[153]The following classification of the Nematoda is employed in this chapter:I.Nematoidea.Ascaridæ.Strongylidæ.Trichinidæ.Filaridæ.Mermithidæ.Anguillulidæ.II.Gordioidea.III.Chætosomoidea.[154]The ova of Anguillula aceti are stated by Hallez to undergo a similar development to those of Cucullanus.[155]The following facts are mainly derived from Leuckart’s exhaustive treatise (No.388).[156]Leuckart does not appear to be satisfied as to the hermaphroditism of these forms; and holds that it is quite possible that the ova may develop parthenogenetically.[157]VideLeuckart,D. men. Par.,Vol.II.p.704.[158]VideD. P. Manson, “On the development of Filaria sanguinis hominis.”Journal of the Linnean Society,Vol.XIV.No.75.[159]Echin. proteus, which is parasitic in the adult state in many freshwater fish, passes through its larval condition in the body cavity of Gammarus pulex. Ech. angustatus, parasitic in the Perch, is found in the larval condition in the body cavity of Asellus aquatious. Ech. gigas, parasitic in swine, is stated by Schneider (No.394) to pass through its larval stages in maggots.
[153]The following classification of the Nematoda is employed in this chapter:
I.Nematoidea.
Ascaridæ.
Strongylidæ.
Trichinidæ.
Filaridæ.
Mermithidæ.
Anguillulidæ.
II.Gordioidea.
III.Chætosomoidea.
[154]The ova of Anguillula aceti are stated by Hallez to undergo a similar development to those of Cucullanus.
[155]The following facts are mainly derived from Leuckart’s exhaustive treatise (No.388).
[156]Leuckart does not appear to be satisfied as to the hermaphroditism of these forms; and holds that it is quite possible that the ova may develop parthenogenetically.
[157]VideLeuckart,D. men. Par.,Vol.II.p.704.
[158]VideD. P. Manson, “On the development of Filaria sanguinis hominis.”Journal of the Linnean Society,Vol.XIV.No.75.
[159]Echin. proteus, which is parasitic in the adult state in many freshwater fish, passes through its larval condition in the body cavity of Gammarus pulex. Ech. angustatus, parasitic in the Perch, is found in the larval condition in the body cavity of Asellus aquatious. Ech. gigas, parasitic in swine, is stated by Schneider (No.394) to pass through its larval stages in maggots.
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 nervecords 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. 168A). 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.
Adult example of Peripatus capensisFig. 167. Adult example of Peripatus capensis, natural size.(From Moseley.)
Fig. 167. Adult example of Peripatus capensis, natural size.(From Moseley.)
Two stages in the development of Peripatus capensisFig. 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.
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.
Embryo of Peripatus capensisFig. 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.
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 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. 168B).
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 theirbases 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.
head of an embryo of Peripatus capensisFig. 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-œsophageal 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.
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-œsophageal 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 areformed as cuticular investments of papillæ in pockets of the skin at the ends of their terminal joints.
Head of an embryo PeripatusFig. 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-œsophageal ganglia. The antennæ, oral cavity, and oral papillæ are also shewn.
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-œsophageal 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 infig. 168B. 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-œsophageal ganglia are formed as thickenings of the epiblast of the ventral side of the procephalic lobes in front of the stomodæum. They are shewn atlinfig. 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 infig. 171[161]. Their openings become closed, and the walls of the invaginations constitute a large part of the embryonic supra-œsophageal ganglia.
Similar epiblastic invaginations assist in forming the supra-œsophageal ganglia of other Tracheata. They are described in the sequel for Insects, Spiders and Scorpions. The position of the supra-œsophageal ganglia on the ventral side of the procephalic lobes is the same as that in other Tracheata.
Section of an embryo of PeripatusFig. 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.
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). Thereare 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-œsophageal 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. 196A) or Spider (fig. 200C) or better still with Metschnikoff’s figure of Geophilus (No.399)Pl.XXI.fig.II,he will be satisfied on this point.
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 (fourthpost-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].
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].
Three stages in the development of Strongylosoma GueriniiFig. 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.
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.
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.
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. 173A,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. 173B). 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. 173C). 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-œsophageal ganglia. The next important change which takes place is the segmentation of the body of the embryo (fig. 174A), 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. Behindthe three existing pairs of limbs there are developed three fresh pairs, ofwhich 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. 174A).
In Polyxenus a cuticular membrane is present as in Strongylosoma, but it is not provided with a tooth-like process. In the same form amœboid cells separate themselves from the blastoderm at an early period. These cells have been compared to the embryonic envelopes of Insects described below.
In Julustwocuticular 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.
Two stages in the development of Strongylosoma GueriniiFig. 174. Two stages in the development of Strongylosoma Guerinii.(After Metschnikoff.)A. A seventeen days’ embryo, already segmented.B. A just hatched larva.
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. 174B) 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 andfourth 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].
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.
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 andMetschnikoff’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 havenotoriginated 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.
5. Two stages in the development of GeophilusFig. 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.
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. 175A). 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 anappearance something like that of many embryo Crustaceans (fig. 175B). 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.
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].
The formation of the embryonic layers in Insects has not been followed out in detail in a large number of types; but, asin 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.
Four embryos of Hydrophilus piceusFig. 176. Four embryos of Hydrophilus piceus viewed from the ventral surface.(After Kowalevsky.)The upper end is the anterior.gg.germinal groove;am.amnion.
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.
Two sections of Hydrophilus piceusFig. 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. 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.
The segmentation has not been studied, but no doubt belongs to the centrolecithal type (videpp.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. 177A). 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. 176B 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. 178A,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. 177B). 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.
Sections of Hydropilus piceusFig. 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.
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 ageneral way in the same manner as the amnion in, for instance, the chick. The folds at their origin are shewn in surface view infig. 176D, am, and in section infig. 177B,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 (videfig. 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) dividedby 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 infig. 178B. The whole of the body of the embryo is formed from the ventral plate, and no part from the amnion or serous envelope.