The Works of Francis Balfour 2-17
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
- The Ovum and Spermatozoon | The Maturation and Impregnation of the Ovum | The Segmentation of the Ovum | Dicyemae and Orthonectidae Dicyema | Porifera | Coelenterata | Platyhelminthes | Rotifera | Mollusca | Polyzoa | Brachiopoda | Chilopoda | Discophora | Gephyrea | Chaetognatha | Nemathelminthes | Tracheata | Crustacea | Pcecilopoda | Echinodermata | Enteropneusta | Bibliography
The Works of Francis Balfour Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol II. A Treatise on Comparative Embryology (1885)
CHAPTER XVII. TRACHEATA
PROTOTRACH EAT A. THE remarkable researches of Moseley (No. 396) on Peripatus
FIG. 167. ADULT EXAMPLE OF PERIPATUS CAPENSIS, natural size. (From Moseley.)
capensis have brought clearly to light the affinities of this form with the tracheate Arthropoda ; and its numerous primitive
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 antennae have become developed. Both figures represent the larva as it appears within the egg.
i and i. First and second post-oral appendages.
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 1 ), 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 prae-oral lobe of Chaetopods 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.
The single pair of antennae 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).
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, i) while in front and behind it are formed the upper and lower lips. These two appendages next turn inwards towards the mouth, and their
FIG. 169. EMBRYO OF PERIPATUS CAPENSIS. Slightly older than A in fig. 168; unrolled. (After Moseley.)
a. antennae ; o. mouth ; i. intestine ; c. procephalic lobe, i, 2, 3, etc., postoral appendages.
1 F. M. Balfour, "On certain points in the Anatomy of Peripatus capensis." Quart. Journ. of Micros. Science, Vol. xix. 1879.
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.
The antennae early become jointed, and fresh joints continue to be added throughout embryonic life ; in the adult there are 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 antennae 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 FIG. 171. HEAD OF AN EMBRYO PERIPA, . . r TUS. (From Moseley.)
and largest pair of appen- The figure shews the jaws (mamlil)lcs)> and
dagCS in the embryo are close to them epiblastic involutions, which
FIG. 170. VENTRAL VIEW OF THE HEAD OF AN EMBRYO OF PERIPATUS CAPENSIS AT A LATE STAGE OF DEVELOPMENT.
/. thickening of epiblast of procephalic lobe to form supra-oesophageal ganglion ; ///. process from procephalic lobe growing over the first post-oral appendage ; o. mouth; e. eye; i and 2, first and second pair of post-oral appendages.
the oral papillae. They
grow into the supra-oesophageal ganglia. The antennae, oral cavity, and oral papilhe are also
are chiefly remarkable for shewn.
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 papillae in pockets of the skin at the ends of their terminal joints.
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-cesophageal ganglia are formed as thickenings of the epiblast of the ventral side of the procephalic lobes in front of the stomodaeum. They are shewn at / 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 l . Their openings become closed, and the walls of the invaginations constitute a large part of the embryonic supra-cesophageal ganglia.
Similar epiblastic invaginations assist in forming the supra-cesophageal ganglia of other Tracheata. They are described in the sequel for Insects, Spiders and Scorpions. The position of the supracesophageal ganglia on the ventral side of the procephalic lobes is the same as that in other Tracheata.
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
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.
me. median section of body cavity.
k. lateral section of body cavity.
v.n. ventral nerve cord.
1 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.
are two special longitudinal septa dividing the body cavity into three compartments, a median (me), containing the mesenteron, and two lateral (Ic) 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 Chaetopoda, 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 supracesophageal ganglia.
The alimentary tract is formed of a mesenteron (fig. 172), a stomodaeum, and proctodaeum. 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 stomodaeum 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 proctodaeum is a moderately deep pit near the hinder end of the body.
The existence of a tracheal system 1 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 Metschnikoffs figure of Geophilus (No. 399) PI. xxi. fig. u,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 papillae to the first pair of maxillae, etc. Moseley is somewhat doubtful about the homologies of the appendages, and hesitates between considering the oral papillae as equivalent to the second pair of maxillae (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
1 The specimens shewing tracheae 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.
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 Chelicerae (mandibles) of Spiders, and there is some evidence in Insects for the existence of a gland belonging to the first pair of maxillae, 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.
(396) H. N. Moseley. "On the Structure and Development of Peripatus capensis." Phil. Trans. Vol. 164, 1874.
MYRIAPODA 1 .
Chilognatha. The first stages in the development of the Chilognatha have been investigated by Metschnikoffand 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 2 .
1 The classification of the Myriapoda employed in the present section is
I. Chilognatha. (Millipedes.) II. Chilopoda. (Centipedes.)
2 Stecker's (No. 400) observations were made on the eggs of Julus fasciatus, Julus fcetidus, 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
FIG. 173. THREE STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUERINII. (After Metschnikoff.)
A. Embryo on eleventh day with commencing ventral flexure (*).
B. Embryo with three pairs of post-oral appendages.
C. Embryo with five pairs of post-oral appendages.
gs. ventral plate; at. antenme; 15 post-oral appendages; x. point of flexure of the ventral plate.
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 Sleeker, 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.
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 antennae, 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 antennae two further pairs become added, and at the same time oral and anal invaginations become formed '(fig- 173 Q. In front of the oral opening an unpaired upper lip is developed. The prse-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-cesophageal 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 antennae 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 whicJi tJie 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.
At the time when the embryo of Strongylosoma is hatched (fig. 174 B) nine post-cephalic segments appear to be present.
FlG. 174. TWO STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUEKINll.
A. A seventeen days' embryo, already segmented.
B. A just-hatched larva.
Of these segments the second is apparently (from MetschnikofT'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 1 .
Julus, at the time it leaves the chorion, is imperfectly segmented, but is provided with antennas, mandibles, and maxillae, 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) 2 . 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 and
1 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.
2 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.
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.
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.
a u . i
FlG. 175. TWO STAGES IN THE DEVELOPMENT OF GEOPHILUS.
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. antenna.-; an.t. proctodseum.
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 antennae 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 maxillae 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 maxillae 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 maxillae of Insects or to the first pair of limbs of Insects? In favour of the former view is the fact (i) that in embryo Insects the second pair of maxillae 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 maxillae 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 maxillae, though they are limb-like and not connected with the head. The subjoined table shews the probable homologies of the appendages.
CHILOGNATHA(Strongylo so ma at time of birth).
CHILOPODA (Scolopendra adult).
ist Post-oral segment.
2nd ,, ,,
Maxillae i. (Four-lobed plate in adult, but a simple pair of appendages in embryo).
Maxillie i. (Palp and bilobed median process).
3rd (probably equivalent to segment bearing 2nd pair of maxillae in Insects).
ist pair of ambulatory limbs.
Limb-like appendages with basal parts in contact.
4th ,, ,,
2nd pair of ambulatory limbs.
ist pair of ambulatory limbs.
4th and sth (rudimentary. )
8th ,, ,,
6th (the 7th pair is developed in this segment later).
,, (last segment 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 stomodacum and proctodaeum.
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 stomodaeum and the proctodasum. The Malpighian bodies arise as outgrowths from the blind extremity of the latter.
(397) G. N e wp or t. " On the Organs of Reproduction and Development of the Myriapoda." Philosophical Transactions, 1841.
(398) E. Metschnikoff. ' ' Embryologie der doppeltflissigen Myriapoden (Chilognatha)." Zeit.f. wiss. Zool., Vol. xxiv. 1874.
(399) ' ' Embryologisches iiber Geophilus." Zeit. f. wiss. ZooL y Vol. xxv.
1875 (400) Anton Stecker. "Die Anlage d. Keimblatter bei den Diplopoden." Archivf. mik. Anatomie, Bd. xiv. 1877.
INSECTA 1 .
The formation of the embryonic layers in Insects has not been followed out in detail in a large number of types ; but, as
1 The following classification of the Insecta is employed in this chapter, ((i) Collembola.
I. Aptera. | (a) Thysanura .
!(i) Orthoptera genuina (Blatta, Locusta, etc.). (2) pseudoneuroptera (Termes, Ephemera,
!(i) Hemiptera heteroptera (Cimex, Notonecta, etc.). (2) ,, homoptera (Aphis, Cicada, etc.).
(3) ,, parasita (Pediculus, etc.).
in so many other instances, some of the most complete histories we have are due to Kowalevsky (No. 416). The development
FiG. 176. FOUR EMBRYOS OF llYDROPHlLUS P1CEUS VIEWED FROM THE
VENTRAL SURFACE. (After Kowalevsky.) The upper end is the anterior, gg. germinal groove; am. amnion.
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.
The segmentation has not been studied, but no doubt belongs to the centrolecithal type (vide pp. no 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.
!(i) Diptera genuina (Musca, Tipula, etc.). (2) aphaniptera (Pulex, etc.). (3) ,, pupipara (Braula, etc.).
v .. ( (i) Neuroptera planipennia (Myrniclcon, etc.) TOptera. j (a) ^ trichoptera (Phryganea, etc.).
VI. Coleoptera. VII. Lepidoptera.
(i) Hymenoptera aculeata (Apis, Formica, etc.). (a) ,, entomophaga (Ichneumon, Platy gaster, etc). (3) ,, phytophaga ( Tenthredo, Sirex, etc.).
FlG. 177. TWO TRANSVERSE SECTIONS THROUGH
EMBRYOS OF HvDROPHiLUS 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.
). 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.
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
gg. germinal groove ; nion ; yk. yolk.
me. mesoblast ; am. am
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 1 . 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
FIG. 178. SECTIONS THROUGH TWO EMBRYOS OF HYDROPHILUS PICEUS. (After Kowalevsky.)
A. Section through the posterior part of the embryo fig. 1 76 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 ; cp. epiblast.
1 The reverse nomenclature to this is rather inconveniently employed by Metschnikoff.
FIG. 179. EMBRYO OF HYDROPHILUS PICEUS
VIEWED FROM THE VEN TRAL SURFACE. (After Kowalevsky.)
pc.L procephalic lobe.
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.
The general history of the succeeding stages may be briefly told.
The appendages appear as very small rudiments at the close of the last stage, but soon become much more prominent (fig. 1 80 A). They are formed as outgrowths of both layers, and arise nearly simultaneously. There are in all eight pairs of appendages. The anterior or antennae (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. 1 80 A, give rise to the tracheae; and the nervous system is formed as two lateral epiblastic thickenings, one on each side of the midventral line. These eventually become split off from the skin ; while between them there passes in a median invagination of the skin
FlG. 1 80. TWO STAGES IN THE DEVELOPMENT OF HYDROPHILUS
PICEUS. (From Gegenbaur, after Kowalevsky.)
Is. labrum ; at. antenna ; md.
(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, Is). A proctodaeum is formed at the hind end of the body slightly later than the stomodaeum. 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, hi]. 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 proctodaeum forms the rectum and Malpighian bodies 1 , and the stomodseum the oesophagus and proventriculus. The two epiblastic sections of the alimentary tract are eventually placed in communication with the mesenteron.
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.
TJte 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,
1 This has not been shewn in the case of Hydrophilus,
which usually contains nuclei, and in the Poduridae 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.
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 thatadouble 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 B. II. 26
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. stomodaeum ; an i. proctodaeum.
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, Lino).
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 Libellulidae 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.
In the Libellulidas (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 laminae, 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 Libellulidae 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).
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
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. vefitral plate; g. edge of ventral plate; am. amnion ; se- serous envelope.
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.
In Hydrophilus and apparently in the Phryganidae 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 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
FlG. 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.
sf. serous envelope ; am. amnion ; ab. abdomen ; v. anterior end of head ; at. antennae ; md. mandible ; mx l . maxilla i ; mx*. maxilla 2 ; p 1 ^. three pairs of legs; oe. oesophagus.
fig. 184 A, doi and in section in fig. 185 A, do. The
FIG. 184. THREE LARVAL STAGES OF HYDROPHILUS FROM THE DORSAL SIDE, SHEWING THE GRADUAL CLOSING IN OF THE DORSAL REGION WITH THE FORMATION < >! THK I'l.CULIAR DORSAL ORGAN
do. (After Kowalevsky.)
do. dorsal organ ; at. antennae.
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 1 .
A. B. C.
FIG. 185. THREE TRANSVERSE SECTIONS THROUGH ADVANCED
EMBRYOS OF HYDROPHILUS.
Section through the posterior part of the body of the same age as fig. 184 A. Section through the embryo of the same age as fig. 184 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 Poduridas 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 Ichneumonidas 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),
1 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.
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 (i) 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 Muscidae 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 1 . 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.
The origin of the hypoblast is still in dispute. It will be remembered (vide pp. 1 14 and 1 16) 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
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
1 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.
many cases by the division of the mesoblastic bands into separate somites. The most anterior line marks off a prae-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.
The appendages arise as paired pouchlike 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 antennae. They have in the embryo a distinctly ventral position as compared to that which they have in the adult.
In the median ventral part of the procephalic region there arises the labrum (fig. 187, Is}. It is formed by the coalescence of a pair of prominences very similar to true appendages, though it is probable that they have not this value 1 .
1 If these structures are equivalent to appendages, they may correspond to one of the pairs of antennae of Crustacea. From a figure by Fritz Miiller of the larva of Calotermes (Jenaische Zeit. Vol. XI. pi. n, fig. 12) it would appear that they lie in front of the true antennae, and would therefore on the above hypothesis correspond to the first pair of antennae of Crustacea. Biitschli (No. 405) describes in the Bee a pair of prominences immediately in front of the mandibles which eventually unite to form a kind of underlip ; they in some ways resemble true appendages.
FIG. 1 86. EMBRYO OF HYDROPHILUS PI CEUS VIEWED FROM THE VENTRAL SURFACE.
(After Kowalevsky.) pc. I. procephalic lobe.
The antennae themselves can hardly be considered to have the same morphological value as the succeeding appendages. They are rather equivalent to paired processes of the prae-oral lobes of the Chaetopoda.
From the first three post-oral segments there grow out the mandibles and two pairs of maxillae, and from the three following segments the three pairs of thoracic appendages. In many Insects (cf. Hydrophilus) a certain .number of appendages of the same nature as the anterior ones are visible in the embryo on the abdominal segments, a fact which shews that Insects are descended from ancestors with more than three pairs of ambulatory appendages.
In Apis according to Biitschli (No. 405) all the abdominal segments are provided with appendages, which always remain in a very rudimentary condition. All trace of them as well as of the thoracic appendages is lost by the time the embryo is hatched. In the phytophagous Hymenoptera the larva is provided with 9 ii pairs of legs.
In the embryo of Lepidoptera there would appear from Kowalevsky's figures to be rudiments of ten pairs of post-thoracic appendages. In the caterpillar of this group there are at the maximum five pairs of such rudimentary feet, viz. a pair on the 3rd, 4th, 5th, and 6th, and on the last abdominal segment. The embryos of Hydrophilus (fig. 187), Mantis, etc. are also provided with additional appendages. In various Thysanura small prominences are present on more or fewer of the abdominal segments (fig. 192), which may probably be regarded as rudimentary feet.
Whether all or any of the appendages of various kinds connected with the hindermost segments belong to the same category as the legs is very doubtful. Their usual absence in the embryo or in any case their late appearance appears to me against so regarding them ; but Biitschli is of opinion that in the Bee the parts of the sting are related genetically to the appendages of the penultimate and antepenultimate abdominal segments, and this view is to some extent supported by more recent
FlG. 187. TWO STAGES IN THE DEVELOPMENT OF HYDROPHILUS
PICEUS. (From Gegenbaur, after Kowalevsky. )
Is. labrum; at. antenna; tnd. mandible; nix. maxilla I.; li. maxilla II.; //>"/" feet; a. anus.
observations (Kraepelin, etc.), and if it holds true for the Bee must be regarded as correct for other cases also.
As to the order of the appearance of the appendages observations are as yet too scanty to form any complete scheme. In many cases all the appendages appear approximately at the same moment, e.g. Hydrophilus, but whether this holds good for all Coleoptera is by no means certain. In Apis the appendages are stated by Biitschli to arise simultaneously, but according to Kowalevsky the two mouth appendages first appear, then the antennae, and still later the thoracic appendages. In the Diptera the mouth appendages are first formed, and either simultaneously with these, or slightly later, the antennae. In the Hemiptera and Libellulidae the thoracic appendages are the first to be formed, and the second pair of maxillae makes its appearance before the other cephalic appendages.
The history of the changes in the embryonic appendages during the attainment of the 'adult con- .
dition is beyond the scope of this treatise, but it may be noted that the second pair of maxillae are relatively very large in the embryo, and not infrequently (Libellula, etc.) have more resemblance to the ambulatory than to the masticatory appendages.
The exact nature of the wings and their relation to the other segments is still very obscure. They appear as dorsal leaf-like appendages on the 2nd and 3rd thoracic segments, and are in many respects similar to the tracheal gills of the larvae of Ephemeridae and Phryganidae (fig. 1 88 A), of which they are supposed by Gegenbaur and Lubbock to be modifications. The undoubtedly secondary character of the closed tracheal system of larvae with tracheal gills tells against this view. Fritz Miiller finds in the larvae of Calotermes ru
FIG. 188. FIGURES ILLUSTRATING AQUATIC RESPIRATION IN INSECTS. (After Gegenbaur.)
A. Hinder portion of the body of Ephemera vulgata. a. longitudinal tracheal trunks; b. alimentary canal ; c. tracheal gills.
B. Larva of ^Eschna grandis. a. superior longitudinal tracheal trunks ; b. their anterior end ; c. portion branching on proctodaeum ; o. eyes.
C. Alimentary canal of the same larva from the side, a, b, and c. as in B ; d. inferior tracheal trunk ; e. transverse branches between upper and lower tracheal trunks.
gosus (one of the Termites) that peculiar and similar dorsal appendages are present on the two anterior of the thoracic segments. They are without tracheae. The anterior atrophies, and the posterior acquires tracheas and gives rise to the first pair of wings. The second pair of wings is formed from small processes on the third thoracic segment like those on the other two. Fritz Miiller concludes from these facts that the wings of Insects are developed from dorsal processes of the body, not equivalent to the ventral appendages. What the primitive function of these appendages was is not clear. Fritz Miiller suggests that they may have been employed as respiratory organs in the passage from an aqueous to a terrestrial existence, when the Termite ancestors lived in moist habitations a function for which processes supplied with blood-channels would be well adapted. The undoubted affinity of Insects to Myriapods, coupled with the discovery by Moseley of a tracheal system in Peripatus, is however nearly fatal to the view that Insects can have sprung directly from aquatic ancestors not provided with tracheae. But although this suggestion of Fritz Miiller cannot be accepted, it is still possible that the processes discovered by him may have been the earliest rudiments of wings, which were employed first as organs of propulsion by a water-inhabiting Insect ancestor which had not yet acquired the power of flying.
The nervous system. The nervous system arises entirely from the epiblast; but the development of the prae-oral and post-oral sections may be best considered separately.
The post-oral section, or ventral cord of the adult, arises as two longitudinal thickenings of the epiblast, one on each side of the median line (fig. 189 B, vn), which are subsequently split ofif from the superficial skin and give rise to the two lateral strands of the ventral cord. At a later period they undergo a differentiation into ganglia and connecting cords.
Between these two embryonic nerve cords there is at first a shallow furrow, which soon becomes a deep groove (fig. 189 C). At this stage the differentiation of the lateral elements into ganglia and commissures takes place, and, according to Hatschek (No. 414), the median groove becomes in the region of the ganglia converted into a canal, the walls of which soon fuse with those of the ganglionic enlargements of the lateral cords, and connect them across the middle line. Between the ganglia on the other hand the median groove undergoes atrophy, becoming first a solid cord interposed between the lateral strands of the nervous system, and finally disappearing without giving rise to any part of the nervous system. It is probable that Hatschek is entirely mistaken about the entrance of a median element into the ventral cord, and that the appearances he has described are due to shrinkage. In Spiders the absence of a median element can be shewn with great certainty, and, as already stated, this element is not present in
Peripatus. Hatschek states that in the mandibular segment the median element is absorbed, and that the two lateral cords of that part give rise to the oesophageal commissures, while the sub-cesophageal ganglion is formed from the fusion of the ganglia of the two maxillary segments.
The prae-oral portion of the nervous system consists entirely of the supra-cesophageal ganglion. It is formed, according to Hatschek, of three parts. Firstly and mainly, of a layer sepa
FIG. 189. THREE TRANSVERSE SECTIONS THROUGH THE EMBRYO OF HYDROPHILUS. (After Kowalevsky.)
A. Transverse section through the larva represented in fig. 187 A.
B. Transverse section through a somewhat older embryo in the region of one of the stigmata.
C. Transverse section through the larva represented in fig. 187 B.
vn. ventral nerve cord; am. amnion and serous membrane ; me. mesoblast ; me.s. somatic mesoblast ; hy. hypoblast (?) ; yk. yolk cells (true hypoblast) ; st. stigma of trachea.
rated from the thickened inner part of the cephalic lobe on each side ; secondly, of an anterior continuation of the lateral cords ; and thirdly, of a pit of skin invaginated on each side close to the
412 IN SECT A.
dorsal border of the antennae. This pit is at first provided with a lumen, which is subsequently obliterated; while the walls of the pit become converted into true ganglion cells. The two supra-cesophageal ganglia remain disconnected on the dorsal side till quite the close of embryonic life.
The tracheae and salivary glands. The tracheae, as was first shewn by Butschli (No. 405), arise as independent segmentally arranged paired invaginations of the epiblast (fig. 189 B and C, st). Their openings are always placed on the outer sides of the appendages of their segments, where such are present.
Although in the adult stigmata are never found in the space between the prothorax and head 1 , in the embryo and the larva tracheal invaginations may be developed in all the thoracic (and possibly in the three jaw-bearing segments) and in all the abdominal segments except the two posterior.
In the embryo of the Lepidoptera, according to Hatschek (No. 414), there are 14 pairs of stigmata, belonging to the 14 segments of the body behind the mouth ; but Tichomiroff states that Hatschek is in error in making this statement for the foremost post-oral segments. The last two segments are without stigmata. In the larvae of Lepidoptera as well as those of many Hymenoptera, Coleoptera and Diptera, stigmata are present on all the postcephalic segments except the 2nd and 3rd thoracic and the two last abdominal. In Apis there are eleven pairs of tracheal invaginations according to Kowalevsky (No. 416), but according to Butschli (No. 405) only ten, the prothorax being without one. In the Bee they appear simultaneously, and before the appendages.
The blind ends of the tracheal invaginations frequently (e.g. Apis) unite together into a common longitudinal canal, which forms a longitudinal tracheal stem. In other cases (eg. Gryllotalpa, Dohrn, No. 408) they remain distinct, and each tracheal stem has a system of branches of its own.
The development of the tracheae strongly supports the view, arrived at by Moseley from his investigations on Peripatus, that they are modifications of cutaneous glands.
The salivary and spinning glands are epiblastic structures, which in their mode of development are very similar to the tracheae, and perhaps have a similar origin. The salivary glands
1 In Smynthurus, one of the Collembola, there are, according to Lubbock, only two stigmata, which are placed on the head.
arise as paired epiblastic imaginations, not, as might be expected, of the Stomodaeum, but of the ventral plate behind the mouth on the inner side of the mandibles. At first independent, they eventually unite in a common duct, which falls into the mouth. The spinning glands arise on the inner side of the second pair of maxillae in Apis and Lepidoptera, and form elongated glands extending through nearly the whole length of the body. They are very similar in their structure and development to salivary glands, and are only employed during larval life. They no doubt resemble the mucous glands of the oral papillae of Peripatus, with which they have been compared by Moseley. The mucous glands of Peripatus may perhaps be the homologous organs of the first pair of maxillae, for the existence of which there appears to be some evidence amongst Insects.
Mesoblast. It has been stated that the mesoblast becomes divided in the region of the body into two lateral bands (fig. 189 A). These bands in many, if not all forms, become divided into a series of somites corresponding with the segments of the body. In each of them a cavity appears the commencing perivisceral cavity which divides them into a somatic plate in contact with the epiblast, and a splanchnic plate in contact with the hypoblast (fig. 189). In the interspaces between the segments the mesoblast is continuous across the median ventral line. The mesoblast is prolonged into each of the appendages as these are formed, and in the appendages there is present a central cavity. By Metschnikoff these cavities are stated to be continuous, as in Myriapods and Arachnida, with those of the somites ; but by Hatschek (No. 414) they are stated to be independent of those in the somites and to be open to the yolk.
The further details of the history of the mesoblast are very imperfectly known, and the fullest account' we have is that by Dohrn (No. 408) for Gryllotalpa. It would appear that the mesoblast grows round and encloses the dorsal side of the yolk earlier than the epiblast. In Gryllotalpa it forms a pulsating membrane. As the epiblast extends dorsalwards the median dorsal part of the membrane is constricted off as a tube which forms the heart. At the same time the free space between the pulsating membrane and the yolk is obliterated, but transverse passages are left at the lines between the somites, through which the blood passes from the ventral part of the body to corresponding openings in the wall of the heart. The greater part of the membrane gives rise to the muscles of the trunk.
Ventrally the mesoblastic bands soon meet across the median line. The cavities in the appendages become obliterated and their mesoblastic walls form the muscles, etc. The cavities in the separate mesoblastic somites also cease to be distinctly circumscribed.
The splanchnic mesoblast follows the hypoblast in its growth, and gives rise to the connective tissue and muscular parts of the walls of the alimentary tract. The mesoblastic wall of the proctodaeum is probably formed independently of the mesoblastic somites. In the head the mesoblast is stated to form at first a median ventral mass, which does not pass into the procephalic lobe ; though it assists in forming both the antennae and upper lip.
The alimentary canal. The alimentary tract of Insects is formed of three distinct sections (fig. 181) a mesenteron or middle section (me), a stomodaeum (st) and a proctodaeum (an). The stomodaeum and proctodaeum are invaginations of the epiblast, while the mesenteron is lined by the hypoblast. The distinction between the three is usually well marked in the adult by the epiblastic derivatives being lined by chitin. The stomodaeum consists of mouth, oesophagus, crop, and proventriculus or gizzard, when such are present. The mesenteron includes the stomach, and is sometimes (Orthoptera, etc.) provided at its front end with pyloric diverticula posteriorly it terminates just in front of the Malpighian bodies. These latter fall into the proctodaeum, which includes the whole of the region from their insertion to the anus.
The oral invagination appears nearly coincidently. with the first formation of segments at the front end of the groove between the lateral nerve cords, and the anal invagination appears slightly later at the hindermost end of the ventral plate.
The Malpighian bodies arise as two pairs of outgrowths of the epiblast of t/te proctodceum, whether solid at first is not certain. The subsequent increase which usually takes place in their number is due to sproutings (at first solid) of the two original vessels.
The glandular walls of the mesenteron are formed from the hypoblast ; but the exact origin of the layer has not been thoroughly worked out in all cases. In Hydrophilus it is stated by Kowalevsky (No. 416) to appear as two sheets split off from the lateral masses of mesoblast, which gradually grow round the yolk, and a similar mode of formation would seem to hold good for Apis. Tichomiroff (No. 420) confirms Kowalevsky on this point,
TR ACHE AT A. 415
and further states that these two masses meet first ventrally and much later on the dorsal side. In Lepidoptera, on the other hand, Hatschek finds that the hypoblast arises as a median mass of polygonal cells in the anterior part of the ventral plate. These cells increase by absorbing material from the yolk, and then gradually extend themselves and grow round the yolk.
Dohrn (No. 408) believes that the yolk cells, the origin of which has already been spoken of, give rise to the hypoblastic walls of the mesenteron, and this view appears to be shared by Graber (No. 412), though the latter author holds that some of the yolk cells are derived by budding from the blastoderm 1 .
From the analogy of Spiders I am inclined to accept Dohrn's and Graber's view. It appears to me probable that Kowalevsky's observations are to be explained by supposing that the hypoblast plates which he believes to be split off from the mesoblast are really separated from the yolk.
.It will be convenient to add here a few details to what has already been stated as to the origin of the yolk cells. As mentioned above, the central yolk breaks up at a period, which is not constant in the different forms, into polygonal or rounded masses, in each of which a nucleus has in many instances been clearly demonstrated although in others such nuclei have not been made out. It is probable however that nuclei are in all cases really present, and that these masses must be therefore regarded as cells. They constitute in fact the yolk cells. The periphery of the yolk breaks up into cells while the centre is still quite homogeneous.
The hypoblastic walls of the mesenteron appear to be formed in the first instance laterally (fig. 189 B and C, hy). They then meet ventrally (fig. 185 A and B), and finally close in the mesenteron on the dorsal side.
The mesenteron is at first a closed sack, independent of both stomodaeum and proctodaeum ; and in the case of the Bee it so remains even after the close of embryonic life. The only glandular organs of the mesenteron are the not unfrequent pyloric tubes, which are simple outgrowths of its anterior end. It is possible that in some instances they may be formed in situ around the lateral parts of the yolk.
In many instances the whole of the yolk is enclosed in the walls of the mesenteron, but in other cases, as in Chironomus and Simulia (Weismann, No. 430 ; Metschnikoff, No. 423), part of the yolk may be left between the ventral wall of the mesenteron and the ventral plate. In Chironomus the
1 Graber's view on this point may probably be explained by supposing that he has mistaken a passage of yolk cells into the blastoderm for a passage of blastoderm cells into the yolk. The former occurrence takes place, as I have found, largely in Spiders, and probably therefore also occurs in Insects.
41 6 INSECTA.
mass of yolk external to the mesenteron takes the form of a median and two lateral streaks. Some of the yolk cells either prior to the establishment of the mesenteron, or derived from the unenclosed portions of the yolk, pass into the developing organs (Dohrn, 408) and serve as a kind of nutritive cell. They also form blood corpuscles and connective-tissue elements. Such yolk cells may be compared to the peculiar bodies described by Reichenbach in Astacus, which form the secondary mesoblast. Similar cells play a very important part in the development of Spiders.
Generative organs. The observations on the development of the generative organs are somewhat scanty. In Diptera certain cells known as the pole cells are stated by both Metschnikoff (No. 423) and Leuckart to give rise to the generative organs. The cells in question (in Chironomus and Musca vomitoria, Weismann, No. 430) appear at the hinder end of the ovum before any other cells of the blastoderm. They soon separate from the blastoderm and increase by division. In the embryo, produced by the viviparous larva of Cecidomyia, there is at first a single pole cell, which eventually divides into four, and the resulting cells become enclosed within the blastoderm. They next divide into two masses, which are stated by Metschnikoff (No. 423) to become surrounded by indifferent embryonic cells 1 . Their protoplasm then fuses, and their nuclei divide, and they give rise to the larval ovaries, for which the enclosing cells form the tunics.
In Aphis Metschnikoff (No. 423) detected at a very early stage a mass of cells which give rise to the generative organs. These cells are situated at the hind end of the ventral plate ; and, except in the case of one of the cells which gives rise by division to a green mass adjoining the fat body, the protoplasm of the separate cells fuses into a syncytium. Towards the close of embryonic life the syncytium assumes a horse-shoe form. The mass is next divided into two, and the peripheral layer of each part gives rise to the tunic, while from the hinder extremity of each part an at first solid duct the egg- tube grows out. The masses themselves form the germogens. The oviduct is formed by a coalescence of the ducts from each germogen.
Ganin derives the generative organs in Platygaster (vide p. 347) from the hind end of the ventral plate close to the proctodaeum ; while Suckow states that the generative organs are outgrowths of the proctodicum. According to these two sets of observations the generative organs would appear to have an epiblastic origin an origin which is not incompatible with that from the pole cells.
In Lepidoptera the genital organs are present in the later periods of embryonic life as distinct paired organs, one on each side of the heart, in the eighth postcephalic segment. They are elliptical bodies with a duct passing off from the posterior end in the female or from the middle in the male. The egg-tubes or seminal tubes are outgrowths of the elliptical bodies.
1 This point requires further observation.
In other Insects the later stages in the development of the generative organs closely resemble those in the Lepidoptera, and the organs are usually distinctly visible in the later stages of embryonic life.
It may probably be laid down, in spite of some of Metschnikoff's observations above quoted, that the original generative mass gives rise to both the true genital glands and their ducts. It appears also to be fairly clear that the genital glands of both sexes have an identical origin.
Special types of larva.
Certain of the Hymenopterous forms, which deposit their eggs in the eggs or larvae of other Insects, present very peculiar modifications in their development. Platygaster, which lays its egg in the larvae of Cecidomyia, undergoes perhaps the most remarkable development amongst these forms. It has been studied especially by Ganin (No. 410), from whom the following account is taken.
The very first stages are unfortunately but imperfectly known, and the interpretations offered by Ganin do not in all cases appear quite satisfactory. In the earliest stage after being laid the egg is enclosed in a capsule produced into a stalk (fig. 190 A). In the interior of the egg there soon appears a single spherical body, regarded by Ganin as a cell (fig. 190 B). In the next stage three similar bodies appear in the vitellus, no doubt derived from the first one (fig. 190 C). The central one presents somewhat different characters to the two others, and, according to Ganin, gives rise to the whole embryo. The two peripheral bodies increase by division, and soon appear as nuclei imbedded in a layer of protoplasm (fig. 190 D, E, F). The layer so formed serves as a covering for the embryo, regarded by Ganin as equivalent to the amnion (? serous membrane) of other Insect embryos. In the embryo cell new cells are stated to be formed by a process of endogenous cell formation (fig. 190 D, E). It appears probable that Ganin has mistaken nuclei for cells in the earlier stages, and that a blastoderm is formed as in other Insects, and that this becomes divided in a way not explained into a superficial layer which gives rise to the serous envelope, and a deeper layer which forms the embryo. However this
B. II. 27
FlG. 190. A SERIES OF STAGES IN THE DEVELOPMENT
OF PLATYGASTER. (From Lubbock ; after Ganin.)
41 8 INSECTA.
may be, a differentiation into an epiblastic layer of columnar cells and a hypoblastic layer of more rounded cells soon becomes apparent in the body of the embryo. Subsequently to this the embryo grows rapidly, till by a deep transverse constriction on the ventral surface it becomes divided into an anterior cephalothoracic portion and a posterior caudal portion (fig. 190 F). The cephalothorax grows in breadth, and near its anterior end an invagination appears, which gives rise to the mouth and cesophagus. On the ventral side of the cephalothorax there is first formed a pair of claw-like appendages on each side of the mouth, then a posterior pair of appendages near the junction of the cephalothorax and abdomen, and lastly a pair of short conical antennae in front.
At the same time the hind end of the abdomen becomes bifid, and gives rise to a fork-like caudal appendage ; and at a slightly later period four grooves make their appearance in the caudal region, and divide this part of the embryo into successive segments. While these changes have been taking place in the general form of the embryo, the epiblast has given rise to a cuticle, and the hypoblastic cells have become differentiated into a central hypoblastic axis the mesenteron and a surrounding layer of mesoblast, some of the cells of which form longitudinal muscles.
With this stage closes what may be regarded as the embryonic development of Platygaster. The embryo becomes free from the amnion, and presents itself as a larva, which from its very remarkable characters has been spoken of as the Cyclops larva by Ganin.
The larvae of three species have been described by Ganin, which are represented in fig. 1 9 1 A, B, C. These larvae are strangely dissimilar to the ordinary Hexapod type, whether larval or adult. They are formed of a cephalothoracic shield with the three pairs of appendages (a, kf, lfg\ the development of which has already been described, and of an abdomen formed of five segments, the last of which bears the somewhat varying caudal appendages. The nervous system is as yet undeveloped.
The larvae move about in the tissues of their hosts by means of their claws.
The first larval condition is succeeded by a second with very different characters, and the passage from the first to the second is accompanied by an ecdysis.
The ecdysis commences at the caudal extremity, and the whole of the last segment is completely thrown off. As the ecdysis extends forwards the tail loses its segmentation and becomes strongly compressed, the appendages of the cephalothorax are thrown off, and the whole embryo assumes an oval form without any sharp distinction into different regions and without the slightest indication of segmentation (fig. 191 D). Of the internal changes which take place during the shedding of the cuticle, the first is the formation of a proctodaeum (gfi) by an invagination, which ends blindly in contact with the mesenteron. Shortly after this a thickening of the epiblast (bsm} appears along the ventral surface, which gives rise mainly to the ventral nerve cord ; this thickening is continuous behind with the
epiblast which is invaginated to form the proctodaeum, and in front is prolonged on each side into two procephalic lobes, in which there are also thickenings of the epiblast (gsae), which become converted into supraoesophageal ganglia, and possibly other parts.
Towards the close of the second larval period the muscles (/;) become segmentally arranged, and give indications of the segmentation which
FlG. 191. A SERIES OF STAGES IN THE DEVELOPMENT OF PLATYGASTER.
(From Lubbock ; after Ganin.)
A. B. C. Cyclops larvae of three species of Platygaster. D. Second larval stage. E. Third larval stage.
mo. mouth ; a. antenna ; kf. hooked feet ; Ifg. lateral feet ; /. branches of tail ; ul. lower lip ; slkf. oesophagus ; gsae. supra- oesophageal ganglion ; bsm. ventral epiblastic plate ; Im. lateral muscles (the letters also point in D to the salivary glands) ; gh. proctodseum ; ga. generative organs ; md. mandibles ; ag. ducts of salivary glands ; sp. (in E) salivary glands ; mis. stomach ; ed. intestine ; ew. rectum ; ao. anus ; tr. tracheae ; fk. fat body.
becomes apparent in the third larval period. The third and last larval stage (fig. 191 E) of Platygaster, during which it still remains in the tissues of its host, presents no very peculiar features. The passage from the second to the third form is accompanied by an ecdysis.
Remarkable as are the larvae just described, there can I think be no reason, considering their parasitic habits, for regarding them as ancestral.
Metamorphosis and heterogamy.
Metamorphosis. The majority of Insects are born in a condition in which they obviously differ from their parents. The extent of this difference is subject to very great variations, but as a rule the larvae pass through a very marked metamorphosis before reaching the adult state. The complete history of this metamorphosis in the different orders of Insects involves a far too considerable amount of zoological detail to be dealt with in this work ; and I shall confine myself to a few observations on the general characters and origin of the metamorphosis, and of the histological processes which take place during its occurrence 1 .
In the Aptera the larva differs from the adult only in the number of facets in the cornea and joints in the antennae.
In most Orthoptera and Hemiptera the larvae differ from the adult in the absence of wings and in other points. The wings, etc., are gradually acquired in the course of a series of successive moultings. In the Ephemeridae and Libellulidae, however, the metamorphosis is more complicated, in that the larvae have provisional tracheal gills which are exuviated before the final moult. In the Ephemeridae there are usually a great number of moultings ; the tracheal gills appear after the second moult, and the rudiments of the wings when the larva is about half grown. Larval life may last for a very long period.
In all the other groups of Insects, viz. the Diptera, Neuroptera, Coleoptera, Lepidoptera, and Hymenoptera, the larva passes with a few exceptions through a quiescent stage, in which it is known as a pupa, before it attains the adult stage. These forms are known as the Holometabola.
In the Diptera the larvae are apodous. In the true flies (Muscidae) they are without a distinct head and have the jaws replaced by hooks. In the Tipulidae there is on the other hand a well-developed head with the normal appendages. The pupae of the Muscidae are quiescent, and are enclosed in the skin of the larva which shrinks and forms a firm oval case. In the
1 For a systematic account of this subject the reader is referred to Lubbock (No. 420) and to Graber (No. 411). He will find in Weismann (Nos. 430 and 431) a detailed account of the internal changes which take place.
TRACHEATA. 42 1
Tipulidae the larval skin is thrown off at the pupa stage, and in some cases the pupae continue to move about.
The larvae of the Neuroptera are hexapodous voracious forms. When the larva becomes a pupa all the external organs of the imago are already established. The pupa is often invested in a cocoon. It is usually quiescent, though sometimes it begins to move about shortly before the imago emerges.
In the Coleoptera there is considerable variety in the larval forms. As a rule the larvae are hexapodous and resemble wingless Insects. But some herbivorous larvae (e.g. the larva of Melolontha) closely resemble true caterpillars, and there are also grub-like larvae without feet (Curculio) which resemble the larvae of Hymenoptera. The pupa is quiescent, but has all the parts of the future beetle plainly visible. The most interesting larvae among the Coleoptera are those of Sitaris, one of the Meloidae (Fabre, No. 409). They leave the egg as active hexapodous larvae which attach themselves to the bodies of Hymenoptera, and are thence transported to a cell filled with honey. Here they eat the ovum of the Hymenopterous form. They then undergo an ecdysis, in which they functionally lose their appendages, retaining however small rudiments of them, and become grubs. They feed on the honey and after a further ecdysis become pupae.
In the Lepidoptera the larva has the well-known form of a caterpillar. The caterpillars have strong jaws, adapted for biting vegetable tissues, which are quite unlike the oral appendages of the adult. They have three pairs of jointed thoracic legs, and a variable number (usually five) of pairs of rudimentary abdominal legs the so-called pro-legs. The larva undergoes numerous ecdyses, and the external parts of the adult such as the wings, etc., are formed underneath the chitinous exoskeleton before the pupa stage. The pupa is known as a chrysalis and in some Lepidoptera is enveloped in a cocoon.
The Hymenoptera present considerable variations in the character of the larvae. In the Aculeata, many Entomophaga, the Cynipidae, etc., the larvae are apodous grubs, incapable of going in search of their food ; but in the Siricidse they are hexapodous forms like caterpillars, which are sometimes even provided with pro-legs. In some of the Entomophaga the larvae have very remarkable characters which have already been described in a special section, 'vide pp. 418, 419.
Before proceeding to the consideration of the value of the various larval forms thus shortly enumerated, it is necessary to say a few words as to the internal changes which take place during the occurrence of the above metamorphosis. In the simplest cases, such as those of the Orthoptera and Hemiptera, where the metamorphosis is confined to the gradual formation of the wings, etc. in a series of moults, the wings first appear as two folds of the epidermis beneath the cuticle on the two posterior thoracic segments. At the next moult these processes
become covered by the freshly formed cuticle, and appear as small projections. At every successive moult these projections become more prominent owing to a growth in the epidermis which has taken place in the preceding interval. Accompanying the formation of such organs as the wings, internal changes necessarily take place in the arrangement of the muscles, etc. of the thorax, which proceed pari passu with the formation of the organs to which they belong. The characters of the metamorphosis in such forms as the Ephemeridae only differ from the above in the fact that provisional organs are thrown off at the same time that the new ones are formed.
In the case of the Holometabola the internal phenomena of the metamorphosis are of a very much more remarkable character. The details of our knowledge are largely due to Weismann (Nos. 430 and 431). The larvae of the Holometabola have for the most part a very different mode of life to the adults. A simple series of transitions between the two is impossible, because intermediate forms would be for the most part incapable of existing. The transition from the larval to the adult state is therefore necessarily a more or less sudden one, and takes place during the quiescent pupa condition. Many of the external adult organs are however formed prior to the pupa stage, but do not become visible on the surface. The simplest mode of Holometabolic metamorphosis may be illustrated by the development of Corethra plumicornis, one of the Tipulidae. This larva, like that of other Tipulidae, is without thoracic appendages, but before the last larval moult, and therefore shortly before the pupa stage, certain structures are formed, which Weismann has called imaginal discs. These imaginal discs are in Corethra simply invaginations of the epidermis. There are in the thorax six pairs of such structures, three dorsal and three ventral. The three ventral are attached to the terminations of the sensory nerves, and the limbs of the imago are formed as simple outgrowths of them, which as they grow in length take a spiral form. In the interior of these outgrowths are formed the muscles, tracheae, etc., of the limbs; which are believed by Weismann (it appears to me without sufficient ground) to be derived from a proliferation of the cells of the neurilemma. The wings are formed from the two posterior dorsal imaginal
discs. The hypodermis of the larva passes directly into that of the imago.
The pupa stage of Corethra is relatively very short, and the changes in the internal parts which take place during it are not considerable. The larval abdominal muscles pass for the most part unchanged into those of the imago, while the special thoracic muscles connected with the wings, etc., develop directly during the latest larval period from cords of cells already formed in the embryo.
In the Lepidoptera the changes in the passage from the larval to the adult state are not very much more considerable than those in Corethra. Similar imaginal discs give rise during the later larval periods to the wings, etc. The internal changes during the longer pupa period are somewhat more considerable. Important modifications and new formations arise in connection with the alimentary tract, the nervous and muscular systems.
The changes which take place in the true flies (Muscidse) are far more complicated than either those in Corethra or in the Lepidoptera. The abdomen of the larva of Musca becomes bodily converted into the abdomen of the imago as in the above types, but the whole epidermis and appendages of the head and thorax are derived from imaginal discs which are formed within and (so far as is known) independently of the epidermis of the larva or embryo. These imaginal discs are simple masses of apparently indifferent cells, which for the most part appear at the close of embryonic life, and are attached to nerves or tracheae. They grow in size during larval life, but during the relatively long pupa stage they unite together to give rise to a continuous epidermis, from which the appendages grow out as processes. The epidermis of the anterior part of the larva is simply thrown off, and has no share in forming the epidermis of the adult.
There are a pair of cephalic imaginal discs and six pairs of thoracic discs. Two pairs, a dorsal and a ventral, give rise to each thoracic ring, and the appendages attached to it.
Though, as mentioned above, no evidence has yet been produced to shew that the imaginal discs of Musca are derived from the embryonic epiblast, yet their mode of growth and
eventual fate proves beyond the shadow of a doubt that they are homologous with the imaginal discs of Corethra. Their earliest origin is well worth further investigation.
The metamorphosis of the internal organs is still more striking than that of the external. There is a disruption, total or partial, of all the internal organs except the generative organs. In the case of the alimentary tract, the Malpighian vessels, the heart and the central nervous system, the disruption is of a partial kind, which has been called by Weismann histolysis. The cells of these organs undergo a fatty degeneration, the nuclei alone in some cases remaining. The kind of plasma resulting from this degeneration retains the shape of the organs, and finally becomes built up again into the corresponding organs of the imago. The tracheae, muscles and peripheral nerves, and an anterior part of the alimentary tract, are entirely disrupted. They seem to be formed again from granular cells derived from the enormous fat body.
The phenomena of the development of the Muscidse are undoubtedly of rather a surprising character. Leaving for the moment the question of the origin of the pupa stage to which I return below, it will be admitted on all hands that during the pupa stage the larva undergoes a series of changes which, had they taken place by slow degrees, would have involved, in such a case as Musca, a complete though gradual renewal of the tissues. Such being the case, the cells of the organs common to the larva and the imago would, in the natural course of things, not be the same cells as those of the larva but descendants of them. We might therefore expect to find in the rapid conversion of the larval organs into those of the adult some condensation, so to speak, of the process of ordinary cell division. Such condensations are probably represented in the histolysis in the case of the internal organs, and in the formation of imaginal discs in the case of the external ones, and I think it probable that further investigation will shew that the imaginal discs of the Muscidae are derivatives of the embryonic epiblast. The above considerations by no means explain the whole of Weismann's interesting observations, but an explanation is I believe to be found by following up these lines.
More or less parallel phenomena to those in Insects are found in the development of the Platyelminthes and Echinoderms. The four disc-like invaginations of the skin in many larval Nemertines (vide p. 198), which give rise to the permanent body wall of the Nemertine, may be compared to the imaginal discs. The subsequent throwing off of the skin of Pilidium or larva of Desor is a phenomenon like the absorption of part of the larval skin of Musca. The formation of an independent skin within the first larval
TR ACHE AT A.
form in the Distomeaeand in the Cestoda may be compared to the apparently independent formation of the imaginal discs in Musca.
The fact that in a majority of instances it is possible to trace an intimate connection between the surroundings of a larva and its organization proves in the clearest way that the characters of the majority of existing larval forms of Insects have owed their origin to secondary adaptations. A few instances will illustrate this point.
In the simplest types of metamorphosis, e.g. those of the Orthoptera genuina, the larva has precisely the same habits as the adult. We find that a caterpillar form is assumed by phytophagous larvae amongst the Lepidoptera, Hymenoptera and Coleoptera. Where the larva has not to go in search of its nutriment the grub-like apodous form is assumed. The existence of such an apodous larva is especially striking in the Hymenoptera, in that rudiments of thoracic and abdominal appendages are present in the embryo and disappear again in the larva. The case of the larva of Sitaris, already described (p. 421), affords another very striking proof that the organization of the larva is adapted to its habits.
It follows from the above that the development of such forms as the Orthoptera genuina is more primitive than that of the holometabolous forms; a conclusion which tallies with the fact
HALF OF CAMPODEA FRAGILIS. (From Gegenbaur; after Palmen.)
a. antennae ; p. feet ; j> ', post-tho feet; s.
that both palaeontological and anatomical evidence shew the Orthoptera to be a very primitive group of Insects.
The above argument probably applies with still greater force to the case of the Thysanura ; and it seems to be probable that this group is more nearly related than any other to the primitive wingless ancestors of Insects 1 . The characters of the oral
1 Brauer and Lubbock (No. 421) have pointed out the primitive characters of these forms, especially of Campodea.
appendages in this group, the simplicity of their metamorphosis, and the presence of abdominal appendages (fig. 192), all tell in favour of this view, while the resemblance of the adult to the larvae of the Pseudoneuroptera, etc., points in the same direction. The Thysanura and Collembola are not however to be regarded as belonging to the true stock of the ancestors of Insects, but as degenerated relations of this stock ; much as Amphioxus and the Ascidians are degenerate relations of the ancestral stock of Vertebrates, and Peripatus of that of the Tracheata. It is probable that all these forms have succeeded in retaining their primitive characters from their degenerate habits, which prevented them from entering into competition in the struggle for existence with their more highly endowed relatives. While in a general way it is clear that the larval forms of Insects cannot be expected to throw much light on the nature of Insect ancestors, it does nevertheless appear to me probable that such forms as the caterpillars of the Lepidoptera are not without a meaning in this respect. It is easy to conceive that even a secondary larval form may have been produced by the prolongation of one of the embryonic stages ; and the general similarity of a caterpillar to Peripatus, and the retention by it of post-thoracic appendages, are facts which appear to favour this view of the origin of the caterpillar form.
The two most obscure points which still remain to be dealt with in the metamorphosis of Insects are (i) the origin of the quiescent pupa stage ; (2) the frequent dissimilarity between the masticatory apparatus of the larva and adult.
These two points may be conveniently dealt with together, and some valuable remarks about them will be found in Lubbock (No. 420).
On grounds already indicated it may be considered certain that the groups of Insects without a pupa stage, and with a larva very similarly organised to the adult, preceded the existing holometabolic groups. The starting-point in the metamorphosis of the latter groups was therefore something like that of the Orthoptera. Suppose it became an advantage to a species that the larva and adult should feed in a somewhat different way, a difference in the character of their mouth parts would soon make itself manifest ; and, since an intermediate type of mouth parts
would probably be disadvantageous, there would be a tendency to concentrate into a single moult the transition from the larval to the adult form of mouth parts. At each ordinary moult there is a short period of quiescence, and this period of quiescence would naturally become longer in the important moult at which the change in the mouth parts was effected. In this way a rudimentary pupa stage might be started. The pupa stage, once started, might easily become a more important factor in the metamorphosis. If the larva and imago diverged still more from each other, a continually increasing amount of change would have to be effected at the pupa stage. It would probably be advantageous to the species that the larva should not have rudimentary functionless wings ; and the establishment of the wings as external organs would therefore become deferred to the pupa stage. The same would probably apply to other organs.
Insects usually pass through the pupa stage in winter in cold climates and during the dry season in the tropics, this stage serving therefore apparently for the protection of the species during the inclement season of the year. These facts are easily explained on the supposition that the pupa stage has become secondarily adapted to play a part in the economy of the species quite different from that to which it owes its origin.
Heterogamy. The cases of alternations of generations amongst Insects all fall under the heading already defined in the introduction as Heterogamy. Heterogamy amongst Insects has been rendered possible by the existence of parthenogenesis, which, as stated in the introduction, has been taken hold of by natural selection, and has led to the production of generations of parthenogenetic forms, by which a clear economy in reproduction is effected. Parthenogenesis without heterogamy occurs in a large number of forms. In Bees, Wasps, and a Sawfly (Nematus ventricosus) the unfertilized ova give rise to males. In two Lepidopterous genera (Psyche and Solenobia) the unfertilized ova give rise mainly, if not entirely, to females. Heterogamy occurs in none of the above types, but in Psyche and Solenobia males are only occasionally found, so that a series of generations producing female young from unfertilized ova are followed by a generation producing young of both sexes from fertilized ova. It
would be interesting to know if the unimpregnated female would not after a certain number of generations give rise to both males and females ; such an occurrence might be anticipated on grounds of analogy. In the cases of true heterogamy parthenogenesis has become confined to special generations, which differ in their character from the generations which reproduce themselves sexually. The parthenogenetic generations generally flourish during the season when food is abundant; while the sexual generations occur at intervals which are often secondarily regulated by the season, supply of food, etc.
A very simple case of this kind occurs, if we may trust the recent researches of Lichtenstein 1 , in certain Gall Insects (Cynipidae). He finds that the female of a form known as Spathegaster baccarum, of which both males and females are plentiful, pricks a characteristic gall in certain leaves, in which she deposits the fertilized eggs. The eggs from these galls give rise to a winged and apparently adult form, which is not, however, Spathegaster, but is a species considered to belong to a distinct genus known as Neuroterus ventricularis. Only females of Neuroterus are found, and they lay unfertilized ova in peculiar galls which develop into Spathegaster baccarum. Here we have a true case of heterogamy, the females which produce parthenogenetically having become differentiated from those which produce sexually. Another interesting type of heterogamy is that which has been long known in the Aphides. In the autumn impregnated eggs are deposited by females, which give rise in the course of the spring to females which produce parthenogenetically and viviparously. The viviparous females always differ from the females which lay the fertilized eggs. The generative organs are of course differently constituted, and the ova of the viviparous females are much smaller than those of the oviparous females, as is generally the case in closely allied viviparous and oviparous forms; but in addition the former are usually without wings, while the latter are winged. The reverse is however occasionally the case. An indefinite number of generations of viviparous females may be produced if they are artificially kept warm and supplied with food ; but in the course of
1 Petites Nouvelles Entomolog iyues, May, 1878.
nature the viviparous females produce in the autumn males and females which lay eggs with firm shells, and so preserve the species through the winter. The heterogamy of the allied Coccidae is practically the same as that of the Aphidae. In the case of Chermes and Phylloxera the parthenogenetic generations lay their eggs in the normal way.
The complete history of Phylloxera quercus has been worked out by Balbiani (No. 401). The apterous females during the summer lay eggs developing parthenogenetically into apterous females, which continue the same mode of reproduction. In the autumn, however, the eggs which are laid give rise in part to winged forms and in part to apterous forms. Both of these forms lay smaller and larger eggs, which develop respectively into very minute males and females without digestive organs. The fertilized eggs laid by these forms probably give rise to the parthenogenetic females.
A remarkable case of heterogamy accompanied by paedogenesis was discovered by Wagner to take place in certain species of Cecydomyia (Miastor), a genus of the Diptera. The female lays a few eggs in the bark of trees, etc. These eggs develop in the winter into larvae, in which ovaries are early formed. The ova become detached into the body cavity, surrounded by their follicles, and grow at the cost of the follicles. They soon commence to undergo a true development, and on becoming hatched they remain for some time in the body cavity of the parent, and are nourished at the expense of its viscera. They finally leave the empty skin of their parent, and subsequently reproduce a fresh batch of larvae in the same way. After several generations the larvae undergo in the following summer a metamorphosis, and develop into the sexual form.
Another case of paedogenesis is that of the larvae of Chironomus, which have been shewn by Grimm (No. 413) to lay eggs which develop exactly in the same way as fertilized eggs into larvae.
(401) M. Balbiani. " Observations s. la reproduction d. Phylloxera du Chene." An. Sc. Nat. Ser. v. Vol. xix. 1874.
(402) E. Bess els. " Studien u. d. Entwicklung d. Sexualdriisen bei den Lepidoptera." Ztit.f. wiss. Zool. Bd. xvii. 1867.
(403) Alex. Brandt. "Beitrage zur Entwicklungsgeschichte d. Libellulida u. Hemiptera, mil besonderer Berucksichtigung d. Embryonalhiillen derselben." Mem. Ac. Petersbourg, Ser. vn. Vol. xm. 1869.
(404) Alex. Brandt. Ueber das Ei u. seine Bildungsstdttt. Leipzig, 1878.
(405) O. Biitschli. "Zur Entwicklungsgeschichte d. Biene." Zeit. f. wiss. Zool. Bd. xx. 1870.
(406) H. Dewitz. "Bau u. Entwicklung d. Stachels, etc." Zeit.f. wiss. Zool. Vols. xxv. and xxvin. 1875 and 1877.
(407) H. Dewitz. "Beitrage zur Kenntniss d. Postembryonalentwicklung d. Gliedmassen bei den Insecten." Zeit.f. wiss. Zool. xxx. Supplement. 1878.
(408) A. Dohrn. "Notizen zur Kenntniss d. Insectenentwicklung." Zeitschrift f. wiss. Zool. Bd. xxvi. 1876.
(409) M. Fabre. " L'hypermetamorphose et lesmoeursdes Meloides." An.Sci. Nat. Series iv. Vol. vn. 1857.
(410) Ganin. " Beitrage zur Erkenntniss d. Entwicklungsgeschichte d. Insecten." Zeit.f. wiss. Zool. Bd. xix. 1869.
(411) V. Graber. Die Insecten. MUnchen, 1877.
(412) V. Graber. "Vorlauf. Ergeb. lib. vergl. Embryologie d. Insecten." Archivf. mikr. Anat. Vol. XV. 1878.
(413) O. v. Grimm. " Ungeschlechtliche Fortpflanzung einer Chironomus Art-u. deren Entwicklung aus dem unbefruchteten Ei." Mem. Acad. Petersbourg. 1870.
(414) B. Hatschek. " Beitrage zur Entwicklung d. Lepidopteren." Jenaische Zeitschrift, Bd. XI.
(415) A. K 6 1 1 i k e r. " Observationes de prima insectorum genese, etc. " Ann. Sc. Nat. Vol. xx. 1843.
(416) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden." Mem. Ac. imp. Petersbourg, Ser. vn. Vol. xvi. 1871.
(417) C. Kraepelin. 4 ' Untersuchungen Ub. d. Bau, Mechanismus u. d. Entwick. des Stachels d. bienartigen Thiere." Zeit.f. wiss. Zool. Vol. xxni. 1873.
(418) C. Kupffer. "Faltenblatt an d. Embryonen d. Gattung Chironomus." Arch.f. mikr. Anat. Vol. u. 1866.
(419) R. Leuckart. Zur Kenntniss d. Generationswechsels u. d. Parthenogenese b. d. Insecten. Frankfurt, 1858.
(420) Lubbock. Origin and Metamorphosis of Insects. 1874.
(421) Lubbock. Monograph on Collembola and Thysanura. Ray Society, 1873.
(422) Melnikow. " Beitrage z. Embryonalentwicklung d. Insecten." Archiv f. Naturgeschichte, Bd. xxxv. 1869.
(423) E. Metschnikoff. "Embryologische Studien an Insecten." Zeit. f. wiss. Zool. Bd. xvi. 1866.
(424) P. Meyer. "Ontogenie und Phylogenie d. Insecten." Jenaische Zeitschrift, Vol. x. 1876.
(425) FritzMiiller. " Beitrage z. Kenntniss d. Termiten." Jenaische Zeitschrift, Vol. IX. 1875.
(426) A. S. Packard. " Embryological Studies on Diplex, Perithemis, and the Thysanurous genus Isotoma." Mem. Peabody Acad. Science, I. i. 1871.
(427) Suckow. " Geschlechtsorgane d. Insecten." Ileusinger's Zeitschrift f. organ. Physik, Bd. n. 1828.
(428) Tichomiroff. " Ueber die Entwicklungsgeschichte des Seidenwiirms." Zoologischer Anzeiger, n. Jahr. No. 20 (Preliminary Notice).
(429) Aug. Weismann. "Zur Embryologie d. Insecten." Archiv f. Anat. und Phys. 1864.
(430) Aug. Weismann. " Entwicklung d. Dipteren." Zeit. f. wiss. Zool. Vols. xin. and xiv. Leipzig, 1863 4.
(431) Aug. Weismann. " Die Metamorphose d. Corethra plumicornis. " Zeit. f. wiss. Zool. Vol. xvi. 1866.
(432) N. Wagner. "Beitrag z. Lehre d. Fortpflanzung d. Insectenlarven." Zeit.f. wiss. Zool. Vol. xin. 1860.
(433) Zaddach. Untersuchungen iib. d. Bau u. d. Entwicklungd. Gliederthiere. Berlin, 1854.
ARACHNIDA 1 .
The development of several divisions of this interesting group has been worked out ; and it will be convenient to deal in the first instance with the special history of each of these divisions, and then to treat in a separate section the development of the organs for the whole group.
Scorpionidae. The embryonic development always takes place within the female Scorpion. In Buthus it takes place within follicle-like protuberances of the wall of the ovary. In Scorpio also development commences while the egg is still in the follicle, but when the trunk becomes segmented the embryo passes into the ovarian tube. The chief authority for the development of the Scorpionidae is Metschnikoff (No. 434).
At the pole of the ovum facing the ovarian tube there is
FIG. 193. OVUM OF SCORPION WITH THE ALREADY -FORMED BLASTODERM SHEWING THE PARTIAL SEGMENTATION. (After Metschnikoff.)
1 The classification of the Arachnida adopted in the present work is shewn below. c Scorpionidse. . . ( Tetrapneumones.
Pedipalpi. IL Aranema - JDipneumones.
I. ArthrOgastra. \ Pseudoscorpionidae.
I Soiifugse. in. Acarina,
formed a germinal disc which undergoes a partial segmentation (fig. 193 bl). A somewhat saucer-shaped one-layered blastoderm is then formed, which soon becomes thickened in the centre and then divided into two layers. The outer of these is the epiblast, the inner the mesoblast. Beneath the mesoblast there subsequently appear granular cells, which form the commencement of the hypoblast 1 .
During the formation of the blastoderm a cellular envelope is formed round the embryo. Its origin is doubtful, though it is regarded by Metschnikoff as probably derived from the blastoderm and homologous with the amnion of Insects. It becomes double in the later stages (fig. 195).
During the differentiation of the three embryonic layers the germinal disc becomes somewhat pyriform, the pointed end being the posterior. At this extremity there is a special thickening which is perhaps equivalent to the primitive cumulus of Spiders. The germinal disc continues gradually to spread over the yolk, but the original pyriform area is thicker than the remainder, and is marked off anteriorly and posteriorly by a shallow furrow. It constitutes a structure corresponding with the ventral plate of other Tracheata. It soon becomes grooved by a FIG. 194. THREE SURFACE VIEWS OF THE
. A ,. , f VENTRAL PLATE OF A DEVELOPING SCORPION.
shallow longitudinal fur- (After Metschnikoff.)
A. Before segmentation.
B. After five segments have become formed.
C. After the appendages have begun to be
row (fig. 194 A) which subsequently becomes less distinct. It is then divided by two transverse lines into three parts 2 .
1 The origin of the hypoblast cells, if such these cells are, is obscure. Metschnikoff doubtfully derives them from the blastoderm cells ; from my investigations on Spiders it appears to me more probable that they originate in the yolk.
- The exact fate of the three original segments is left somewhat obscure by
In succeeding stages the anterior of the three parts is clearly marked out as the procephalic lobe, and soon becomes somewhat broader. Fresh segments are added from before backwards, and the whole ventral plate increases rapidly in length (fig. 194 B).
When ten segments have become formed, appendages appear as paired outgrowths of the nine posterior segments (fig. 194 C). The second segment bears the pedipalpi, the four succeeding segments the four ambulatory appendages, and the four hindermost segments smaller provisional appendages which subsequently disappear, with the possible exception of the second. The foremost segment, immediately behind the procephalic lobes, is very small, and still without a rudiment of the chelicerae, which are subsequently formed on it. It would appear from Metschnikoff's figures to be developed later than the other post-oral segments present at this stage. The still unsegmented tail has become very prominent and makes an angle of 180 with the remainder of the body, over the ventral surface of which it is flexed.
By the time that twelve segments are definitely formed, the procephalic region is distinctly bilobed, and in the median groove extending along it the stomodaeum has become formed (fig. 196 A). The chelicerae (ck) appear as small rudiments on the first post-oral segment, and the
FlG. 195. A FAIRLY-ADVANCED EMBRYO OF THE SCORPION ENVELOPED IN
ITS MEMBRANES. (After Metschnikoff. )
ch. chelicerae ; pd. pedipalpi ; p^p 4 . ambulatory appendages ; al>. post-abdomen (tail).
Metschnikoff. He believes however that the anterior segment forms the procephalic lobes, the posterior probably the telson and five adjoining caudal segments, and the middle one the remainder of the body. This view does not appear to me quite satisfactory, since on the analogy of Spiders and other Arthropoda the fresh somites ought to be added by a continuous segmentation of the posterior lobe.
B. II. 28
434 1 SEUDOSCORPIONID^E.
nerve cords are distinctly differentiated and ganglionated. In the embryonic state there is one ganglion for each segment. The ganglion in the first segment (that bearing the chelicerse) is very small, but is undoubtedly post-oral.
At this stage, by a growth in which all the three germinal layers have a share, the yolk is completely closed in by the blastoderm. It is a remarkable fact with only few parallels, and those amongst the Arthropoda, that the blastopore, or point where the embryonic membranes meet in closing in the yolk, is situated on the dorsal surface of the embryo.
The general relations of the embryo at about this stage are shewn in fig. 195, where the embryo enclosed in its double cellular membrane is seen in a side view. This embryo is about the same age as that seen from the ventral surface in fig. 196 A.
The general nature of the further changes may easily be gathered from an inspection of fig. 196 B and C, but a few points may be noted.
An upper lip or labrum is formed as an unpaired organ in the line between the procephalic lobes. The pedipalpi become chelate before becoming jointed, and the chelicerae also early acquire their characteristic form. Rudimentary appendages appear on the six segments behind the ambulatory legs, five of which are distinctly shewn in fig. 195 ; they persist only on the second segment, where they appear to form the comb-like organs or pectines. The last abdominal segment, Le. that next the tail, is without provisional appendages. The embryonic tail is divided into six segments including the telson (fig. 196 C, ab). The lungs (st) are formed by paired invaginations, the walls of which subsequently become plicated, on the four last segments which bear rudimentary limbs, and simultaneously with the disappearance of the rudimentary limbs.
PseudoscorpionidaB. The development of Qielifer has been investigated by Metschnikoff (436), and although (except that it is provided with tracheae instead of pulmonary sacks) it might be supposed to be closely related to Scorpio, yet in its development is strikingly different.
The eggs after being laid are carried by the female attached to the first segment of the abdomen. The segmentation (vide p. 93) is intermediate between the types of complete and superficial segmentation. The ovum, mainly formed of food-yolk, divides into two, four, and eight equal segments
(fig. 197 A). There then appear one or more clear segments on the surface of these, and finally a complete layer of cells is formed round the central yolk spheres (fig. 197 B), which latter subsequently agglomerate into a central mass. The superficial cells form what may be called a blastoderm, which soon becomes divided into two layers (fig. 197 C). There now appears a single pair of appendages (the pedipalpi) (fig. 198 A,/^/), while at the same time the front end of the embryo grows out into a remarkable proboscis-like prominence a temporary upper lip (concealed in the figure
FIG. 196. THREE STAGES IN THE DEVELOPMENT OF THE SCORPION. THE
EMBRYOS ARE REPRESENTED AS IF SEEN EXTENDED ON A PLANE.
ch. chelicerae ; pd. pedipalpi ; p l />*. ambulatory appendages ; pe. pecten ; st. stigmata ; ab. post abdomen (tail).
behind the pedipalpus), and the abdomen (ab) becomes bent forwards towards the ventral surface. In this very rudimentary condition, after undergoing an ecdysis, the larva is hatched, although it still remains attached to its parent. After hatching it grows rapidly, and becomes filled with a peculiar transparent material. The first pair of ambulatory appendages is formed behind the pedipalpi and then the three suceeding pairs, while at the same time the chelicerae appear as small rudiments in front. External signs of segmentation have not yet appeared, but about this period the nervous system is formed. The supra-cesophageal ganglia are especially distinct, and provided with a central cavity, probably formed by an invagination, as in other Arachnida. In the succeeding stages (fig. 198 B) four provisional
pairs of appendages (shewn as small knobs at ati] appear behind the ambulatory feet. The abdomen is bent forwards so as to reach almost to the pedipalpi. In the later stages (fig. 198 C) the adult form is gradually attained. The enormous upper lip persists for some time, but subsequently atrophies and is replaced by a normal labrum. The appendages behind the
FIG. igj. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER.
In A the ovum is divided into a number of separate segments. In B a number of small cells have appeared (bl) which form a blastoderm enveloping the large yolk spheres. In C the blastoderm has become divided into two layers.
ambulatory feet atrophy, and the tail is gradually bent back into its final position. The segmentation and the gradual growth of the limbs do not call for special description, and the formation of the organs, so far as is known, agrees with other types.
The segmentation of Chthonius is apparently similar to that of Chelifer (Stecker, No. 437).
Phalangidae. Our knowledge of the development of Phalangium is unfortunately confined to the later stages (Balbiani, No. 438). These stages do not appear however to differ very greatly from those of true Spiders.
Araneina. The eggs of true Spiders are either deposited in nests made specially for them, or are carried about by the females. Species belonging to a considerable number of genera, viz. Pholcus, Epeira, Lycosa, Clubione, Tegenaria and Agelcna
have been studied by Claparede (No. 442), Balbiani (No. 439), Barrois (No. 441) and myself (No. 440), and the close similarity between their embryos leaves but little doubt that there are no great variations in development within the group.
The ovum is enclosed in a delicate vitelline membrane, enveloped in its turn by a chorion secreted by the walls of the oviduct. The chorion is covered by numerous rounded prominences, and occasionally exhibits a pattern corresponding with the areas of the cells which formed it. The segmentation has already been fully described, pp. 1 18 and 1 19. At its close there is present an enveloping blastoderm formed of a single layer of large flattened cells. The yolk within is formed of a number of
' r .v-ii~-cr^ ^H 1
FIG. 198. THREE STAGES IN THE DEVELOPMENT OF CHELIFKR.
(After Metschnikoff.) pd. pedipalpi ; ab. abdomen ; an.i. anal invagination ; c/i. chelicerse.
large polygonal segments ; each of which is composed of large yolk spherules, and contains a nucleus surrounded by a layer of protoplasm, which is prolonged into stellate processes holding together the yolk spherules. The nucleus, surrounded by the major part of the protoplasm of each yolk cell, appears, as a rule,
to be situated not at the centre, but on one side of its yolk segment.
The further description of the development of Spiders applies more especially to Agelena labyrinthica, the species which formed the subject of my own investigations.
The first differentiation of the blastoderm consists in the cells of nearly the whole of one hemisphere becoming somewhat more columnar than those of the other hemisphere, and in the cells of a small area near one end of the thickened hemisphere becoming distinctly more columnar than elsewhere, and two layers thick. This area forms a protuberance on the surface of the ovum, originally discovered by Claparede, and called by him the primitive cumulus. In the next stage the cells of the thickened hemisphere of the blastoderm become still more columnar; and a second area, at first connected by a whitish streak with the cumulus, makes its appearance. In the second area the blastoderm is also more than one cell deep (fig. 199). It will be noticed that the blastoderm, though more than one cell thick over a large part of the ventral surface, is not divided into distinct layers. The second area appears as a white patch and soon becomes more distinct, while the streak continued to it from the cumulus is no longer visible. It is shewn in surface view in fig. 200 A. Though my observations on this stage are not quite satisfactory, yet it appears to me probable that there is a longitudinal thickened ridge of the blastoderm extending from the primitive cumulus to the large white area. The section represented in fig. 199, which I believe to be oblique, passes through this ridge at its most projecting part.
The nuclei of the yolk cells during the above stages multiply rapidly, and cells are formed in the yolk which join the blastoderm ; there can however be no doubt that the main increase in the cells of the blastoderm has been due to the division of the original blastoderm cells.
In the next stage I have been able to observe there is, in the place of the previous thickened half of the blastoderm, a well developed ventral plate with a procephalic lobe in front, a caudal lobe behind, and an intermediate region marked by about three transverse grooves, indicating a division into segments. This plate is throughout two or more rows of
FIG. 199. SECTION THROUGH THE EMBRYO OF AGELENA LABYRINTHICA.
The section is from an embryo of the same age as fig. 200 A, and is represented with the ventral plate upwards. In the ventral plate is seen a keel-like thickening, which gives rise to the main mass of the mesoblast.
yk. yolk divided into large polygonal cells, in several of which nuclei are shewn.
cells thick, and the cells which form it are divided into two distinct layers a columnar superficial layer of epiblast cells, and a deeper layer of mesoblast cells (fig. 203 A). In the latter layer there are several very large cells which are in the act of passing from the yolk into the blastoderm. The identification of the structures visible in the previous stage with those visible in the present stage is to a great extent a matter of guess-work, but it appears to me probable that the primitive cumulus is still present as a slight prominence visible in surface views on the caudal lobe, and that the other thickened patch persists as the procephalic lobe. However this may be, the significance of the primitive cumulus appears to be that it is the part of the blastoderm where two rows of cells become first established \
The whole region of the blastoderm other than the ventral plate is formed of a single row of flattened epiblast cells. The yolk retains its original constitution.
By this stage the epiblast and mesoblast are distinctly differentiated, and the homologue of the hypoblast is to be sought for in the yolk-cells. The yolk-cells are not however entirely hypoblastic, since they continue for the greater part of the development to give rise to fresh cells which join the mesoblast.
The Spider's blastoderm now resembles that of an Insect (except for the amnion) after the establishment of the mesoblast, and the mode of origin of the mesoblast in both groups is very similar, in that the longitudinal ridge-like thickening of the
1 Various views have been put forward by Claparfede and Balbiani about the position and significance of the primitive cumulus. For a discussion of which vide self, No. 440.
mesoblast shewn in fig. 199 is probably the homologue of the mesoblastic groove of the Insects' blastoderm.
The ventral plate continues to grow rapidly, and at a somewhat later stage (fig. 200 B) there are six segments interposed between the procephalic and caudal lobes. The two anterior of these (ch and pd), especially the foremost, are less distinct than the remainder ; and it is probable that both of them, and in any case the anterior one, are formed later than the three segments following. These two segments are the segments of the chelicenc and pedipalpi. The four segments following belong to the four pairs of ambulatory legs. The segments form raised transverse bands separated by transverse grooves. There is at this stage a faintly marked groove extending along the median line of the ventral plate. This groove is mainly caused by the originally single mesoblastic plate having become divided throughout the whole region of the ventral plate, except possibly the procephalic lobes, into two bands, one on each side of the middle line (fig. 203 B).
The segments continue to increase in number by the continuous addition of fresh segments between the one last formed and the caudal lobe. By the stage with nine segments the first rudiments of the limbs make their appearance. The first rudiments to appear are those of the pedipalpi and four ambulatory limbs : the chelicerae, like the segment to which they belong, lag behind in development. The limbs appear as small protuberances at the borders of their segments. By the stage when they are formed the procephalic region has become bilobed, and the two lobes of which it is composed are separated by a shallow groove.
By a continuous elongation the ventral plate comes to form a nearly complete equatorial ring round the ovum, the procephalic and caudal lobes being only separated by a very narrow space, the undeveloped dorsal region of the embryo. This is shewn in longitudinal section in fig. 204. In this condition the embryo may be spoken of as having a dorsal flexure. By the time that this stage is reached (fig. 200 C) the full number of segments and appendages has become established. There are in all sixteen segments (including the caudal lobe). The first six of these bear the permanent appendages of the adult ; the
next four are provided with provisional appendages ; while the last six are without appendages. The further features of this stage which deserve notice are (i) the appearance of a shallow depression (st) the rudiment of the stomodaeum between the hinder part of the two procephalic lobes ; (2) the appearance of
FIG. aoo. FOUR STAGES IN THE DEVELOPMENT OF AGELENA LABYRINTHICA.
A. Stage when the ventral plate is very imperfectly differentiated, pr.c. primitive cumulus.
B. Ovum viewed from the side when the ventral plate has become divided into six segments, ch. segment of chelicerae imperfectly separated from procephalic lobe ; pd. segment of pedipalpi.
C. Ventral plate ideally unrolled after the full number of segments and appendages are established, st. stomodoeum between the two proe-oral lobes. Behind the six pairs of permanent appendages are seen four pairs of provisional appendages.
D and E. Two views of an embryo at the same stage. D ideally unrolled, E seen from the side. st. stomodseum ; ch. chelicerse ; on their inner side is seen the ganglion belonging to them. pd. pedipalpi ; pr.p. provisional appendages.
raised areas on the inner side of the six anterior appendagebearing segments. These are the rudiments of the ventral ganglia. It deserves to be especially noted that the segment of
44 2 AKANEINA.
the chelicera, like the succeeding segments, is provided with ganglia ; and that the ganglia of the chelicerae are quite distinct from the supra-cesophageal ganglia derived from the procephalic lobes. (3) The pointed form of the caudal lobe. In Pholcus (Claparede, No. 442) the caudal lobe forms a projecting structure which, like the caudal lobe of the Scorpion, bends forward so as to face the ventral surface of the part of the body immediately in front. In most Spiders such a projecting caudal lobe is not found. While the embryo still retains its dorsal flexure considerable changes are effected in its general constitution. The appendages (fig. 200 D and E) become imperfectly jointed, and grow inwards so as to approach each other in the middle line. Even in the stage before this, the ventral integument between the rudiments of the ganglia had become very much thinner, and had in this way divided the ventral plate into two halves. At the present stage the two halves of the ventral plate are still further separated, and there is a wide space on the ventral side only covered by a delicate layer of epiblast. This is shewn in surface view (fig. 200 D) and in section in fig. 203 C.
The stomodaeum (j/) is much more conspicuous, and is bounded in front by a prominent upper lip, and by a less marked lip behind. The upper lip becomes less conspicuous in later stages, and is perhaps to be compared with the provisional upper lip of Chelifer. Each procephalic lobe is now marked by a deep semicircular groove.
The next period in the development is characterised by the gradual change in the flexure of the embryo from a dorsal to a ventral one ; accompanied by the division of the body into an abdomen and cephalo-thorax, and the gradual assumption of the adult characters.
The change in the flexure of the embryo is caused by the elongation of the dorsal region, which has hitherto been hardly developed. Such an elongation increases the space on the dorsal surface between the procephalic and caudal regions, and therefore necessarily separates the caudal and procephalic lobes ; but, since the ventral plate does not become shortened in the process, and the embryo cannot straighten itself in the egg-shell, it necessarily becomes ventrally flexed.
If there were but little food yolk this flexure would naturally
cause the whole embryo to be bent in so as to have the ventral surface concave. But instead of this the flexure is at first confined to the two bands which form the ventral plate. These bands, as shewn in fig. 201 A, acquire a true ventral flexure, but the yolk forms a projection a kind of yolk sack as Barrois (No. 441) calls it distending the thin integument between the two ventral bands. This yolk sack is shewn in surface view in
FlG. 201. TWO LATE STAGES IN THE DEVELOPMENT OF AGELENA LABYRINTHICA.
A. Embryo from the side at the stage when there is a large ventral protuberance of yolk. The angle between the line of insertion of the permanent and provisional appendages shews the extent of the ventral flexure.
B. Embryo nearly ready to be hatched. The abdomen which has not quite acquired its permanent form is seen to be pressed against the ventral side of the thorax.
prJ. procephalic lobe; pd. pedipalpi ; ch. chelicerae ; c,L caudal lobe; pr.p. provisional appendages.
fig. 20 1 A and in section in fig. 206. At a later period, when the yolk has become largely absorbed, the true nature of the ventral flexure becomes quite obvious, since the abdomen of the young Spider, while still in the egg, is found to be bent over so as to press against the ventral surface of the thorax (fig. 201 B). The general character of the changes which take place during this period in the development is shewn in fig. 201 A and B representing two stages in it. In the first of these stages there is no constriction between the future thorax and abdomen.
The four pairs of provisional appendages exhibit no signs of atrophy ; and the extent of the ventral flexure is shewn by the angle formed between the line of their insertion and that of the appendages in front. The yolk has enormously distended the integument between the two halves of the ventral plate, as is illustrated by the fact that, at a somewhat earlier stage than that figured, the limbs cross each other in the median ventral line, while at this stage they do not nearly meet The limbs have acquired their full complement of joints, and the pedipalpi bear a cutting blade on their basal joint.
The dorsal surface between the prominent caudal lobe and the procephalic lobes forms more than a semicircle. The terga are fully established, and the boundaries between them, especially in the abdomen, are indicated by transverse markings. A large lower lip now bounds the stomodaeum, and the upper lip has somewhat atrophied. In the later stage (fig. 201 B) the greater part of the yolk has passed into the abdomen, which is now to some extent constricted off from the cephalo-thorax. The appendages of the four anterior abdominal somites have disappeared, and the caudal lobe has become very small. In front of it are placed two pairs of spinning mammillae. A delicate cuticle has become established, which is very soon moulted.
Acarina. The development of the Acarina, which has been mainly investigated by Claparede (No. 446), is chiefly remarkable from the frequent occurrence of several larval forms following each other after successive ecdyses. The segmentation (vide p. 116) ends in the formation of a blastoderm of a single layer of cells enclosing a central yolk mass.
A ventral plate is soon formed as a thickening of the blastoderm, in which an indistinct segmentation becomes early observable. In Myobia, which is parasitic on the common mouse, the ventral plate becomes divided by five constrictions into six segments (fig. 202 A), from the five anterior of which paired appendages very soon grow out (fig. 202 B) The appendages are the chelicerae (ch} and pedipalpi (pd] and the first three pairs of limbs (p^fi 1 }. On the dorsal side of the chelicerae a thickened prominence of the ventral plate appears to correspond to the procephalic lobes of other Arachnida. The part of the body behind the five primitive appendage-bearing segments appears to become divided into at least two segments. In other mites the same appendages are formed as in Myobia, but the preceding segmentation of the ventral plate is not always very obvious.
In Myobia two moultings take place while the embryo is still within the primitive egg-shell. The first of these is accompanied by the apparently total disappearance of the three pediform appendages, and the complete
TRACK EAT A.
coalescence of the two gnathiform appendages into a proboscis (fig. 202 C). The feet next grow out again, and a second ecdysis then takes place. The embryo becomes thus inclosed within three successive membranes, viz. the original egg-shell and two cuticular membranes (fig. 202 D). After the second ecdysis the appendages assume their final form, and the embryo leaves the egg as an hexapodous larva. The fourth pair of appendages is
FIG. 202. FOUR SUCCESSIVE STAGES IN THE DEVELOPMENT OF MYOBIA MUSCULI. (After Claparede.)
J 1 j 4 . post-oral segments ; ch. chelicerae ; pd. pedipalpi ; pr. proboscis formed by the coalescence of the chelicerse and pedipalpi ; p l , /*, etc. ambulatory appendages.
acquired by a post-embryonic metamorphosis. From the proboscis are formed the rudimentary palpi of the second pair of appendages, and two elongated needles representing the chelicerae.
In the cheese mite (Tyroglyphus) the embryo has two ecdyses which are not accompanied by the peculiar changes observable in Myobia : the cheliceras and pedipalpi fuse however to form the proboscis. The first larval form is hexapodous, and the last pair of appendages is formed at a subsequent ecdysis.
In Atax Bonzi, a form parasitic on Unio, the development and metamorphosis are even more complicated than in Myobia. The first ecdysis occurs before the formation of the limbs, and shortly after the ventral plate has become divided into segments. Within the cuticular membrane resulting from the first ecdysis the anterior five pairs of limbs spring out in the usual fashion. They undergo considerable differentiation ; the chelicerae and pedipalpi approaching each other at the anterior extremity of the body, and the three ambulatory legs becoming segmented and clawed. An oesophagus, a stomach, and an oesophageal nerve-ring are also formed. When the larva
has attained this stage the original egg-shell is split into two valves and eventually cast off, but the embryo remains enclosed within the cuticular membrane shed at the first ecdysis. This cuticular membrane is spoken of by Claparede as the deutovum. In the deutovum the embryo undergoes further changes ; the chelicerae and pedipalpi coalesce and form the proboscis ; a spacious body cavity with blood corpuscles appears ; and the alimentary canal enclosing the yolk is formed.
The larva now begins to move, the cuticular membrane enclosing it is ruptured, and the larva becomes free. It does not long remain active, but soon bores its way into the gills of its host, undergoes a fresh moult, and becomes quiescent. The cuticular membrane of the moult just effected swells up by the absorption of water and becomes spherical. Peculiar changes take place in the tissues, and the limbs become, as in Myobia, nearly absorbed, remaining however as small knobs. The larva swims about as a spherical body within its shell. The feet next grow out afresh, and the posterior pair is added. From the proboscis the palpi (of the pedipalpi) grow out below. The larva again becomes free, and amongst other changes the chelicerae grow out from the proboscis. A further ecdysis, with a period of quiescence, intervenes between this second larval form and the adult state.
The changes in the appendages which appear common to the Mites generally are (i) the late development of the fourth pair of appendages, which results in the constant occurrence of an hexapodous larva ; and (2) the early fusion of the chelicerae and pedipalpi to form a proboscis in which no trace of the original appendages can be discerned. In most instances palpi and stilets of variable form are subsequently developed in connexion with the proboscis, and, as indicated in the above descriptions, are assumed to correspond with the two original embryonic appendages.
TJie history of tJie germinal layers.
It is a somewhat remarkable fact that each of the groups of the Arachnida so far studied has a different form of segmentation. The types of Chelifer and the Spiders are simple modifications of the centrolecithal type, while that of Scorpio, though apparently meroblastic, is probably to be regarded in the same light (vide p. 120 and p. 434). The early development begins in the Scorpion and Spiders with the formation of a ventral plate, and there can be but little doubt that Chelifer is provided with an homologous structure, though very probably modified, owing to the small amount of food-yolk and early period of hatching.
The history of the layers and their conversion into the organs has been studied in the case of the Scorpion (Metschnikoff, No.
434), and of the Spiders ; and a close agreement has been found to obtain between them.
It will be convenient to take the latter group as type, and simply to call attention to any points in which the two groups differ.
The epiblast. The epiblast, besides giving rise to the skin (hypodermis and cuticle), also supplies the elements for the nervous system and organs of sense, and for the respiratory sacks, the stomodaeum and proctodaeum.
At the period when the mesoblast is definitely established, the epiblast is formed of a single layer of columnar cells in the region of the ventral plate, and of a layer of flat cells over other parts of the yolk.
When about six segments are present the first changes take place. The epiblast of the ventral plate then becomes somewhat thinner in the median line than at the two sides (fig. 203 B). In succeeding stages the contrast between the median and the lateral parts becomes still more marked, so that the epiblast becomes finally constituted of two lateral thickened bands, which meet in front in the procephalic lobes, and behind in the caudal lobe, and are elsewhere connected by a very thin layer (fig. 203 C). Shortly after the appendages begin to be formed, the first rudiments of the ventral nerve-cord become established as epiblastic thickenings on the inner side of each of the lateral bands. The thickenings of the epiblast of the two sides are quite independent, as may be seen in fig. 203 C, vn, taken from a stage somewhat subsequent to their first appearance. They are developed from before backwards, but either from the first, or in any case very soon afterwards, cease to form uniform thickenings, but constitute a linear series of swellings the future ganglia connected by very short less prominent thickenings of the epiblast (fig. 200 C). The rudiments of the ventral nerve-cord are for a long time continuous with the epiblast, but shortly after the establishment of the dorsal surface of the embryo they become separated from the epiblast and constitute two independent cords, the histological structure of which is the same as in other Tracheata (fig. 206, vn\
The ventral cords are at first composed of as many ganglia as there are segments. The foremost pair, belonging to the
segment of the chelicerae, lie immediately behind the stomodaeum, and are as independent of each other as the remaining ganglia. Anteriorly they border on the supra-cesophageal ganglia. When the yolk sack is formed in connection with the ventral flexure of the embryo, the two nerve-cords become very widely separated (fig. 206, vn) in their middle region. At a later period, at the stage represented in fig. 201 B, they again become approximated in the ventral line, and delicate commissures are formed uniting
FIG. 203. TRANSVERSE SECTIONS THROUGH THE VENTRAL PLATE OF AGELBNA LABYRINTHICA AT THREE STAGES.
A. Stage when about three segments are formed. The mesoblastic plate is not divided into two bands.
B. Stage when six segments are present (fig. ?oo B). The mesoblast is now divided into two bands.
C. Stage represented in fig. 200 D. The ventral cords have begun to be formed on thickenings of the epiblast, and the limbs are established.
ep. epiblast ; me. mesoblast ; me.s. mesoblastic somite ; 7>n. ventral nerve-cord ; yk. yolk.
the ganglia of the two sides, but there is no trace at this or any other period of a median invagination of epiblast between the two cords, such as Hatschek and other observers have attempted to establish for various Arthropoda and Chaetopoda. At the stage represented in fig. 201 A the nerve ganglia are still present in the abdomen, though only about four ganglia can be distinguished. At a later stage these ganglia fuse into two continuous
cords, united however by commissures corresponding with the previous ganglia.
The ganglia of the chelicerae have, by the stage represented in fig. 20 1 B, completely fused with the supra-oesophageal ganglia and form part of the oesophageal commissure. The cesophageal commissure is however completed ventrally by the ganglia of the pedipalpi.
The supra-cesophageal ganglia are formed independently of the ventral cords as two thickenings of the procephalic lobes (fig. 205). The thickenings of the two lobes are independent, and each of them becomes early marked out by a semicircular groove (fig. 200 D) running outwards from the upper lip. Each thickening eventually becomes detached from the superficial epiblast, but before this takes place the two grooves become deeper, and on the separation of the ganglia from the epiblast, the cells lining the grooves become involuted and detached from the skin, and form an integral part of the supra-oesophageal ganglia.
At the stage represented in fig. 201 B the supra-oesophageal ganglia are completely detached from the epiblast, and are constituted of the following parts : (i) A dorsal section formed of two hemispherical lobes, mainly formed of the invaginated lining of the semicircular grooves. The original lumen of the groove is still present on the outer side of these lobes. (2) Two central masses, one for each ganglion, formed of punctiform tissue, and connected by a transverse commissure. (3) A ventral anterior lobe. (4) The original ganglia of the chelicerae, which form the ventral parts of the ganglia 1 .
The later stages in the development of the nervous system have not been worked out.
The development of the nervous system in the Scorpion is almost identical with that in Spiders, but Metschnikoff believes, though without adducing satisfactory evidence, that the median integument between the two nerve cords assists in forming the ventral nerve cord. Grooves are present in the supra-cesophageal ganglia similar to those in Spiders.
The mesoblast. The history of the mesoblast, up to the formation of a ventral plate subjacent to the thickened plate of epiblast, has been already given. The ventral plate is shewn in fig. 203 A. It is seen to be formed mainly of small cells,
1 For further details vide self, No. 440. B. II. 29
but some large cells are shewn in the act of passing into it from the yolk. During a considerable section of the subsequent development the mesoblast is confined to the ventral plate.
The first important change takes place when about six somites are established ; the mesoblast then becomes divided
FIG. 204. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELENA
The section is through an embryo of the same age as that represented in fig. 200 C, and is taken slightly to one side of the middle line so as to shew the relation of the mesoblastic somites to the limbs. In the interior are seen the yolk segments and their nuclei.
i 16. the segments; pr.l. procephalic lobe ; do. dorsal integument.
into two lateral bands, shewn in section in fig. 203 B, which meet however in front in the procephalic lobes, and behind in the caudal lobes. Very shortly afterwards these bands become broken up into a number of parts corresponding to the segments, each of which soon becomes divided into two layers, which enclose a cavity between them (vide fig. 204 and fig. 207). The outer layer (somatic) is thicker and attached to the epiblast, and the inner layer (splanchnic) is thinner and mainly, if not entirely, derived (in Agelena) from cells which originate in the yolk. These structures constitute the mesoblastic somites. In the appendage-bearing segments the somatic layer of each of them, together with a prolongation of the cavity, is continued
into the appendage (fig. 203 C). Since the cavity of the mesoblastic somites is part of the body cavity, all the appendages contain prolongations of the body cavity. Not only is a pair of mesoblastic somites formed for each segment of the body, but also for the procephalic lobes (fig. 205). The mesoblastic somites for these lobes are established somewhat later than for the true segments, but only differ from them in the fact that the somites of the two sides are united by a median bridge of undivided mesoblast. The development of a somite for the procephalic lobes is similar to what has been described by Kleinenberg for Lumbricus (p. 339), but must not be necessarily supposed to indicate that the procephalic lobes form a segment equivalent to the segments of the trunk. They are -rather equivalent to the
FIG. 205. SECTION THROUGH THE PROCEPHALIC LOBES OF AN EMBRYO OF AGELENA LABYRINTHICA.
The section is taken from an embryo of the same age as fig. 200 D.
Drae oral lobe of g roove
stomodseum ; gr. section through semi-circular procephalic lobe ; ce.s. cephalic section of body cavitv.
Chaetopod larvae. When the dorsal surface of the embryo is established a thick layer of mesoblast becomes formed below the epiblast. This layer is not however derived from an upgrowth of the mesoblast of the somites, but from cells which originate in the yolk. The first traces of the layer are seen in fig. 204, do, and it is fully established as a layer of large round cells in the stage shewn in fig. 206. This layer of cells is seen to be quite independent of the mesoblastic somites (ine.s). The mesoblast of the dorsal surface becomes at the stage represented in fig. 201 B divided into splanchnic and somatic layers, and in the abdomen at any rate into somites continuous with those of the ventral part of the mesoblast. At the lines of junction of successive somites the splanchnic layer of mesoblast dips into the yolk, and forms a number of transverse septa, which do not reach the middle of the yolk, but leave a central part free, in which the mesenteron is subsequently formed. At the insertion of these septa there
are developed widish spaces between the layers of somatic and splanchnic mesoblast, which form transversely directed channels passing from the heart outwards. They are probably venous. At a later stage the septa send out lateral offshoots, and divide the peripheral part of the abdominal cavity into a number of compartments filled with yolk. It is probable that the hepatic diverticula are eventually formed in these compartments.
The somatic layer of mesoblast
FIG. 206. TRANSVERSE SECTION THROUGH THE THORACIC REGION OF AN EMBRYO OF AGELENA LABYRINTHICA.
The section is taken from an embryo of the same age as fig. 201 A, and passes through the maximum protuberance of the ventral yolk sack.
vn. ventral nerve cord ; yk. yolk ; me.s. mesoblastic somite ; ao. aorta.
is converted into the muscles, both of the limbs and trunk, the superficial connective tissue, nervous sheath, etc. It probably also gives rise to the three muscles attached to the suctorial apparatus of the oesophagus.
The heart and aorta are formed as a solid rod of cells of the dorsal mesoblast, before it is distinctly divided into splanchnic and somatic layers. Eventually the central cells of the heart become blood corpuscles, while its walls are constituted of an outer muscular and inner epithelioid layer. It becomes functional, and acquires its valves, arterial branches, etc., by the stage represented in fig. 201 B.
The history of the mesoblast, more especially of the mesoblastic somites, of the Scorpion is very similar to that in Spiders : their cavity is continued in the same way into the limbs. The general character of the somites in the tail is shewn in fig. 207. The caudal aorta is stated by MetschnikofT to be formed from part of the mesenteron, but this is too improbable to be accepted without further confirmation.
The hypoblast and alimentary tract. It has already been stated that the yolk is to be regarded as corresponding to the hypoblast of other types.
For a considerable period it is composed of the polygonal yolk cells already described and shewn in figs. 203, 204, and 205. The yolk cells divide and become somewhat smaller as development proceeds ; but the main products of the division of the yolk nuclei and the protoplasm around them are undoubtedly cells which join the mesoblast (fig. 203 A). The permanent alimentary tract is formed of three sections, viz. stomodaeum, proctodaeum, and mesenteron. The stomodaeum and proctodaeum are both formed before the mesenteron. The stomodaeum is formed as an epiblastic pit between the two procephalic lobes (figs. 200 and 205, st). It becomes deeper, and by the latest stage figured is a deep pit lined by a cuticle and ending blindly. To its hinder section, which forms the suctorial apparatus of the adult, three powerful muscles (a dorsal and two lateral) are attached.
The proctodaeum is formed considerably later than the stomodaeum. It is a comparatively shallow involution, which forms the rectum of the adult. It is dilated at its extremity, and two Malpighian vessels early grow out from it.
The mesenteron is formed in the interior of the yolk. Its walls are derived from the cellular elements of the yolk, and the first section to be formed is the hinder extremity, which appears as a short tube ending blindly behind in contact with the proctodaeum, and open to the yolk in front. The later history of the mesenteron has not been followed, but it undoubtedly includes
FlG. 207. TAIL OF AN ADVANCED EMBRYO OF THE SCORPION TO ILLUSTRATE THE STRUCTURE OF THE MESOBLASTIC
SOMITES. (After Metschnikoff.)
al. alimentary tract; an.i. anal invagination ; ep. epiblast ; me.s. mesoblastic somite.
the whole of the abdominal section of the alimentary canal of the adult, except the rectum, and probably also the thoracic section. The later history of the yolk which encloses the mesenteron has not been satisfactorily studied, though it no doubt gives rise to the hepatic tubes, and probably also to the thoracic diverticula of the alimentary tract.
The general history of the alimentary tract in Scorpio is much the same as in Spiders. The hypoblast, the origin of which as mentioned above is somewhat uncertain, first appears on the ventral side and gradually spreads so as to envelop the yolk, and form the wall of the mesenteron, from which the liver is formed as a pair of lateral outgrowths. The proctodaeum and stomodseum are both short, especially the former (vide fig. 207).
Summary and general conclusions.
The embryonic forms of Scorpio and Spiders are very similar, but in spite of the general similarity of Chelifer to Scorpio, the embryo of the former differs far more from that of Scorpio than the latter does from Spiders. This peculiarity is probably to be explained by the early period at which Chelifer is hatched ; and though a more thorough investigation of this interesting form is much to be desired, it does not seem probable that its larva is a primitive type.
The larvae of the Acarina with their peculiar ecdyses are to be regarded as much modified larval forms. It is not however easy to assign a meaning to the hexapodous stage through which they generally pass.
With reference to the segments and appendages, some interesting points are brought out by the embryological study of these forms.
The maximum number of segments is present in the Scorpion, in which nineteen segments (not including the praeoral lobes, but including the telson) are developed. Of these the first twelve segments have traces of appendages, but the appendages of the six last of these (unless the pecten is an appendage) atrophy. In Spiders there are indications in the embryo of sixteen segments ; and in all the Arachnida, except the Acarina, at the least four segments bear appendages in the embryo which are without them in the adult. The morphological bearings of this fact are obvious.
It deserves to be noted that, in both Scorpio and the Spider, the chelicerae are borne in the embryo by the first post-oral segment, and provided with a distinct ganglion, so that they cannot correspond (as they are usually supposed to do) with the antennae of Insects, which are always developed on the prae-oral lobes, and never supplied by an independent ganglion.
The chelicerae would seem probably to correspond with the mandibles of Insects, and the antennae to be absent. In favour of this view is the fact that the embryonic ganglion of the mandibles of Insects is stated (cf. Lepidoptera, Hatschek, p. 340) to become, like the ganglion of the chelicerae, converted into part of the cesophageal commissure.
If the above considerations are correct, the appendages of the Arachnida retain in many respects a very much more primitive condition than those of Insects. In the first place, both the chelicerae and pedipalpi are much less differentiated than the mandibles and first pair of maxillae with which they correspond. In the second place, the first pair of ambulatory limbs must be equivalent to the second pair of maxillae of Insects, which, for reasons stated above, were probably originally ambulatory. It seems in fact a necessary deduction from the arguments stated that the ancestors of the present Insecta and Arachnida must have diverged from a common stem of the Tracheata at a time when the second pair of maxillae were still ambulatory in function.
With reference to the order of the development of the appendages and segments, very considerable differences are noticeable in the different Arachnoid types. This fact alone appears to me to be sufficient to prove that the order of appearance of the appendages is often a matter of embryonic convenience, without any deep morphological significance. In Scorpio the segments develop successively, except perhaps the first postoral, which is developed after some of the succeeded segments have been formed. In Spiders the segment of the chelicerae, and probably also of the pedipalpi, appears later than the next three or four. In both these types the segments arise before the appendages, but the reverse appears to be the case in Chelifer. The permanent appendages, except the chelicerae, appear simultaneously in Scorpions and Spiders. The second pair appears long before the others in Chelifer, then the third, next the first, and finally the three hindermost.
(434) El. Metschnikoff. " Embryologie des Scorpions." Zeit.f.wiss. Zool. Bd. xxi. 1870.
(435) H. Rathke. Reisebemerkungen aus Taurien (Scorpio), Leipzig, 1837.
(436) El. Metschnikoff. " Entwicklungsgeschichte d. Chelifer." Zeit.f.wiss. Zool., Bd. xxi. 1870.
(437) A. Stecker. " Entwicklung der Chthonius-Eier im Mutterleibe und die Bildung des Blastoderms." Sitzung. konigl. bohmisch. Gesellschaft Wissensch., 1876, 3. Heft, and Aimed, and Mag. Nat. History, 1876, xvm. 197.
(438) M. Balbiani. " Memoire sur le developpement des Phalangides." Ann. Scien. Nat. Series v. Vol. xvi. 1872.
(439) M. Balbiani. "Memoire sur le developpement des Araneides." Ann. Scien. Nat. Series v. Vol. xvn. 1873.
(440) F. M. Balfour. "Notes on the development of the Araneina." Quart. Journ. of Micr. Science, Vol. xx. 1880.
(441) J. Barrois. " Recherches s. 1. developpement des Araigndes. " Journal de 1'Anat. et de la Physiol. 1878.
(442) E. Claparede. Recherches s. t evolution des Araignees. Utrecht, 1862.
(443) Hero Id. De generatione Araneorum in Ovo. Marburg, 1824.
(444) H. Ludwig. "Ueber die Bildung des Blastoderms bei den Spinnen." Zeit.f. wiss. Zool., Vol. xxvi. 1876.
(445) P. van Beneden. " Developpement de 1'Atax ypsilophora." Acad. Bruxelles, t. xxiv.
(446) Ed. Claparede. "Studien iiber Acarinen." Zeit.f. wiss. Zool., Bd. xvm. 1868.
Formation of the layers and the embryonic envelopes in the
There is a striking constancy in the mode of formation of the layers throughout the group. In the first place the hypoblast is not formed by a process which can be reduced to invagination : in other words, there is no gastrula stage.
Efforts have been made to shew that the mesoblastic groove of Insects implies a modified gastrula, but since it is the essence of a gastrula that it should directly or indirectly give rise to the archenteron, the groove in question cannot fall under this category. Although the mesoblastic groove of Insects is not a gastrula, it is quite possible that it is the rudiment of a blastopore, the gastrula corresponding to which has now vanished from the development. It would thus be analogous to the primitive streak of Vertebrates 1 .
The growth of the blastoderm over the yolk in Scorpions admits no doubt of being regarded as an epibolic gastrula. The blastopore would however be situated dorsally, a position which it does not occupy in any gastrula type so far dealt with. This fact, coupled with the consideration that the partial segmentation of Scorpio can be derived without difficulty from the ordinary Arachnidan type (vide p. 120), seems to shew that there is no true epibolic invagination in the development of Scorpio.
On the formation of the blastoderm traces of two embryonic layers are established. The blastoderm itself is essentially the epiblast, while the central yolk is the hypoblast. The formation of the embryo commences in connection with a thickening of the blastoderm, known as the ventral plate. The mesoblast is formed as an unpaired plate split off from the epiblast of the ventral plate. This process takes place in at any rate two ways. In Insects a groove is formed, which becomes constricted off to form the mesoblastic plate : in Spiders there is a keel-like thickening of the blastoderm, which takes the place of the groove.
The unpaired mesoblastic plate becomes in all forms very soon divided into two mesoblastic bands.
The mesoblastic bands are very similar to, and probably homologous with, those of Chaetopoda ; but the different modes by which they arise in these two groups are very striking, and probably indicate that profound modifications have taken place in the early development of the Tracheata. In the Chaetopoda the bands are from the first widely separated, and gradually approach each other ventrally, though without meeting. In the Tracheata they arise from the division of an unpaired ventral plate.
The further history of the mesoblastic bands is nearly the
1 The primitive streak of Vertebrates, as will appear in the sequel, has no connection with the medullary groove, and is the rudiment of the blastopore.
same for all the Tracheata so far investigated, and is also very much the same as for the Chaetopoda. There is a division into somites; each containing a section of the body cavity. In the cephalic section of the mesoblastic bands a section of the body cavity is also formed. In Arachnida, Myriapoda, and probably also Insecta, the body cavity is primitively prolonged into the limbs.
In Spiders at any rate, and very probably in the other groups of the Tracheata, a large part of the mesoblast is not derived from the mesoblastic plate, but is secondarily added from the yolk-cells.
In all Tracheata the yolk-cells give rise to the mesenteron which, in opposition, as will hereafter appear, to the mesenteron of the Crustacea, forms the main section of the permanent alimentary tract.
One of the points which is still most obscure in connection with the embryology of the Tracheata is the origin of the embryonic membranes. Amongst Insects, with the exception of the Thysanura, such membranes are well developed. In the other groups definite membranes like those of Insects are never found, but in the Scorpion a cellular envelope appears to be formed round the embryo from the cells of the blastoderm, and more or less similar structures have been described in some Myriapods (vide p. 390). These structures no doubt further require investigation, but may provisionally be regarded as homologous with the amnion and serous membrane of Insects. In the present state of our knowledge it does not seem easy to give any explanation of the origin of these membranes, but they may be in some way derived from an early ecdysis.