Chapter XXVII: Part II: Embryology of Insects (1)

_a._ The egg

FIG. 482.—Female Dyticus, laying eggs: _A_, ovipositor extended. _B_,
egg of Notonecta, attached to stem of rush. _C_, egg of Dyticus,
laid in excavation in rush.—After Régimbart, from Miall.
]

Insects as a rule arise from eggs which are laid in a great variety of situations, those species which are viviparous being exceedingly few in number compared with the class as a whole. It is noteworthy that Leydig has found in the same Aphis, and even in the same ovary, an egg-tube producing eggs, while a neighboring tube was producing viviparous individuals.[77] The viviparous species are confined to certain May-flies, the Aphidæ, Diptera (Sarcophaga, Tachinidæ, Œstridæ, and Pupipara), and to certain Coleoptera (Stylopidæ and some Staphylinidæ).

The number of eggs laid varies from a very few, as in the Collembola and in the Psocidæ, or 15 or even less in certain fossorial wasps, and from 20 to 35 in some locusts to many thousands in the social insects, the honey-bee laying by estimate over 1,000,000 eggs in the course of her life. Dr. Sharp thinks that from 50 to 100 may perhaps be taken as an average number for one female to produce. The eggs of insects with a complete metamorphosis are said by Brauer to be smaller in proportion to the parent than those laid by ametabolous or heterometabolous insects. In this respect the insects are paralleled by the birds, the highest forms laying smaller eggs than the water birds, ostrich, Apteryx, etc.

FIG. 483.—Eggs (_e_) of Hydrobius (?) and their capsules, from which
the larva, Fig. 452, hatched.—Emerton _del._
]

The egg, or ovum, when laid is not always ripe or perfect, but, as in those of ants, continues to grow after oviposition. Others are laid some time after the embryo has begun to form; and in the flesh-flies the larva hatches before the egg is deposited.

FIG. 484.—Egg-masses of Chironomus: _A_, string of eggs of _C.
dorsalis_, divided into sections to show both sides. _B_, twisted
fibres which traverse the string of eggs. _C_, egg-mass of
Chironomus (_sp_). _D_, egg-mass of a third species. _E_, part of
_D_, more highly magnified. _F_, developing eggs, two stages.—After
Miall.
]

Insects as a rule instinctively lay their eggs near or upon objects destined to be the food of the larva; those of caterpillars on leaves, those of many flies on meat or carrion, those of Copris and other dung-beetles in dung, those of aquatic insects in water, while many oviposit in the earth or in plants (Fig. 482), or in the bodies of animals destined to be the hosts of the parasitic larvæ. As the eggs are preyed upon by mites and other animals, the contrivances and modifications of the mode of egg-laying, and the situations in which they are placed, are almost endless. Many insects lay their eggs in a mass, covered with a gummy substance; or those laid in the water, as the eggs of dragon-flies, caddis-flies, Chironomus (Fig. 484), etc., are enveloped by a jelly-like mass.

FIG. 485.—Egg-capsule of _Periplaneta americana_: _a_, side; _b_, end
view; _c_, natural size.—After Howard and Marlatt, Bull. 4, Div.
Ent. U. S. Dept. Agr.
]

The _oötheca_ of the cockroach (Fig. 485) is a solid, dense case, which, after being carried about by the mother, can be left without harm in the crevices of the floors of houses. The oötheca of Mantis (Fig. 486) is formed by a large mass of frothy matter, which hardens and is attached to stems of plants.

FIG. 486.—Egg-capsules of _Mantis carolina_.—After Riley.
]

On the other hand, the female “walking-stick” (_Diapheromera
femoratum_) drops her eggs, says Riley, loosely upon the ground,
from whatever height she may happen to be, and “one hears a constant
pattering, not unlike drops of rain, that results from the abundant
dropping of these eggs, which, in places, lay so thick among and
under the dead leaves that they may be scraped up in great
quantities.” (Report for 1879.)

The eggs of the lace-winged flies are supported on pedicels, above the reach of ovivorous mites.

The female Chrysopa usually lays between 40 and 50 eggs. In one
case, we observed that 18 egg-stalks were deposited, but there were
only nine well-formed eggs in the batch, and nine eggless stalks,
some only half the usual height, others with the knob of cement at
the end to which the egg is ordinarily fastened. The eggs are
evidently stuck on to the end of the pedicel after the latter has
been formed, as, in one instance, an egg was glued to the stalk very
much out of centre, the insect’s abdomen not having been aimed
straight, so to speak, at the mass of cement.

FIG. 487.—Eggs of Chrysopa, with larva and fly.
]

The eggs of Rhodites are fixed to a long stalk thickened at the end;
those of Inquilines and certain Chalcids (_Leucospis gigas_, Fig.
489, _A_) are also stalked; and the use of this stalk in the eggs of
Cynips (_E_) is thought by Adler to be respiratory, while, also, he
states that the egg-cavity communicates with the egg-stalk, so that
a part of the egg-contents can pass into the latter, and this
happens at the laying of each egg. The egg of certain ichneumons
(Paniscus, Fig. 488) ends in a short stalk, which is inserted in the
skin of the caterpillar destined to serve as the host of the
parasite, the eggs, as stated by De Geer, being retained more firmly
in the integument by the stalk so swelling as to form two knobs
(Fig. 498, _c_).

FIG. 488.—Young larva of Paniscus in position of feeding on the skin
of a caterpillar: _a_, the egg-shell.—After Newport, from Sharp.
]

Certain Homoptera also have stalked eggs, as those of _Psylla
pyricola_ (Fig. 489, _B_), those of _Aleyrodes citri_ (_C_, _a_,
_b_), and of an allied form, _Aleurodicus cocois_ (_D_), and those
of Corixa (Fig. 493).

FIG. 489.—Stalked eggs: _A_, of a Chalcid (after Fabre); _B_, of
Psylla (after Slingerland); _C_, of Aleyrodes; _D_, of Aleurodicus
(after Riley and Howard); _E_, of _Dryophanta scutellaris_ (after
Adler).
]

FIG. 490.—Eggs of ox bot-fly, enlarged.—After Riley.
]

Reference should also be made to the eggs of lice, which are oval
and attached to the hairs of their host. Those of the ox bot-fly
(_Hypoderma lineata_) are usually placed four to six together, and
fastened to a hair. The lower portion of the egg is admirably
adapted for clasping a hair. “It consists of two lobes, forming a
bulbous enlargement, which is attached to the egg by a broad, but
rather thin, neck, so that, when the latter is viewed sidewise, it
appears as a slender pedicel” (Fig. 490, _a-d_). (Riley in Insect
Life, iv, p. 307.) The egg of another fly (_Drosophila ampelophila_,
Fig. 491) bears a pair of long, slender appendages near the anterior
end. “The egg is inserted into the soft pulp of the decaying fruit;
these appendages leave the ovipositor last, and are spread out upon
the surface of the mass. They, in this way, serve to keep the egg in
place, and thus insure the emergence of the larva into the open air
instead of into the more or less fluid mass in which the egg is
situated. The larva issues from the egg just above the base of these
appendages.” (Comstock.)

FIG. 491.—Egg of Drosophila.—After Comstock.
]

=Mode of deposition.=—The exact process of oviposition has been rarely observed, or at least not observed in detail, and further observations are much needed. In the cockroach (Phyllodromia), Wheeler has seen the eggs pass out of the oviduct and become arranged in the oötheca, in a way similar to that in the account published by Kadyi on Periplaneta.

FIG. 492.—Rocky Mountain locust (_aa_) depositing its eggs (_c_); _d_,
the earth partially removed, showing (_e_) an egg-mass already in
place, and (_d_) one being placed; _f_ shows where such a mass has
been covered over. _A_, oviposition; _j_, position of oviduct; _g_,
egg-guide; _e_, egg. _B_, egg-mass of the same; _a_, from side, _b_,
from beneath, _c_, from above.—After Riley.
]

“When about to form the capsule, the female Blatta closes the
genital armature, and the two folds of the white membrane which
lines the oöthecal cavity close vertically in the middle line. Then
some of the contents of the colleterial glands are poured into the
chamber, and bathe the inner surface of the posterior wall. The
first egg glides down the vagina from the left ovary, describes an
arc, still keeping its germarium-pole uppermost, after having
pressed the micropylar area against the mouth of the spermatheca,
passes to the right side of the back of the chamber, and is placed
perpendicularly two-thirds to the right of the longitudinal axis of
the insect’s body. The next egg comes from the right ovary,
describes an arc to the opposite side of the body, decussating with
the path of the first egg, and is placed completely on the left side
of the median line. The third egg comes from the left ovary, and is
made to lie completely on the right side of the median line; and so
the process continues, the ovaries discharging the eggs alternately,
and each egg describing an arc to the opposite side of the capsule.
The oöthecal chamber soon becomes too small to contain all the
constantly accumulating eggs, so the anal armature opens and allows
the end of the capsule to project. A raised line, the impression of
the edges of the white membrane, runs down the end of the capsule.
The last egg deposited comes from the right ovary, and lies
two-thirds on the left, and one-third to the right, of the median
line. As soon as the egg is laid, a further discharge from the
colleterial glands spreads over the vaginal or anterior wall of the
cavity, and becomes evenly continuous with the secretion which has
before been spread over the back and the sides of the capsule by the
white membrane.

“The crista, a cord-like ridge running the full length of the dorsal
surface of the capsule, is a thick-walled tube, either half of which
is formed by the edge of the side walls of the capsule split into
two laminæ. The rhythmical clasping of the three pairs of palpi
which guard the vaginal opening is registered in an exquisite
pattern on the inner face of either half of the crista.”[78]

The mode of oviposition in the locust has been fully described by Riley, who states that the eggs pass down and out of the oviduct, and “guided by a little finger-like style” (Fig. 298), they pass in between the horny valves of the ovipositor, and issue at their tips amid the mucous fluid which forms the egg-capsule (Fig. 492).

=Vitality of eggs.=—It is well known that the eggs of phyllopod and other fresh-water Crustacea have wonderful vitality, withstanding extreme dryness for several years, at least from two to ten. Such cases are unknown among insects. It has been observed, however, by T. W. Brigham, and also by L. Trouvelot, that the eggs of the walking-stick (_Diapheromera femorata_) for the most part hatch only after the interval of two years.[79]

The eggs of Bittacus are said by Brauer to lie over unhatched for two years; indeed, the first condition of their hatching is a complete drying of the earth in which the eggs lie, the second is a succeeding thorough wetting of the ground in spring.

_Appearance and structure of the ripe egg._—The eggs of insects are on the whole rather large in proportion to the size of the parent, especially so in many minute forms, as the fleas, lice, etc.

Their general shape is spherical or oval, often cylindrical; where the eggs are long and cylindrical a dorsal and ventral side can be distinguished (Fig. 502). They are in the Tortricidæ and Limacodid moths flattened, thin, and scale-like. In the eggs of locusts and grasshoppers, as well as certain Diptera, the ventral side of the embryo corresponds to the convex side, and the concave side of the egg to the dorsal region of the embryo (Figs. 502 and 493).

There is an anterior and posterior end or pole, the anterior end being that which in the body of the parent lies towards her head, or towards the upper or distal end of the ovarian tube. Towards this end lies in the later stages of embryonic life the head-end of the embryo, while the posterior end of the embryo is turned towards the hinder pole of the egg (Figs. 493 and 520).

=The egg-shell and yolk-membrane.=—The ripe egg is protected by two membranes: 1, an inner or _vitelline membrane_ or _oölemma_ (_dh_) (Fig. 500, _d_), produced in the egg by a hardening of the outer layer, and 2, the outer or _chorion_ (_c_), which is secreted by the cells of the ovarian follicle. The latter is divided into two layers: an inner, the _endochorion_, and an outer, the _exochorion_.

FIG. 503.—Fertilization of the egg of a round-worm (_Ascaris
megalocephala_): _A_, the ends (centrosomes) of the spindle formed.
_B_, the spindle completed; _sp_, sperm-nucleus, with its
chromosomes; _ei_, egg-nucleus; _p_, polar bodies.—After Boveri,
from Field’s Hertwig.
]

FIG. 493.—Eggs of Corixa: _A_, early stage before formation of the
embryo, from one side. _B_, the same viewed in the plane of
symmetry. _C_, the embryo in its final position; _a_, anterior, _p_,
posterior, end; _l_, left, _r_, right, _v_, ventral, _d_, dorsal,
aspect. (The letters refer to the _final_ position of the embryo,
which is nearly diametrically opposite to that in which it first
develops); _m_, micropyle; _p_, pedicle.—After Metschnikoff, from
Wilson.
]

FIG. 494.—Eggs of Phasmidæ: _A_, _Lonchodes duivenbodi_. _B_,
_Platycrania edulis_. _C_, _Haplopus grayi_. _D_, _Phyllium
siccifolium_.—After Kaup, from Sharp.
]

While the yolk-membrane is usually a completely homogeneous, thin, structureless membrane, the chorion or shell of the egg is usually covered with a network of ridges enclosing polygonal areas, varying in shape according to the species or genus. These external markings are due to the impress of the cellular structure of the epithelium of the ovarian follicle.

In the chorion of the cockroach the surface appears to be finely granular, the finest granules being arranged in large, more or less regularly hexagonal areas, which are bounded by narrow, dark spaces, containing somewhat larger though less dense granules. The surface of the eggs of certain Phasmids are variously sculptured (Fig. 494).

The true structure of the chorion can only be, as Wheeler observes,
seen in cross-sections, as shown by Blochmann, and also by Wheeler.
The chorion consists of two chitinous laminæ kept in close
apposition by means of numerous minute trabeculæ or pillars. It is
the ends of these pillars that look like granules. In the spaces
between the hexagonal areas, the trabeculæ are more scattered and
individually thicker than those of the hexagons.

FIG. 495.—Egg of cotton-worm moth, Aletia: _a_, top view, showing the
micropyle.—After Comstock.
]

FIG. 496.—Egg of _Danais archippus_.—After Riley.
]

These markings are of singular beauty and complexity in the eggs of many Lepidoptera, whose ova are variously ribbed, forming a beautiful fretwork of raised lines (Figs. 495 and 496), while in the Diptera and Hymenoptera the chorion is less solid, and usually smooth under low powers. The exochorion of the egg of the house and meat fly (_C. vomitoria_) is pitted with elongated hexagonal depressions, which cross the egg transversely. That of the honey-bee is also divided into long hexagonal areas (Fig. 497).

FIG. 497.—Egg with embryo of honey-bee, × 40: _ch_, chorion; _ga_,
ganglia; _s. ga_, brain; _jm_, jaw-muscles forming; _c_, œsophageal
collar; _fb_, fore intestine; _mb_, mid-intestine; _ab_,
hind-intestine.—After Cheshire.
]

FIG. 498.—Micropyle (_Mk_) of eggs; _a_, of a fly, Antomyia; _b_,
_Drosophila cellaris_; _c_, stalked egg of _Paniscus
testaceus_.—After Leuckart, from Perrier.
]

When the eggs are deposited in exposed places, and remain in such situations for several days, or weeks, or even through the winter, the shell is either solid and strengthened by the ribs and ridges; or the shell, if of winter eggs, is unornamented, and is dense and solid, to resist extremes in temperature or the attacks of egg-eating birds, mites, etc.

=The micropyle.=—This is an opening or canal, or, as in most insects, a group of canals situated at the anterior end of the egg for the entrance of the spermatozoa during the process of fertilization of the ovum (Fig. 498). In Acrydians, however, the micropyle is situated at the posterior end of the egg. The micropyle (Fig. 499) is a complicated apparatus within whose circumference the vitelline membrane appears to be firmly attached to the chorion, so that the perforation passes through the chorion as well as the yolk-membrane.

The micropyles of the cockroach are probably as simple and generalized as in any insect. Wheeler states that they are in Phyllodromia scattered over the end of the egg, “over a quadrant of the upper hemisphere, where the beautiful hexagonal pattern of the chorion gives away to an even trabeculation.” The micropyles are wide-mouthed, very oblique, funnel-shaped canals, perforating the chorion, the apertures of the funnels appearing under a low power as clear, oval spots, the long axis of which is parallel to the long axis of the egg.

FIG. 499.—_a_, fragment of a micropylar papilla, showing its lumen;
_b_, optical section of another papilla, in this one the lumen
extends to the vitelline membrane, but does not pass beyond it; _c_,
_d_, _e_, and _f_, papillæ of different forms. _A_, anterior end of
an ovarian egg, showing mode of growth of the micropylar papillæ:
_a_, _b_, two successive stages; _c_, surface view of modified
papillæ from the lower edges of the cap; _d_, tunica propria of the
ovariole; _e_, remnant of the cell-mass that secreted (?) the
micropylar cap.—After Ayers.
]

FIG. 500.—Egg of _Perla maxima_: _c_, chorion; _d_, oölemma; _gs_,
glass-like covering of micropyle; _l_, cavity under same; _g_,
canals penetrating chorion.—After Imhof, from Sharp.
]

“With a higher power the tube of each funnel is clearly visible as a
thin canal which dilates rapidly into the large oval aperture on the
outer face of the chorion. The narrow tube is sometimes fully as
long as the large orifice. The micropylar perforations are all
directed from the germarium to the vaginal pole of the egg. Hence a
line, the hypothetical path of the spermatozoön, drawn through one
of these oblique micropyles, and continued into the egg, would
strike the equatorial plane. The female pronucleus, as we shall see
further on, moves in this plane.” (Wheeler, p. 289.)

FIG. 501.—Micropyles: _a_, of _Nepa cinerea_; _b_, of _Locusta
viridissima_; _c_, of a bug (_Pyrrhocoris apterus_).—From
Gerstäcker.
]

The micropylar region is generally, at least in Orthoptera and Odonata, covered by a gelatinous cap (Figs. 499 and 500, _gs_), which may form a covering membrane which extends over a large part of the egg, or may envelop the entire outer surface. In some cases micropyles are scattered over the entire surface of the egg, but usually the perforation is situated at the end, and is often guarded by raised processes, either one or several, like bristles, or toadstools, etc., these being especially characteristic of the eggs of certain Hemiptera (Nepa, Fig. 501, _a_, and Ranatra), or the region is variously sculptured, as in the eggs of butterflies. In the micropylar apparatus of Œcanthus the papillæ have a distinct lumen (Fig. 499), or a channel for the ingress of the male filament.

FIG. 502.—Diagrammatic median section through egg of Musca in stage of
fertilization (incorporating the figures of Henking and Blochmann):
_ch_, chorion; _d_, dorsal; _v_, ventral side of the egg; _dh_,
yolk-membrane; _do_, nutritive yolk; _g_, gelatinous cap over the
micropyle (_m_); _K_, outer layer of plasma (Keimhautblastem); _p_,
male and female pronucleus before copulation; _r_, directive body
(Richtungskörper).—After Korschelt and Heider.
]

Another use of the micropylar apparatus noticed by Ayers in the egg of the tree-cricket is that it “serves as a thick, roughened plate, against which the insect may push when ovipositing, without injury to the egg, and without danger that the ovipositor slips from its place.” In Chrysopa eggs the micropyle forms a conspicuous button-like knob, resembling the finely milled head of a certain kind of screw.

=Internal structure of the egg.=—The egg-contents are surrounded by an outer layer of protoplasm or formative yolk, which is separate from the inner parts of the egg (Fig. 502, _do_), the latter being mostly composed of the nutritive yolk-element. The superficial protoplasmic layer, called by Weismann _Keimhautblastem_ (_K_) is, in a few cases, afterwards entirely lost, but in most instances forms a very thin layer of clear protoplasm, slight in extent compared with the yolk-mass within.

The eggs of insects are rich in yolk, only certain eggs, such as those of the Aphides and the egg parasites (Proctotrypidæ) being poor in yolk. The eggs of heterometabolous insects have been said by Brauer to contain relatively more yolk than those of the Metabola, particularly the Diptera; though, as Wheeler observes, this rule has some exceptions, the eggs of the 17–year Cicada being very numerous and small.

This he thinks is a greater advantage to the insect than the
production of a few large eggs, “when we consider the extremely long
period of larval life and the vicissitudes to which the larvæ may be
subjected during all this time.” “Similarly, _Meloë angusticollis_
produces a large number of very small eggs, while the eggs of the
smaller beetles (Doryphora, _e.g._) are much larger. But Meloë is a
parasitic form, and probably only a few of its many offspring ever
succeed in gaining access to the egg of the bee.”

In the eggs of Chrysopa the yolk-granules are remarkably small, so that the primitive band is in strong contrast to the yolk in color and density. When crushed, the yolk does not flow out as a liquid, but in a pasty mass, and we have questioned whether, as in the eggs of Limulus, whose yolk is solid with fine granules, the denseness of the yolk is not connected in the way of cause and effect with their exposed situation.

The central or yolk-mass (Fig. 502, _do_) consists chiefly of rounded masses of yolk, with fat-globules, between which extends a fine network of protoplasm.

The elements of the yolk are spherical and strongly refractive, by pressure becoming polygonal structureless homogeneous bodies.

The germinal vesicle of the ripe insect-egg lies in the centre of the yolk, where it appears as a large vesicle-like cell-nucleus containing a few chromatin elements.

_b._ Maturation or ripening of the egg

Before the eggs of animals can be fertilized, they require in some observed cases, and probably in animals in general, to undergo a series of changes, which, as observed in the starfish, etc., consists in the replacement of the germinal vesicle by a very much smaller egg-nucleus, and also at the same time the construction at one pole of the egg of the directive or polar bodies (Fig. 502, _r_). Towards the end of the ripening process of the insect egg this vesicle, according to Blochmann, passes to the dorsal surface of the egg, and is transformed into the directive spindles (_Richtungspindel_).

_c._ Fertilization of the egg

The egg next requires the penetration and admission into the yolk-interior of a spermatozoön.

This process is essentially in insects, as in other animals, the fusion of the sperm-nucleus with the nucleus of the egg. Under normal conditions but a single spermatozoön is required for fertilization. As shown by Hertwig, in the sea-urchin, after the spermatozoön has penetrated into the egg, the head, and the small rounded body, called a _centrosome_, can still be recognized, but the tail becomes fused with the yolk of the egg. In the protoplasm of the egg (called _cytoplasm_) the achromatic end of the sperm-nucleus gives rise to conspicuous rays, like those observed in ordinary cell-division. Preceded by these rays, the sperm-nucleus or male pronucleus (Fig. 502, _p_) moves towards the nucleus of the egg, and finally fuses with it, thus forming a new single nucleus. This latter, which is called “the cleavage nucleus,” rapidly forms a nuclear or “cleavage spindle” (Fig. 503). This act gives an impulse to the cleavage of the egg, which is the first step in the formation of the embryo. All these changes have yet to be worked out in detail in insects by microscopic sections of the egg, whose generally hard and opaque egg-shells present great obstacles to such work.

_d._ Division and formation of the blastoderm[80]

In insects as in most other Arthropoda the segmentation of the yolk is superficial and not total. The ovum is _centrolicithal_, _i.e._ the yolk is concentrated at the centre of the egg, and surrounded by a peripheral layer of transparent protoplasm (the _Keimhautblastem_).

FIG. 504.—Formation of the blastoderm of _Pieris cratægi_: _A_,
longitudinal section through the egg, with two masses of protoplasm
in the yolk. _B_, a blastoderm-cell at the upper end. _C_, a later
stage, with more blastoderm-cells.—After Bobretsky.
]

The first step in segmentation is the movement of the first division-nucleus (_i.e._ that in the fertilized egg arising from the union of the sperm-nucleus with the female pronucleus) towards the interior of the egg in order to multiply itself by the mode of indirect nuclear division (Figs. 504, _A_, and 507).

FIG. 505.—Embryology of the mole-cricket: 1, egg in which the amœboid
nuclei (_abc_) are moving toward the surface; 2, egg in which the
nuclei (_abc_) have reached the surface, and show an active
nucleus-formation; 3, the blastoderm-cells have no nucleus, and are
placed at equal distances apart; 4, the blastoderm-cells now forming
a continuous layer; 5, cross-section of the egg with blastodermic
disk, also showing the disposition of the endodermal cells; 6,
cross-section of the blastodermic disk, with the myoblast cells
(_mb_) already formed; 7, cross-section through the thorax of the
embryo, the body-cavity extended into the limbs.
]

LETTERING.

_abc_, amœboid blastodermic cells.

_bc_, blastoderm-cells.

_bl_, blastoderm.

_en_, endodermal cells.

_M″_, cavity of the myoblast.

_mb_, myoblast cells.

_N_, nerve-furrow.

_P_, primitive groove.

_pd_, primitive disk.

FIG. 505 _concluded_.—Later stages in the embryology of the
mole-cricket: 8, longitudinal section of the embryo; the
yolk-pyramids (_yp_) form a common inner yolk-mass (_y_). 9, section
through the heart; _H_, cavity of the heart; the two halves of the
heart-sinuses having united dorsally, ventrally they are still open
and are bounded by the walls of the mesenteron. 10, cross-section of
an embryo, showing the blood-lacunæ separated on the back by the
dorsal organ (_do_); the intestinal fasciated layer
(_Darmfaserblatt_) has not completely enclosed the yolk. 11, embryo
completely segmented, with the rudiments of the appendages, labrum
(_lab_), and nervous ganglia (_pc-ng_). 12, a more advanced embryo,
showing the stomodæum (_st_) indicated as a frontal protuberance.
13, section through the recently hatched larva, showing the cells of
the mesenteron or chyle-stomach, and the cellular layer on the front
surface, also the proventriculus or crop.
]

LETTERING.

_ant_, antenna.

_ar_, arterial sinus.

_bl_, blastoderm.

_bla_, abdominal vesicles.

_cr_, proventriculus, or crop.

_dm_, ventral diaphragm.

_do_, dorsal organ.

_d pm_, dorsal diaphragm.

_ent_, enteric layer.

_f_, fat-body.

_g_, ventral ganglion.

_H_, _ht_, heart.

_l_, lacuna.

_m_, mouth.

_md_, mandible.

_m.en_, mesenteron.

_mx′_, 1st maxilla.

_mx″_, labium, or 2d maxilla.

_ml_, leaf-like portion of mesenteron.

_oe_, œsophagus.

_pc_, procerebrum.

_pm_, proctodæum.

_sg_, subœsophageal ganglion.

_sm_, stomodæum.

_tg_, thoracic ganglion.

_vm_, ventral muscle.

_y_, yolk.

_yp_, yolk-pyramids.

_I_, 1st pair of feet.

_II_, 2d pair of feet.

_III_, 3d pair of feet.

—After Korotneff.

The origin of numerous division-nuclei as the offspring of the first has been observed to take place in the eggs of those insects (Aphides, Cecidomyia, and Cynips) which have a slight amount of yolk. Yet in the large, ordinary eggs of insects with an abundance of yolk there is no doubt, say Korschelt and Heider, that these numerous division-nuclei, which soon after the process of oviposition are scattered within the egg between the yolk-spheres, and are enveloped by a star-shaped protoplasmic layer, and which constitute the formative elements of the blastoderm,—there is no doubt but that they have practically arisen through indirect nuclear division from the first division-nucleus.

The process of formation of the blastoderm in ordinary eggs with abundant yolk was first observed by Bobretsky in the eggs of a moth (Porthesia) and Pieris, also by Graber, and more recently by Blochmann in Musca, and by Heider in Hydrophilus.

In the earliest stage observed by Bobretsky there first appear after fertilization a few (the smallest number four) cell-like, minute amœboid masses of protoplasm, each with a distinct nucleus. A few (one at least) of these bodies gradually pass out of the centre of the yolk to the surface of the egg (Fig. 504, _A_, _n_), these becoming larger and rounder, and from one or two of these nuclei (_B_, _bc_) the blastoderm originates (_C_, _bl_). Those nuclei remaining in the yolk increase in number and afterwards become the nuclei of rounded masses of yolk-granules, forming the so-called yolk-spheres which Bobretsky regards as true cells.

To the few blastoderm cells situated on the upper end of the egg are added others which continue to pass from the yolk to the periphery, and then the blastoderm spreads out farther and farther from the upper end of the egg until finally it covers or envelops the whole yolk. This layer of cells is called the _blastoderm_.

As to the origin of the primitive amœboid cells, Bobretsky is in doubt, but is disposed to think that they are the result of the subdivision of the germinative vesicle or nucleus of the ovarian egg-cell. In this connection may be quoted the observations of Graber, who states that an examination of the ovarian cell at an early period has revealed the presence, in the centre of the yolk, of a number of amœboid cells, which appear to have been formed by the division of the germinal vesicle. These “primary embryonic cells” have a relatively large nucleus and a number of nucleoli. Several may be seen to unite with one another by means of their pseudopodia, and they may also be observed to undergo division. With this account may be compared the results obtained by Korotneff in his work on the embryology of the mole-cricket (Fig. 505).

FIG. 506.—Four successive stages in the formation of the blastoderm of
_Calliphora vomitoria_ (the figures represent segments of
cross-sections through the fly’s egg): _A_, the nuclei of the
division-cells have arranged themselves parallel with the outer
surface of the egg. _B_, the division-cells fused with the
“keimhautblastem.” _C_, the outer surface becomes furrowed by
indentations; all the nuclei of the blastoderm-cells in process of
division. _D_, the blastoderm-cells form a high cylinder-epithelium:
_b_, “keimhautblastem”; _bz_, blastoderm-cells; _d_, nutritive yolk;
_dz_, yolk-cell; _fz_, so-called division-cell; _i_, inner
“keimhautblastem.”—After Blochmann, from Korschelt and Heider.
]

The result of these and of later observations, especially those of Blochmann on Musca, and those of Heider on Hydrophilus, show that the division-nuclei lie near the centre of the egg, along the longitudinal axis (Fig. 507, _A_). Each of these nuclei is enveloped by a star-shaped mass of protoplasm, and on the whole resembles a wandering amœboid cell. These isolated masses of protoplasm are all connected by a fine network of rays, which unite to form within the yolk a syncytium. Afterwards, in the later stages, these division-cells, as they may be, though somewhat incorrectly, regarded, move nearer the periphery and arrange themselves into a plane parallel with the surface (Figs. 506, _A_, 507, _B_). Continuing to divide, they reach the surface and fuse with the peripheral protoplasmic layer (Figs. 506, _B_, 507, _C_). Then follows the division into single cell-territories (Figs. 506, _B_, 507, _C_), corresponding to the division-nuclei, through the appearance of furrows which pass in from the outer surfaces of the egg into the interior and gradually penetrate the entire “keimhautblastem.” In this way the surface of the egg is covered with an epithelium (blastoderm). In many insects the so-called inner “keimhautblastem” (Fig. 506, _D_, _i_) is formed by the separation of a layer of protoplasm which contains larger granules and are accumulated between the blastoderm and the upper surface of the central nutritive yolk-mass. By the addition of this plasmic layer the cells of the blastoderm increase in height, and now form a cubical or cylinder epithelium, which continuously envelops the surface of the egg. (Korschelt and Heider.)

_e._ Formation of the first rudiments of the embryo, and of the
embryonic membranes

FIG. 507.—Formation of the blastoderm in Hydrophilus: _b_, completed
blastoderm; _d_, yolk; _f_, so-called division-cells; _k_,
“keimhautblastem”; _z_, yolk-cells.—After Heider, from Korschelt and
Heider.
]

The embryo first arises as a whitish streak or band-like thickening on the ventral side of the egg, and is variously called the “primitive streak,” “primitive band,” “germinal band,” or “embryonal streak.” In most cases the primitive band is divided at regular intervals by transverse furrows, indicating the limits of what are to be the body segments.

Cross-sections (Fig. 509) show that the band is composed of several layers, _i.e._ an outer layer (ectoderm) and an inner layer which comprises the endoderm and mesoderm, and so long as these two layers are not sharply differentiated from one another, this second layer may be called, with Kowalevsky, “the inner lower layer, or ento-mesoderm” (Figs. 508, 509, _B_, _C_, _u_).

It is characteristic of insects, only rarely occurring in other arthropods (_e.g._ the scorpion), that the primitive streak is not situated on the surface of the egg, but becomes overgrown by a folded structure (Fig. 508, _af_) rising from its edges, the amnion-fold, so that it appears somewhat depressed or sunken in under the upper surface of the yolk. While the amnion-folds are extending from all sides over the primitive band, there becomes formed under it, by the invagination of the outer surface of the egg, a cavity, the amnion-cavity (_ah_), which, when the amnion-fold has completely overgrown the primitive band and united together (Fig. 509, _C_), appears completely closed from without.

FIG. 508.—Two schematic median sections through an insect-embryo to
represent the development of the embryonal membranes. In _A_ the
primitive streak is not wholly overgrown by the amnion-fold. In _B_
the amnion-folds have united with each other and completely
overgrown the primitive streak: _a_, fore, _b_, hind, egg-pole; _v_,
ventral side; _d_, dorsal side; _af_, amnion-folds; _ah_,
amnion-cavity; _am_, amnion; _do_, yolk; _ec_, ectoderm; _k_,
head-end, _k′_, hinder-end, of the primitive streak; _s_, the part
of the serosa arising from the amnion-fold; _s′_, the part of the
serosa arising from the unaltered blastoderm; _u_, lower
layer.—After Korschelt and Heider.
]

=Formation of the embryonic membranes.=—The amnion-folds finally completely overgrow the primitive band (Fig. 509, _B_ and _C_), and form the embryonal membranes. The primitive band is seen after its completion to be overgrown by a double cellular epithelial membrane. The outer of these two membranes, that which arises from the outer leaf or layer of the amnion-fold, is the _serosa_ (Figs. 508, _B_; 509, _C_, _s_; 510). This passes continuously into the unchanged part of the blastoderm, which has no part in the formation of the primitive band and germ-layers, and which covers the outer surface of the yolk. Thus the _serosa_, which is usually held to include this portion also of the blastoderm, forms a closed sac which covers the whole surface of the egg, with one part extending over the surface of the yolk, and the other over the primitive band (Fig. 510).

FIG. 509.—Diagrammatic cross-section through three successive stages
of the primitive streak, and growing embryonal membranes of
insect-embryos. _A_, formation of the ventral plate and of the
gastrula invagination (_g_). _B_, upward growth of the amnion-folds
(_af_). _C_, complete overgrowth of the primitive band through the
amnion-folds: _v_, ventral side; _d_, dorsal side; _af_,
amnion-folds; _ah_, amnion-cavity; _am_, amnion; _bl_, blastoderm;
_bp_, ventral plate; _do_, yolk; _ec_, ectoderm; _s_, serosa; _u_,
under or inner layer.—After Korschelt and Heider.
]

The inner of the two layers, called the _amnion_ (Fig. 509, _am_), is more closely connected with the embryo. The amnion and ectoderm of the primitive band together form a completely closed sac, whose lumen forms the amniotic cavity. Originally connected with the serous membrane, it splits off from the primitive band about the time the appendages begin to bud out, and continues to closely envelop the body and appendages, as seen in Fig. 509. Both of these membranes are, before the time of hatching, either absorbed, or, as in Lepidoptera, retained. The amnion is retained until after hatching in the locust, etc. In certain Coleoptera the serosa is retained, and the amnion is absorbed (Fig. 532), while in Chironomus and the Trichoptera the serosa is absorbed, and the amnion retained, with the egg-shell or chorion. Hence we have eight layers in the winged insects[81] during embryonic life:

FIG. 510.—Surface view of fresh serosa from an Œcanthus, treated with
acetic carmine; the blastoderm completely formed, × 500: _p_, polar
body; _rf_, radiating fibres; _nls_, nuclear substance; _nlm_,
nuclear membrane.—After Ayers.
]

1. Exochorion. (Remains of the epithelium of the ovarian follicle.)
2. Chorion. (Egg-shell or cuticle secreted in the ovarian
follicle.) 3. Vitelline membrane. (Primary egg-membrane.
Yolk-skin or membrane.) 4. Serous or outer germ-membrane.
(Serosa.) } 5. Amnion or inner germ-membrane. } Derived from the
blastoderm. 6. Ectoderm. } } 7. Mesoderm. } Embryo. } 8.
Endoderm. } }

In the embryo of Xiphidium and Orchelimum Wheeler has found and
described with much detail a membranous structure which he calls the
_indusium_. “The organ,” he says, “appears to have been retained by
the Locustidæ, and completely lost by the embryos of other winged
insects.” It arises in Xiphidium, as a simple circular thickening of
the blastoderm, between and a little in front of the procephalic
lobes (Figs. 511, 512, _A-E_), and afterwards spreads over nearly
the whole surface of the egg, leaving the poles uncovered, as in
Fig. 513, where it is divided into two further membranes, the inner
and outer indusium, the former lying in contact with the amnion.
After this the serosa “is excluded from taking any part in the
development of the embryo; both its position and function are now
usurped by the inner indusium.”

Hence in an egg of the Locustidæ Wheeler distinguishes, passing from
within outward in a median transverse section of the egg, the
following envelopes:

1. The chorion. 2. The blastoderm-skin-like cuticle secreted by
the serosa. 3. The serosa. 4. The outer indusium. 5. A layer
of dark granular secretion (probably some urate). 6. The
cuticle secreted by the inner indusium. 7. The inner indusium.
8. The amnion. While envelopes 1–7 invest the whole egg; layer
8, the amnion, covers only the embryo.

FIG. 511.—Diagrams illustrating the movements and envelopes of the
embryo of Xiphidium: _A_, after the closure of the amnioserosal
folds. _B_, during the embryo’s passage to the dorsal surface.
_C_, just after the straightening of the embryo on the dorsal
surface; _ind_, indusium afterwards forming _ind^1_, the inner,
and _ind^2_, the outer indusium; _ch_, chorion; _sr_, serosa;
_am_, amnion; _gb_, germ-band; _v_, yolk; _bl. c_, blastoderm
membrane.
]

Wheeler further suggests that the so-called micropyle of the
Collembola (Anurida), which has been homologized with the “dorsal
organ” of Crustacea, is a possible homologue of the indusium, as
also the “primitive cumulus” of spiders, and the “facette” or
“cervical cross” of Pentastomids described by Leuckart and also by
Stiles.

=The gastrula stage.=—The primitive band invaginates so as to give the opportunity for the formation of the inner layer. This invagination, which at a certain stage is established along the whole length of the primitive band, forms a median furrow and may be regarded as the gastrula-invagination of insects. The lower (inner) layer thus arising afterwards spreads out under the entire primitive band (Fig. 509, _B_ and _C_), the edges of which become bordered by the growing amnion-fold. (Korschelt and Heider.)

In certain forms the primitive band arises from several separate
rudiments which afterwards unite. Thus in Musca and Hydrophilus the
anterior and posterior ends develop first, and in Hydrophilus the
procephalic lobes originate independently of the rest of the band.
In the Aphides, also, according to Will, these lobes arise
independently, afterwards uniting with the primitive band.

FIG. 512.—Diagrams illustrating the movements and envelopes of the
embryo of Xiphidium: _D_, the stage of the shortened embryo on the
the dorsal yolk. _E_, embryo returning to the ventral surface. _F_,
embryo nearly ready to hatch; _ch_, chorion; _b. lc_, blastoderm
membrane; _sr_, serosa; _ind^1_, outer indusium; _ind^2_, inner
indusium; _ind^2 + am_, inner indusium and amnion fused; _am_,
amnion; _ind^1 c_, cuticle of the inner indusium; _ind^2 s_,
granular secretion of the inner indusium; _am. s_, amniotic
secretion; _v_, yolk; _cl_, columella; _gb_, primitive band.
]

=Division of the embryo or primitive band into body-segments.=—Meanwhile the primitive band grows at the expense of the yolk, spreading out more and more over its surface, until in certain cases (Coleoptera, Diptera, Siphonaptera, and Trichoptera) it lies like a broad ribbon over the yolk, so that the two ends nearly meet on the dorsal side. By this time it becomes divided by transversely impressed lines into segments, which correspond to those of the larva and adult. The first of these segments is divided into two broad and flaring flaps, which are called the procephalic lobes. It becomes the antennal segment.

FIG. 513.—Two stages in the spreading of the indusium. _A_, lateral
view of egg just after the arrival of the embryo on the dorsal yolk.
_B_, lateral view of the egg with the indusium nearly reaching the
poles. _C_, same egg seen from the dorsal surface.
]

The mouth (_stomodæum_) now develops, and is situated at the anterior,[82] and the rectum (_proctodæum_,) at the posterior pole, or end of the primitive band.

FIG. 514.—Median section of the egg of _Anurida maritima_: _do_,
“micropyle”; _bld_, blastoderm.—This and Figs. 511–513, after
Wheeler.
]

In Blatta, Hydrophilus, the Trichoptera, and the Lepidoptera the hindermost part of the primitive band is turned in ventrally (Figs. 534, _C_).

The preceding account of the relations of the primitive band to the
yolk does not apply to all insects, since there are variations which
appear to depend on the form of the egg, and on the amount and
distribution of the yolk-masses. In certain Coleoptera, the
primitive band sinks down and thus becomes immersed into the yolk.
In Donacia (Kölliker and Melnikow) and Hydrophilus (Heider), and in
the Chrysomelidæ and Attelabus, a weevil, as we have observed, the
primitive band rests on the outside of the yolk, but in _Telephorus
fraxini_ it is immersed. In the Hemiptera it is immersed (Fig. 516),
but there is much variation in this respect, the degree of immersion
being most marked in the Coccidæ (Aspidiotus), and least so in
Corixa. Besides the position of the primitive band, there are in
Odonata and Hemiptera differences in the origin of the primitive
band itself and of the embryonic membranes.

FIG. 515.—Ventral view of five developmental stages of Hydrophilus:
_a_ and _b_, places at which the blastopore contracts; _af_, edge
of the amnion-fold; _af′_, caudal fold; _af″_, paired head-fold of
the amnion; _an_, antenna; _es_, last segment; _g_, pit-like
invagination (first indication of the amniotic cavity); _k_,
head-lobes; _r_, furrow-like invagination; _s_, portion of the
primitive streak covered by the amnion.—After Heider, from Lang.
]

FIG. 516.—Embryo of the louse: _am_, serosa; _db_, amnion; _as_,
antenna; _vk_, clypeus.—After Melnikow.
]

Korschelt and Heider divide the early embryo of insects into two
types:

1. Into those with a superficial primitive band; viz., where there
is no passage of yolk-elements into the space between the amnion and
serosa. The primitive band has in such cases a relatively
superficial position (Figs. 508, 509, 521, 535). Examples are
certain Orthoptera (Blatta, Œcanthus, Mantis, Gryllotalpa), also
certain Hemiptera (Corixa), certain Coleoptera, and the Trichoptera,
Diptera, and Hymenoptera.

2. Into those with an immersed primitive band, with the space
between the serosa and amnion filled with yolk (Figs. 517, 518,
534). Examples are the orthopterous Stenobothrus, Odonata, many
Hemiptera (the Pediculina and Pyrrhocoris), the Coleoptera already
mentioned, and Lepidoptera.

It should be observed, however, that these differences are of little
phylogenetic or taxonomic value, since genera of the same order,
notably the Coleoptera, differ as to the position of the primitive
band, so also two orders so nearly allied as the Trichoptera and
Lepidoptera.

=Differences between the invaginated and overgrown primitive
band.=—In respect to the mode of origin of the primitive band and
its relative position, there are two opposite types, though
connected by transitional forms. In the one case the primitive band,
_i.e._ its ventral portion, the “ventral plate” (Fig. 518, _b_, _p_)
is pushed in or invaginated in the interior of the egg; in the other
case it becomes overgrown by the folds of the amnion arising from
its edges.

FIG. 517.—Primitive streak of a lepidopter in cross-section: _ah_,
amniotic cavity; _am_, amnion; _c_, cœlomic cavity; _do_,
nutritive yolk, divided into single nucleated masses; _ec_,
ectoderm; _m_, mesoderm; _pr_, primitive thickenings of the
ventral nervous cord; _s_, serosa.—Combined figure after those of
Brobretsky and Hatschek, from Korschelt and Heider.
]

FIG. 518.—Five diagrammatic median sections representing the growth
of a dragon-fly (Calopteryx). _A-C_, development of the primitive
streak (_k_, _k′_) by invagination. _D_, the amnion-fold (_af_),
growing over the head-end of the primitive streak. _E_, closing of
the opening of the amnion-cavity (_ah_): _v_, ventral, _d_, dorsal
side; _a_, fore, _b_, hind end of egg; _bl_, blastoderm; _bp_,
ventral plate; _do_, yolk; _k_, head-end, _k′_, caudal end, of the
primitive streak; _kh_, germinal thickening or initial point of
invagination; _s_, serosa.—After Brandt, from Korschelt and
Heider.
]

In insects with an overgrown primitive band, the band at the
beginning is generally short and always situated on the ventral side
of the egg, with the head-end looking forward, and remains in this
position throughout embryonic life. There is no revolution of the
embryo. The embryonal membranes arise through the formation of
folds.

FIG. 519.—Three embryonic stages of Calopteryx: _am_, amnion; _g_,
edge of the ventral plate; _ps_, germ of primitive band; _se_,
serosa.—After Brandt, from Balfour.
]

FIG. 520.—Three farther stages of growth of Calopteryx. _B_ and _C_
show the inversion of the embryo: _a_, opening of the
amniotic-cavity, out of which the embryo emerges; _ab_, abdomen;
_am_, amnion; _at_, antenna; _md_, mandible; _mx^1_, _mx^2_, 1st
and 2d maxillæ; _œ_, œsophagus; _p^1_, _p^2_, _p^3_, legs; _se_,
serosa; _v_, anterior end of the primitive streak.—After Brandt,
from Balfour.
]

In insects with an invaginated primitive band, of which the Odonata
afford examples, the first rudiment of the primitive band is in the
form of a ventral plate of slight extent passing ventrally in the
hinder half of the egg, in whose posterior section a process of
invagination (Fig. 518, _A_, _kh_), soon occurs. The cavity of this
invagination is the first indication of the amnion-cavity (Fig. 518,
_B_, _ah_), while its wall in its thickened ventral part (_K′_) is
concerned in the formation of the primitive band, and, in its dorsal
thin part, in the formation of the amnion (_B_, _C_, _am_).

=Revolution of the embryo where the primitive band is
invaginated.=—At first the head-end of the embryo is directed
towards the posterior end of the egg, as in dragon-flies (Fig. 518).
Also that surface of the primitive band which afterwards faces the
ventral, is at first turned towards the dorsal side of the egg. In
order to bring the primitive band into the later relations, there
must occur the process of revolution, or turning, of the embryo. The
somewhat advanced embryo of the Odonata, after the appearance of the
head and thoracic appendages, undergoes a rotating motion around its
transverse axis, and at the same time turns out of the amniotic
cavity (Fig. 520, _B_). This process is so managed that near the
head-region, the amnion and serosa, there closely situated to each
other, are fused together, and at this place tear or burst open.
Through this rent (_a_), in the same place in which the original
invagination-opening was situated, the amniotic cavity again opens,
and through the opening thus formed first the head and then the
succeeding segments of the primitive band (Fig. 520, _B_) pass out,
and remain there while the head passes on to the anterior pole of
the egg on the ventral side, the embryo thus assuming a position
like that of other insects. (Kowalevsky.)

In the parasitic Hemiptera (Pediculina), according to Melnikow, the
opening in the membranes near the head remains permanent, and the
embryo becomes everted through it, while the yolk, enclosed in the
continuous membrane formed by the amnion and serous membrane, forms
a yolk-sac on the dorsal surface. The same process occurs in
Mallophaga, and also in Œcanthus, as described by Ayers (Fig. 521).
Generally as soon as the embryo passes out of the amniotic cavity
the latter soon becomes smaller and finally completely disappears.

FIG. 521.—Revolution of the embryo of Œcanthus (diagrammatic): _a_,
fore, _b_, hind end of egg; _am_, amnion; _d_, dorsal, _v_,
ventral side of egg; _k_, primitive streak; _r_, dorsal plate
(originating by the contraction of the serosa (_s_)).—After Ayers,
from Korschelt and Heider.
]

As the embryo grows, and the sides grow up and the back closes over,
the contents of the yolk-sac are soon taken up and absorbed in the
intestinal cavity, which communicates with it.

In Phyllodromia, according to Wheeler, the process of revolution is
“hurried through by the embryo from the beginning of the 16th to the
end of the 17th day.” Several successive stages are represented in
Fig. 522. In the 15th day the embryo still occupies the middle of
the ventral surface of the egg. Soon the envelopes (amnion and
serosa, _as_) rupture, an irregular slit being formed, and soon the
egg and embryo are as seen in Fig. 522, _B_, the embryo standing out
free from its envelopes on the yolk, and the edges of its dorsal
growing walls (_b_) are distinctly marked. The tail now lies at the
caudal end of the egg (Fig. 522, _C_). By the 17th day the walls
have closed in the median dorsal line, and the embryo has grown in
length to such an extent as to bring its head to the cephalic pole
(Fig. 522, _E_).

Korschelt and Heider consider, since the primitive band of the
chilopod myriopods (Geophilus) is curved in at the middle and sinks
into the interior of the yolk, that in insects the invaginated
primitive band is the ancestral or primitive one, the overgrown
primitive band being derived from it. The overgrown primitive band
by its position may also be better insured against certain
mechanical attacks, perhaps also against the danger of drying up.

FIG. 522.—Embryo of Phyllodromia, 15 days old; revolution about to
begin. The stages in revolution are represented, after the rupture
of the amnion and serosa, in _A_ to _E_, which are from embryos 16,
16½, 16¾, and 17 days old respectively: _as_, amnion and serosa;
_s_, edge of serosa; _b_, dorsal growing body-wall; _d.o_, dorsal
organ; _x_, clear zone covered with scattered amniotic nuclei.—After
Wheeler.
]

_f._ Formation of the external form of the body

=Origin of the body-segments.=—As we have seen, the first traces of segments appear very early, the primitive band being divided by superficial transverse furrows into segments. This segmentation into arthromeres (somites or metameres) can be observed in Hydrophilus and Chalicodoma at a time when gastrulation begins (Figs. 515, 536). The segmentation extends not only across the median portion of the primitive band, through whose invagination the inner layer (endomesoderm) results, but also to the lateral portions which become a part of the ectoderm of the primitive band. These transverse furrows correspond to thinner places in the epithelium, which in this stage forms the embryonal rudiment. It thus happens that, in the forms named, after the end of gastrulation not only the ectoderm, but also the endomesoderm, is already segmented.

So early an appearance of segmentation as that observed in
Hydrophilus and Chalicodoma we must regard as a falsification of the
process of development due to heterochrony. We must consider the
conditions observed in other forms as the primitive ones, in which
(as, for example, in Lina and in Stenobothrus, according to Graber)
the gastrulation and separation of the ectoderm occurs in the still
unsegmented primitive band, the division into segments occurring in
later stages (Fig. 524). In these forms, then, the segmentation
affects the invaginated endomesoderm, as well as the ectoderm.
(Korschelt and Heider, p. 789.)

FIG. 523.—Diagrammatic cross-section through three successive
stages of Gryllotalpa, showing the formation of the heart.
(Compare Fig. 505.) The germs of the glandular intestinal layer
(_darmdrüsenblatt_) are omitted. _A_, earliest stage; the
primitive streak extends from _*x_ to _y*_. The embryonal
membranes are torn and pressed against the back: _am_, edge of
the rent; _rp_, dorsal plate (serosa); _l_, lamella (amnion
turned up) standing in connection with the ectoderm of the
primitive streak. _B_, second stage; the primitive streak has
completely grown around the yolk; the dorsal organ is absorbed.
_C_, third stage, dorsal portion; the formation of the heart is
finished: _am_, vestige of the amnion-fold; _bs_, blood-sinus;
_dd_, rudiment of the dorsal diaphragm; _dv_, ventral diaphragm
(compare Fig. 505); _do_, yolk; _dz_, yolk-cells; _ec_,
ectoderm; _gr_, vascular groove (rudiment of the heart); _l_,
lamella of the upturned amnion; _lh_, definite body-cavity; _m_,
transverse muscle; _n_, nervous cord; _r_, heart; _rp_, dorsal
plate; _sp_, splanchnic; _so_, somatic layer of the mesoderm;
_us_, primitive segmental cavity; _*x_, _y*_, lateral
terminations of the primitive streak.—After Korotneff, from
Korschelt and Heider.
]