egg, and the cephalic extremity is situated at the point of origin of the infolding.

The further history is however somewhat peculiar. The amnion is at first (fig. 182 C) continuous with the serous envelope on the posterior side only, so that the serous envelope does not form a continuous sack, but has an opening close to the head of the embryo. In the Hemiptera parasita this opening (Melnikow, No. 422) remains permanent, and the embryo, after it has reached a certain stage of development, becomes everted through it, while the yolk, enclosed in the continuous membrane formed by the amnion and serous envelope, forms a yolk-sack on the dorsal surface. In the Libellulidæ however and most Hemiptera, a fusion of the two limbs of the serous membrane takes place in the usual way, so as to convert it into a completely closed sack (fig. 183 A). After the formation of the appendages a fusion takes place between the amnion and serous envelope over a small area close to the head of the embryo. In the middle of this area a rupture is then effected, and the head of the embryo followed by the body is gradually pushed through the opening (fig. 183 B and C). The embryo becomes in the process completely rotated, and carried into a position in the egg-shell identical with that of the embryos of other orders of Insects (fig. 183 C).

[FIG. 183. THREE STAGES IN THE DEVELOPMENT OF CALOPTERYX. (After
Brandt.)

The embryo is represented in the egg-shell; B. and C. shew the
inversion of the embryo.
_se._ serous envelope; _am._ amnion; _ab._ abdomen; _v._ anterior
end of head; _at._ antennæ; _md._ mandible; _mx1._ maxilla 1;
_mx2._ maxilla 2; _p1-p3._ three pairs of legs;
_oe._ oesophagus.]

Owing to the rupture of the embryonic envelopes taking place at the point where they are fused into one, the yolk does not escape in the above process, but is carried into a kind of yolk-sack, on the dorsal surface of the embryo, formed of the remains of the amnion and serous envelope. The walls of the yolk-sack either assist in forming the dorsal parietes of the body, or are more probably enclosed within the body by the growth of the dorsal parietes from the edge of the ventral plate.

[FIG. 184. THREE LARVAL STAGES OF HYDROPHILUS FROM THE DORSAL SIDE,
SHEWING THE GRADUAL CLOSING IN OF THE DORSAL REGION WITH THE
FORMATION OF THE PECULIAR DORSAL ORGAN _do._ (After Kowalevsky.)

_do._ dorsal organ; _at._ antennæ.]

In Hydrophilus and apparently in the Phryganidæ also, there are certain remarkable peculiarities in the closure of the dorsal surface. The fullest observations on the subject have been made by Kowalevsky (No. 416), but Dohrn (No. 408) has with some probability thrown doubts on Kowalevsky's interpretations. According to Dohrn the part of the serous envelope which covers the dorsal surface becomes thickened, and gives rise to a peculiar dorsal plate which is shewn in surface view in fig. 184 A, _do_, and in section in fig. 185 A, _do._ The ventral parts of the amnion and serous membrane have either been ruptured or have disappeared. While the dorsal plate is being formed, the mesoblast, and somewhat later the lateral parts of the epiblast of the ventral plate gradually grow towards the dorsal side and enclose the dorsal plate, the wall of which in the process appears to be folded over so as first of all to form a groove and finally a canal. The stages in this growth are shewn from the surface in fig. 184 B and C and in section in fig. 185 B, _do._ The canal is buried on the dorsal part of the yolk, but for some time remains open by a round aperture in front (fig. 184 C). The whole structure is known as the dorsal canal. It appears to atrophy without leaving a trace. The heart when formed lies immediately dorsal to it[170].

[170] According to Kowalevsky the history of the dorsal plate is
somewhat different. He believes that on the absorption of the
amnion the ventral plate unites with the serous membrane, and
that the latter directly gives rise to the dorsal integument,
while the thickened part of it becomes involuted to form the
dorsal tube already described.

[FIG. 185. THREE TRANSVERSE SECTIONS THROUGH ADVANCED EMBRYOS OF
HYDROPHILUS.

A. Section through the posterior part of the body of the same age as
fig. 184 A.
B. Section through the embryo of the same age as fig. 184 C.
C. Section through a still older embryo.
_do._ dorsal plate; _vn._ ventral nerve cord; _al._ mesenteron;
_ht._ heart.
The large spaces at the sides are parts of the body cavity.]

In the Poduridæ the embryonic membranes appear to be at any rate imperfect. Metschnikoff states in his paper on Geophilus that in some ants no true embryonic membranes are found, but merely scattered cells which take their place. In the Ichneumonidæ the existence of two embryonic membranes is very doubtful.

Formation of the embryonic layers. The formation of the layers has been studied in sections by Kowalevsky (No. 416), Hatschek (No. 414), and Graber (No. 412), etc. From their researches it would appear that the formation of the mesoblast always takes place in a manner closely resembling that in Hydrophilus. The essential features of the process (figs. 177 and 178) appear to be that a groove is formed along the median line of the ventral plate, and that the sides of this groove either (1) simply close over like the walls of the medullary groove in Vertebrates, and so convert the groove into a tube, which soon becomes solid and forms a mass or plate of cells internal to the epiblast; or (2) that the cells on each side of the groove grow over it and meet in the middle line, forming a layer external to the cells which lined the groove. The former of these processes is the most usual; and in the Muscidæ the dimensions of the groove are very considerable (Graber, No. 411). In both cases the process is fundamentally the same, and causes the ventral plate to become divided into two layers[171]. The external layer or epiblast is an uniform sheet forming the main part of the ventral plate (fig. 178 B, _ep_). It is continuous at its edge with the amnion. The inner layer or mesoblast constitutes an independent plate of cells internal to the epiblast (fig. 178 B, _me_). The mesoblast soon becomes divided into two lateral bands.

[171] Tichomiroff (No. 420) denies the existence of a true
invagination to form the mesoblast, and also asserts that a
separation of mesoblast cells from the epiblast can take place at
other parts besides the median ventral line.

The origin of the hypoblast is still in dispute. It will be remembered (_vide_ pp. 114 and 116) that after the segmentation a number of nuclei remain in the yolk; and that eventually a secondary segmentation of the yolk takes place around these nuclei, and gives rise to a mass of yolk cells, which fill up the interior of the embryo. These cells are diagrammatically shewn in figs. 181 and 189, and it is probable that they constitute the true hypoblast. Their further history is given below.

_Formation of the organs and their relation to the germinal
layers._

The segments and appendages. One of the earliest phenomena in the development is the appearance of transverse lines indicating segmentation (fig. 186). The transverse lines are apparently caused by shallow superficial grooves, and also in many cases by the division of the mesoblastic bands into separate somites. The most anterior line marks off a præ-oral segment, which soon sends out two lateral wings--the procephalic lobes. The remaining segments are at first fairly uniform. Their number does not, however, appear to be very constant. So far as is known they never exceed seventeen, and this number is probably the typical one (figs. 186 and 187).

In Diptera the number appears to be usually fifteen though it may be only fourteen. In Lepidoptera and in Apis there appear to be sixteen segments. These and other variations affect only the number of the segments which form the abdomen of the adult.

[FIG. 186. EMBRYO OF HYDROPHILUS PICEUS VIEWED FROM THE VENTRAL
SURFACE. (After Kowalevsky.)

_pc.l._ procephalic lobe.]

The appendages arise as paired pouch-like outgrowths of the epiblast and mesoblast; and their number and the order of their appearance are subject to considerable variation, the meaning of which is not yet clear. As a rule they arise subsequently to the segmentation of the parts of the body to which they belong. There is always formed one pair of appendages which spring from the lateral lobes of the procephalic region, or from the boundary line between these and the median ventral part of this region. These appendages are the antennæ. They have in the embryo a distinctly ventral position as compared to that which they have in the adult.