Embryo Of Hydrophilus piceusFig. 179. Embryo Of Hydrophilus piceus viewed from the ventral surface.(After Kowalevsky.)pc.l.procephalic lobe.
Fig. 179. Embryo Of Hydrophilus piceus viewed from the ventral surface.(After Kowalevsky.)
pc.l.procephalic lobe.
The general history of the succeeding stages may be briefly told.
Illustration: TitleFig. 180. Two stages in the development of Hydrophilus piceus.(From Gegenbaur, after Kowalevsky.)ls.labrum;at.antenna;md.mandible;mx.maxilla I.;li.maxilla II.;p´ p´´p´´´. feet;a.anus.
Fig. 180. Two stages in the development of Hydrophilus piceus.(From Gegenbaur, after Kowalevsky.)
ls.labrum;at.antenna;md.mandible;mx.maxilla I.;li.maxilla II.;p´ p´´p´´´. feet;a.anus.
The appendages appear as very small rudiments at the close of the last stage, but soon become much more prominent (fig. 180A). They are formed as outgrowths of both layers, and arise nearly simultaneously. There are in all eight pairs of appendages. The anterior or antennæ (at) spring from the procephalic lobes, and the succeeding appendages from the segments following. The last pair of embryonic appendages, which disappears very early, is formed behind the third pair of the future thoracic limbs. Paired epiblastic involutions, shewn as pits in the posterior segments infig. 180A, give rise to the tracheæ; and the nervous system is formed as two lateral epiblastic thickenings, one on each side of the mid-ventral line. These eventually become split off from the skin; while between them there passes in a median invagination of the skin(fig. 189C). The two nervous strands are continuous in front with the supra-œsophageal ganglia, which are formed of the epiblast of the procephalic lobes. These plates gradually grow round the dorsal side of the embryo, and there is formed immediately behind them an oral invagination, in front of which there appears an upper lip (fig. 180,ls). A proctodæum is formed at the hind end of the body slightly later than the stomodæum. The mesoblast cells become divided into two bands, one on each side of the middle line (fig. 189A), and split into splanchnic and somatic layers. The central yolk mass at about the stage represented infig. 179begins 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. 185al). 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 onp.404. The heart is formed from the mesoblastic layers, where they meet in the middle dorsal line (fig. 185C,ht). The somatic mesoblast gives rise to the muscles and connective tissue, and the splanchnic mesoblast to the muscular part of the wall of the alimentary tract, which accompanies the hypoblast in its growth round the yolk. The proctodæum forms the rectum and Malpighian bodies[169], and the stomodæum the œsophagus 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.
The embryonic membranes and the formation of the layers.
All Insects have at the close of segmentation a blastoderm formed of a single row of cells enclosing a central yolk mass,which usually contains nuclei, and in the Poduridæ is divided up in the ordinary segmentation into distinct yolk cells. The first definite structure formed is a thickening of the blastoderm, which forms a ventral plate.
The ventral plate is very differently situated in relation to the yolk in different types. In most Diptera, Hymenoptera and (?) Neuroptera (Phryganea) it forms from the first a thickening extending over nearly the whole ventral surface of the ovum, and in many cases extends in its subsequent growth not only over the whole ventral surface, but over a considerable part of the apparent dorsal surface as well (Chironomus, Simulia, Gryllotalpa, etc.). In Coleoptera, so far as is known, it commences as a less extended thickening either of the central part (Donacia) or posterior part (Hydrophilus) of the ventral surface, and gradually grows in both directions, passing over to the dorsal surface behind.
Embryonic membranes.In the majority of Insects there are developed enveloping membranes like those of Hydrophilus.
Sections of an insect embryoFig. 181. Diagrammatic longitudinal sections of an Insect embryo at two stages to shew the development of the embryonic envelopes.In A the amniotic folds have not quite met so as to cover the ventral plate. The yolk is represented as divided into yolk cells. In B the sides of the ventral plate have extended so as nearly to complete the dorsal integument. The mesenteron is represented as a closed sack filled with yolk cells.am.amnion;se.serous envelope;v.p.ventral plate;d.i.dorsal integument;me.mesenteron;st.stomodæum;an i.proctodæum.
Fig. 181. Diagrammatic longitudinal sections of an Insect embryo at two stages to shew the development of the embryonic envelopes.
In A the amniotic folds have not quite met so as to cover the ventral plate. The yolk is represented as divided into yolk cells. In B the sides of the ventral plate have extended so as nearly to complete the dorsal integument. The mesenteron is represented as a closed sack filled with yolk cells.am.amnion;se.serous envelope;v.p.ventral plate;d.i.dorsal integument;me.mesenteron;st.stomodæum;an i.proctodæum.
The typical mode of formation of these membranes is represented diagrammatically infig. 181A and B. A fold of the blastoderm arises round the edge of the ventral plate. This fold, like the amniotic fold of the higher Vertebrata, is formed of two limbs, an outer, the serous membrane (se), and an inner, the true amnion (am). Both limbs extend so as to cover over the ventral plate, and finally meet and coalesce, so that a double membrane is present over the ventral plate. At the same time (fig. 181B) the point where the fold originates is carried dorsalwards by thedorsal 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. Infig. 181B the embryo is represented at the stage immediately preceding the closure of the dorsal surface.
By the time that these changes are effected, the serous membrane and amnion are both very thin and not easily separable. The amnion appears to be usually absorbed before hatching; but in hatching both membranes, if present, are either absorbed, or else ruptured and thrown off.
The above mode of development of the embryonic membranes has been especially established by the researches of Kowalevsky (No.416) and Graber (No.412) for various Hymenoptera (Apis), Diptera (Chironomus), Lepidoptera and Coleoptera (Melolontha,Lina).
Considerable variations in the development of the enveloping membranes are known.
When the fold which gives rise to the membranes is first formed, there is, as is obvious infig. 181A, a perfectly free passage by which the yolk can pass in between the amnion and serous membrane. Such a passage of the yolk between the two membranes takes place posteriorly in Hydrophilus and Donacia: in Lepidoptera the yolk passes in everywhere, so that in this form the ventral plate becomes first of all imbedded in the yolk, and finally, on the completion of the dorsal integument, the embryo is enclosed in a complete envelope of yolk contained between the amnion and the serous membrane. During the formation of the dorsal integument the external yolk-sack communicates by a dorsally situated umbilical canal with the yolk cavity within the body. On the rupture of the amnion the embryo is nourished at the expense of the yolk contained in the external yolk-sack.
In the Hemiptera and the Libellulidæ the ventral plate also becomes imbedded in the yolk, but in a somewhat different fashion to the Lepidoptera, which more resembles on an exaggerated scale what takes place in Hydrophilus.
Three stages of CalopteryxFig. 182. Three stages in the development of the embryo of Calopteryx.(After Brandt.)The embryo is represented in the egg-shell.A. Embryo with ventral plate.B. Commencing involution of ventral plate.C. Involution of ventral plate completed.ps.ventral plate;g.edge of ventral plate;am.amnion;se.serous envelope.
Fig. 182. Three stages in the development of the embryo of Calopteryx.(After Brandt.)
The embryo is represented in the egg-shell.A. Embryo with ventral plate.B. Commencing involution of ventral plate.C. Involution of ventral plate completed.
ps.ventral plate;g.edge of ventral plate;am.amnion;se.serous envelope.
In the Libellulidæ (Calopteryx) there is first of all formed (Brandt,No.403) a small ventral and posterior thickening of the blastoderm (fig. 182A). The hinder part of this becomes infolded into the yolk as a projection (fig. 182B), which consists of two laminæ, an anterior and a posterior, continuous at the apex of the invagination. The whole structure, which is completely imbedded within the yolk, rapidly grows in length, and turns towards the front end of the egg (fig. 182C). Its anterior lamina remains thick and gives rise to the ventral plate (ps), the posterior (am) on the other handbecomes 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. 182C) continuous with the serous envelope on the posterior side only, so that the serous envelope does not form a continuous sack, but has an opening close to the head of the embryo. In the Hemiptera parasita this opening (Melnikow,No.422) remains permanent, and the embryo, after it has reached a certain stage of development, becomes everted through it, while the yolk, enclosed in the continuous membrane formed by the amnion and serous envelope, forms a yolk-sack on the dorsal surface. In the Libellulidæ however and most Hemiptera, a fusion of the two limbs of the serous membrane takes place in the usual way, so as to convert it into a completely closed sack (fig. 183A). 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. 183B 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. 183C).
Development of CalopteryxFig. 183. Three stages in the development of Calopteryx.(After Brandt.)The embryo is represented in the egg-shell; B. and C. shew the inversion of the embryo.se.serous envelope;am.amnion;ab.abdomen;v.anterior end of head;at.antennæ;md.mandible;mx1.maxilla 1;mx2.maxilla 2;p1-p3.three pairs of legs;oe.œsophagus.
Fig. 183. Three stages in the development of Calopteryx.(After Brandt.)
The embryo is represented in the egg-shell; B. and C. shew the inversion of the embryo.
se.serous envelope;am.amnion;ab.abdomen;v.anterior end of head;at.antennæ;md.mandible;mx1.maxilla 1;mx2.maxilla 2;p1-p3.three pairs of legs;oe.œsophagus.
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. Thewalls 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.
Three larval stages of HydrophilusFig. 184. Three larval stages of Hydrophilus from the dorsal side, shewing the gradual closing in of the dorsal region with the formation of the peculiar dorsal orgando.(After Kowalevsky.)do.dorsal organ;at.antennæ.
Fig. 184. Three larval stages of Hydrophilus from the dorsal side, shewing the gradual closing in of the dorsal region with the formation of the peculiar dorsal orgando.(After Kowalevsky.)
do.dorsal organ;at.antennæ.
In Hydrophilus and apparently in the Phryganidæ also, there are certain remarkable peculiarities in the closure of the dorsal surface. The fullest observations on the subject have been made by Kowalevsky (No.416), but Dohrn (No.408) has with some probability thrown doubts on Kowalevsky’s interpretations. According to Dohrn the part of the serous envelope which covers the dorsal surface becomes thickened, and gives rise to a peculiar dorsal plate which is shewn in surface view infig. 184A,do, and in section infig. 185A,do.The ventral parts of the amnion and serous membrane have either been ruptured or have disappeared. While the dorsal plate is being formed, the mesoblast, and somewhat later the lateral parts of the epiblast of the ventral plate gradually grow towards the dorsal side and enclose the dorsal plate, the wall of which in the process appears to be folded over so as first of all to form a groove and finally a canal. The stages in this growth are shewn from the surface infig. 184B and C and in section infig. 185B,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. 184C). The whole structure is known as the dorsal canal. It appears to atrophy without leaving a trace. The heart when formed lies immediately dorsal to it[170].
Section of Hydrophilus embryosFig. 185. Three transverse sections through advanced embryos of Hydrophilus.A. Section through the posterior part of the body of the same age as fig. 184 A.B. Section through the embryo of the same age as fig. 184 C.C. Section through a still older embryo.do.dorsal plate;vn.ventral nerve cord;al.mesenteron;ht.heart.The large spaces at the sides are parts of the body cavity.
Fig. 185. Three transverse sections through advanced embryos of Hydrophilus.
A. Section through the posterior part of the body of the same age as fig. 184 A.B. Section through the embryo of the same age as fig. 184 C.C. Section through a still older embryo.
do.dorsal plate;vn.ventral nerve cord;al.mesenteron;ht.heart.
The large spaces at the sides are parts of the body cavity.
In the Poduridæ the embryonic membranes appear to be at any rate imperfect. Metschnikoff states in his paper on Geophilus that in some ants no true embryonic membranes are found, but merely scattered cells which take their place. In the Ichneumonidæ the existence of two embryonic membranes is very doubtful.
Formation of the embryonic layers.The formation of the layers has been studied in sections by Kowalevsky (No.416),Hatschek (No.414), and Graber (No.412), etc. From their researches it would appear that the formation of the mesoblast always takes place in a manner closely resembling that in Hydrophilus. The essential features of the process (figs. 177and178) appear to be that a groove is formed along the median line of the ventral plate, and that the sides of this groove either (1) simply close over like the walls of the medullary groove in Vertebrates, and so convert the groove into a tube, which soon becomes solid and forms a mass or plate of cells internal to the epiblast; or (2) that the cells on each side of the groove grow over it and meet in the middle line, forming a layer external to the cells which lined the groove. The former of these processes is the most usual; and in the Muscidæ the dimensions of the groove are very considerable (Graber,No.411). In both cases the process is fundamentally the same, and causes the ventral plate to become divided into two layers[171]. The external layer or epiblast is an uniform sheet forming the main part of the ventral plate (fig. 178B,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. 178B,me). The mesoblast soon becomes divided into two lateral bands.
The origin of the hypoblast is still in dispute. It will be remembered (videpp.114and116) 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 infigs. 181and189, and it is probable that they constitute the true hypoblast. Their further history is given below.
Formation of the organs and their relation to the germinal layers.
The segments and appendages.One of the earliest phenomena in the development is the appearance of transverse lines indicating segmentation (fig. 186). The transverse lines are apparently caused by shallow superficial grooves, and also inmany cases by the division of the mesoblastic bands into separate somites. The most anterior line marks off a præ-oral segment, which soon sends out two lateral wings—the procephalic lobes. The remaining segments are at first fairly uniform. Their number does not, however, appear to be very constant. So far as is known they never exceed seventeen, and this number is probably the typical one (figs. 186and187).
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.
Embryo of Hydrophilus piceusFig. 186. Embryo of Hydrophilus piceus viewed from the ventral surface.(After Kowalevsky.)pc.l.procephalic lobe.
Fig. 186. Embryo of Hydrophilus piceus viewed from the ventral surface.(After Kowalevsky.)
pc.l.procephalic lobe.
The appendages arise as paired pouch-like outgrowths of the epiblast and mesoblast; and their number and the order of their appearance are subject to considerable variation, the meaning of which is not yet clear. As a rule they arise subsequently to the segmentation of the parts of the body to which they belong. There is always formed one pair of appendages which spring from the lateral lobes of the procephalic region, or from the boundary line between these and the median ventral part of this region. These appendages are the antennæ. They have in the embryo a distinctly ventral position as compared to that which they have in the adult.
In the median ventral part of the procephalic region there arises the labrum (fig. 187,ls). 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[172].
Development of Hydrophilus piceusFig. 187. Two stages in the development of Hydrophilus piceus.(From Gegenbaur, after Kowalevsky.)ls.labrum;at.antenna;md.mandible;mx.maxilla I.;li.maxilla II.;p´ p´´ p´´´.feet;a.anus.
Fig. 187. Two stages in the development of Hydrophilus piceus.(From Gegenbaur, after Kowalevsky.)
ls.labrum;at.antenna;md.mandible;mx.maxilla I.;li.maxilla II.;p´ p´´ p´´´.feet;a.anus.
The antennæ themselves can hardly be considered to have the same morphological value as the succeeding appendages. They are rather equivalent to paired processes of the præ-oral lobes of the Chætopoda.
From the first three post-oral segments there grow out the mandibles and two pairs of maxillæ, 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 Bütschli (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 9‑11 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 Bütschli 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 recentobservations (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 Bütschli to arise simultaneously, but according to Kowalevsky the two mouth appendages first appear, then the antennæ, and still later the thoracic appendages. In the Diptera the mouth appendages are first formed, and either simultaneously with these, or slightly later, the antennæ. In the Hemiptera and Libellulidæ the thoracic appendages are the first to be formed, and the second pair of maxillæ makes its appearance before the other cephalic appendages.
Aquatic respiration in insectsFig. 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 Æschna grandis.a.superior longitudinal tracheal trunks;b.their anterior end;c.portion branching on proctodæum;o.eyes.C. Alimentary canal of the same larva from the side.a, b,andc.as in B;d.inferior tracheal trunk;e.transverse branches between upper and lower tracheal trunks.
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 Æschna grandis.a.superior longitudinal tracheal trunks;b.their anterior end;c.portion branching on proctodæum;o.eyes.C. Alimentary canal of the same larva from the side.a, b,andc.as in B;d.inferior tracheal trunk;e.transverse branches between upper and lower tracheal trunks.
The history of the changes in the embryonic appendages during the attainment of the adult condition is beyond the scope of this treatise, but it may be noted that the second pair of maxillæ 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 larvæ of Ephemeridæ and Phryganidæ (fig. 188A), of which they are supposed by Gegenbaur and Lubbock to be modifications. The undoubtedly secondary character of theclosedtracheal system of larvæ with tracheal gills tells against this view. Fritz Müller finds in the larvæ of Calotermes rugosus(one of the Termites) that peculiar and similar dorsal appendages are present on the two anterior of the thoracic segments. They are without tracheæ. The anterior atrophies, and the posterior acquires tracheæ 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 Müller 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 Müller 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 tracheæ. But although this suggestion of Fritz Müller 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 præ-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. 189B,vn), which are subsequently split off 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. 189C). 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 inPeripatus. 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 œsophageal commissures, while the sub-œsophageal ganglion is formed from the fusion of the ganglia of the two maxillary segments.
Three transverse sections through the embryo of HydrophilusFig. 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.
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.
The præ-oral portion of the nervous system consists entirely of the supra-œsophageal ganglion. It is formed, according to Hatschek, of three parts. Firstly and mainly, of a layer separated 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 thedorsal border of the antennæ. 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-œsophageal ganglia remain disconnected on the dorsal side till quite the close of embryonic life.
The tracheæ and salivary glands.The tracheæ, as was first shewn by Bütschli (No.405), arise as independent segmentally arranged paired invaginations of the epiblast (fig. 189B 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[173], 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 larvæ 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 Bütschli (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 (e.g.Gryllotalpa,Dohrn,No.408) they remain distinct, and each tracheal stem has a system of branches of its own.
The development of the tracheæ 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 tracheæ, and perhaps have a similar origin. The salivary glandsarise as paired epiblastic invaginations, not, as might be expected, of the Stomodæum, 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 maxillæ 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 papillæ 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 maxillæ, 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. 189A). 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 proctodæum 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 antennæ 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 stomodæum (st) and a proctodæum (an). The stomodæum and proctodæum 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 stomodæum consists of mouth, œsophagus, 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 proctodæum, 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 astwo pairs of outgrowths of the epiblast of the proctodæum, 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,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[174].
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. 189B and C,hy). They then meet ventrally (fig. 185A and B), and finally close in the mesenteron on the dorsal side.
The mesenteron is at first a closed sack, independent of both stomodæum and proctodæum; 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 formedin situaround 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 themass 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[175]. Their protoplasm then fuses, and their nuclei divide, and they give rise to the larval ovaries, for which the enclosing cells form the tunics.
InAphisMetschnikoff (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 (videp.347) from the hind end of the ventral plate close to the proctodæum; while Suckow states that the generative organs are outgrowths of the proctodæum. 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.
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 thatthe genital glands of both sexes have an identical origin.
Special types of larvæ.
Certain of the Hymenopterous forms, which deposit their eggs in the eggs or larvæ of other Insects, present very peculiar modifications in their development. Platygaster, which lays its egg in the larvæ 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.