the growth of the primitive band around the yolk, the band widening, so
that its edges behind the head extend up, and finally meet on the back,
forming the back or tergum of the embryo, thus enclosing the yolk (Fig.
530, _F_). The tergal wall of the head is due to the dorsal growth of
the head-lobes, and of the clypeo-labral region. In the course of this
process the anterior end of the primitive band becomes turned up
dorsally, forming a dorsal curve or bend. By this bending up of the
primitive band the forehead nearest the mouth forms a transverse ridge,
the labrum, while the basal or earlier part of the forehead now is
differentiated into the clypeus. This clypeo-labral region likewise
forms the roof or palatal region of the mouth. The head-lobes cause by
this dorsal growth a rotating motion which carries the rudimental
antennæ back over the mouth.
[Illustration:
FIG. 530.—Diagram of the formation of the dorsal organ in Hydrophilus.
_A_, cross-section through an egg, whose primitive streak is still
covered over by amnion (_a_) and serosa (_s_). _B_, amnion and
serosa are grown together in the middle line, then separated and
drawn back to form a fold on each side. _C_, by the contraction of
the serosa (_s_),which becomes converted into the dorsal plate, the
folds become drawn up dorsally. _D_, the contracted serosa becomes
partly overgrown by the folds. _E_, the folds grow together to form
the dorsal tube. _F_, the mid-gut has closed over dorsally and
enclosed the dorsal tube (_s_): _a_, amnion; _d_, yolk; _ec_,
ectoderm; _h_, heart; _l_, body-cavity; _m_, rudiment of the
mid-gut; _n_, nervous system; _s_, serosa (in _C_ and _D_ = dorsal
plate, in _E_ and _F_, dorsal tube); _tr_, the chief tracheal
stem.—After Graber and Kowalevsky, from Lang, and Korschelt and
Heider.
]
The gnathal or post-antennal segments at first bear but a small part in
completing the tergal region of the head, but shortly before hatching
the mandibles and their muscles enlarge, giving fulness to the upper and
back part of the head.
_i._ Dorsal closure and involution of the embryonic membranes
[Illustration:
FIG. 531.—Schematic figure of the formation of the dorsal tube by
invagination of the dorsal plate (transformed serosa); following
after stage Fig. 520, _C_, and Fig. 521, _D_; _am_, amnion (now
forming the provisional dorsal closure); _r_, dorsal tube, whose
cells are already breaking away.—After Korschelt and Heider.
]
In most other Arthropoda (Crustacea, Arachnida, Myriopoda, etc.)
development goes on by the formation of a so-called primitive band, but
without the appearance of peculiar embryonic membranes. The outer
surface of the entire egg becomes, then, in part covered by the
band-like embryonic germ, and partly by a portion of the blastoderm
which remains unchanged. The dorsal region is formed by the widening and
spreading of the primitive band over the greater part of the surface of
the egg, while the area of the unchanged section of the blastoderm
continually becomes more restricted. It is generally accepted that the
latter is concerned in the dorsal closure, because, together with a
histological transformation, it becomes involved in the formation of the
ectoderm of the primitive band.
A similar form of retrograde structure possibly occurs in the
embryos of Poduridæ, in which a dorsal organ has been observed to
develop in an early embryonic stage, which bears some relation to
the cuticula enveloping the embryo, but whose significance is in
general rather obscure.
In most insects the relations are more complicated, since in such
cases, the amnion-folds rise on the edges of the primitive band and
of the unchanged section of the blastoderm, whose retrograde
development is intimately connected with the closure of the back.
A very simple case of dorsal closure, but which certainly is not a
primitive one, occurs in Muscidæ and certain other Diptera whose
amnion-folds are developed in a rudimentary way. In this case
(according to Kowalevsky and Graber), the amnion-folds become
smoothed out again. Amnion and serosa become then a simple
epithelium, which throughout corresponds to the unmodified type of
blastoderm of Crustacea, Arachnida, and Myriopoda, and here seems to
share in the formation of the back. More complicated and very
manifold relations of dorsal closure and involution of the embryonal
membranes occur in other insects, of which Korschelt and Heider
distinguish four different types:
1. Involution under the formation of a continuous dorsal
amnion-serosa-sac (Odonata).
2. Involution with exclusively dorsal absorption of the amnion
(Doryphora).
3. Involution with exclusively dorsal absorption of serosa and
separation of the amnion (Chironomus and Trichoptera).
[Illustration:
FIG. 532.—Diagram of the formation of the dorsal walls in Doryphora
in cross-sections: _am_, amnion; in _B_, serving as a provisional
dorsal closure, in _C_, about to break up; _k_, primitive band;
_s_, serosa.—After Wheeler, from Korschelt and Heider.
]
4. Involution with separation of both embryonic membranes
(Lepidoptera and Hymenoptera, Hylotoma).
[Illustration:
FIG. 533.—Involution of the embryonic membranes of Chironomus: _am_,
amnion; _r_, dorsal umbilicus; _s_, serosa, which has withdrawn
into the region of the dorsal umbilicus, and in _C_ has passed
into the interior of the embryo.—After Graber, from Korschelt and
Heider.
]
The first type occurs in the most primitive order of winged insects.
The second type (Coleoptera) appears to be an independently
inherited form of dorsal closure. In the first type, the formation
of the amnion-serosa-sac is initiated by a rupture of the two fused
embryonic membranes. This rupture in the ventral middle line occurs
in Odonata only in the region of the head-section. In the second
type only the amnion, in the third only the serosa are concerned in
this rupture, while in the fourth type both membranes remain intact
until the slipping out of the larva. (Korschelt and Heider.)
_j._ Formation of the germ-layers
[Illustration:
FIG. 534.—Diagram showing the formation of the embryonic membranes in
Lepidoptera (_A_, after Kowalevsky, _B_ and _C_, after Tichomiroff):
_k_, primitive band; _am_, amnion: _se_, serosa; _do_, yolk; _vd_,
invagination of the fore-gut, _ed_, of the hind-gut; _m_, mouth;
_an_, anus; _x_, dorsal umbilical passage.—From Korschelt and
Heider.
]
The older views on the structure of the layers of the primitive band of
insects were thoroughly unsatisfactory. Bütschli first found that in the
bee, by a kind of folding process, an inner layer of the primitive band
arose. Soon afterwards Kowalevsky, by the employment of section-cutting
and thorough researches, laid the foundation of a more exact knowledge
of these layers. He found that in Hydrophilus a furrow extended along
the whole length of the primitive band (Fig. 515, _A_, _B_, _r_), which,
while invaginating or sinking in, gave rise to the inner layer of the
primitive band, i.e. the common rudiment of endoderm and mesoderm (Fig.
539, _A-C_).
Kowalevsky also found similar conditions in the honey-bee (Apis),
Lepidoptera, and other forms. The furrow above mentioned must be
regarded as a very long gastrula invagination, extending along the
entire ventral side of the embryo, and the edges of the furrow as a
long-drawnout blastopore. The tube arising in Hydrophilus through the
closing of the furrow we may regard as a primitive intestinal canal.
The first rudiment of the gastrula furrow appears in insects as two
folds extending along both sides of the median line in the thickened
ventral plate (Fig. 536, _f_), through whose formation a more median
section of the ventral plate, the so-called middle plate (_m_), becomes
separated from the side plates (_s_). As the middle plate curves in and
becomes overgrown by the folds forming the edges of the blastopore, the
gastrula-tube (Fig. 539, _A_, _r_) is formed, and furnishes the
rudiments of the lower (inner) layer. The ectoderm, then, according to
Heider, arises from the lateral plates of the primitive band. The growth
of the edges of the blastopore, by which the closure of the
gastrula-tube is effected, takes place latest in the region of the most
anterior part of the furrow (Fig. 515, _B_ and _C_), corresponding to
that place in the primitive band in which the stomodæum afterwards
develops.
[Illustration:
FIG. 535.—Two embryonic stages of a saw-fly (_Hylotoma berberidis_) in
schematic median section: _a^1–a^{10}_, 1st to 10th abdominal
segments; _bg_, ventral nervous cord; _og_, brain; _ol_, germ of
labrum; _sp_, salivary gland; _ed_, hind-gut; _x_, _x′_, inner folds
of amnion: other letters as before.—After Graber, from Korschelt and
Heider.
]
[Illustration:
FIG. 536.—Gastrula stage of the wall-bee (Chalicodoma), so-called
flask-shaped stage: _f_, folds which on each side border the middle
plate (edge of the blastopore); _m_, the partly segmented middle
plate (here = rudiment of the mesoderm); _s_, the segmented lateral
plate (becoming afterwards the ectoderm of the primitive band);
_ve_, fore, _he_, hinder entodermal rudiment.—After Carrière, from
Korschelt and Heider.
]
[Illustration:
Fig. 537.—Two successive stages in the gastrulation of Apis.
Cross-section through the primitive band: _b_, lower (inner) layer;
_ec_, ectoderm.—After Grassi, from Korschelt and Heider.
]
During the invagination of the middle plate and its transformation into
the gastrula-tube a change takes place in its histological character
(Fig. 539, _A_ and _B_). While it originally consists of a high cylinder
epithelium, which after farther changes becomes divided into several
layers, since the wedge-shaped single cells push themselves over each
other, the cells in later stages become more and more cubical or
irregularly polygonal (Fig. 539, _B_), and are irregularly arranged. At
the same time the gastrula-tube is compressed in a dorso-ventral
direction. While it in this way spreads out laterally under the side
plates (ectoderm), its originally circular primitive lumen passes into
the form of a horizontal fissure, which in Hydrophilus long remains as
the boundary between the two layers of the inner (or lower) membrane.
(Korschelt and Heider.)
There are numerous variations of the process of gastrulation, which
are by Korschelt and Heider divided into three types, as follows:—
1. Through invagination and formation of a tube (Fig. 539, _A_,
Hydrophilus, Musca, Pyrrhocoris, etc.).
2. By a lateral overgrowth (Fig. 537, Lepidoptera and Hymenoptera).
3. By an inward growth of cells from a median furrow (Aphides and
Trichoptera).
In Doryphora and Lina (Fig. 524) the hinder end of the gastrula
furrow is forked.
[Illustration:
FIG. 538.—Diagrammatic sketch of the formation of the germinal layers
in Doryphora: _A_, view of upper surface. _B_, cross-section through
the fore end of the primitive streak at the line _a-a_. _C_, section
through the middle of the primitive streak corresponding to the line
_b-b_. _D_, section through the hinder end of the primitive band
corresponding to the line _c-c_: _bl_, blastopore; _ec_, ectoderm;
_en′_, anterior U-shaped; _en″_, hinder U-shaped germ of the
endoderm; _ms_, mesoderm.—After Wheeler, from Korschelt and Heider.
]
The cellular layer arising from the gastrula invagination (lower layer)
forms the common germ of the endoderm and mesoderm. It has only recently
become known how these two germ-layers of insects have become
differentiated. Kowalevsky first discovered in Musca that the greatest
part of the lower (inner) layer yielded mesoderm exclusively, and that a
cell-mass only corresponding to the most anterior and posterior end of
the primitive band was used in the formation of the endoderm. We must
therefore, in insects, speak of a fore and a hinder endodermal rudiment.
In proportion, now, as the ectodermal invaginations, which are destined
to form the stomodæum and the proctodæum sink beneath the surface of the
embryo, the cell-masses of which the two endodermal rudiments are
composed are pushed farther in, and a separation between them and the
mesoderm is thus effected. The two endodermal rudiments now form
accumulations of cells which lie closely adjacent to the blind ends of
the stomodeal and the proctodeal invaginations. They soon widen out into
two hour-glass-shaped rudiments, which are directed with their
concavities towards each other, but with their convex side towards the
nearest pole of the egg. They soon change their form; two lateral
stripes grow out from them, and each now assumes the form of a U (Fig.
538, _en′_). The limbs of the fore and hind U-shaped rudiment are
directed toward each other, and grow towards each other until they meet,
and are fused together. Thus the endodermal rudiments arising out of the
fusion of the two U-shaped rudiments form two stripes extending along
the primitive band and situated mostly under the primitive segments. At
the two ends the endodermal rudiment fuses with the stomodeal and
proctodeal invaginations. These lateral endodermal streaks now spread
out, and gradually begin to grow over the yolk, on whose outer surface
they lie. This overgrowth makes the greater advance on the ventral side,
so that the two endodermal streaks first unite in the ventral median
line and afterward in the dorsal. The yolk in this way passes completely
into the interior of the rudiment of the mid-intestine.
Kowalevsky has already proved that it is the median parts only of
the inner layer which at the two ends of the primitive band become
separated as endodermal rudiments through the advance of the
stomodeal and proctodeal invaginations: the lateral portions become
mesoderm.
Kowalevsky has compared the germ-layers of insects with those of
Sagitta. This comparison is supported by the later researches of
Heider and of Wheeler on Coleoptera. (See Korschelt and Heider, p.
809.)
Relations somewhat different from the common type of formation of
germ-layers occur in Hymenoptera. Kowalevsky and also Grassi agree
that here also the endoderm originally forms a part of the lower
(inner) layer. But the separation of the endoderm from the mesoderm
goes on in Apis in such a way that the two ends of the inner layer
pass up to the dorsal side of the egg, where the fore and hind
rudiments of the endoderm extending along the back of the embryo
grow together. When the two horseshoe-shaped rudiments have met each
other and become fused, the enclosing of the yolk begins, which
accordingly here proceeds from the dorsal towards the ventral side,
instead of _vice versa_. As a result the endodermal cell-layer in
Apis (and also Chalicodoma) at first does not lie under the
primitive band, but on the dorsal side of the egg under that flat
epithelium, which, arising from the amnion-fold, completes the
provisional closure of the back.
The yolk-cells and secondary yolk-segmentation are discussed by
Korschelt and Heider at this point. The yolk-cells are elements
scattered throughout the yolk and which partly remain in the yolk
during the formation of the blastoderm (Fig. 507, _C_ and _D_), but
which in part through a later immigration pass out of the blastoderm
into the yolk. Graber has proved the fact of the migration of cells
from the lower layer into the yolk, and his observations have been
confirmed by other authors. Indeed, in certain cases (Melolontha),
these later immigrant cells are clearly distinguishable by their
histological characters from those originally found in the yolk.
The yolk-cells are regularly scattered throughout the yolk. Their
use to the embryo lies in the fact that they absorb the particles of
yolk, which they digest and thus reduce to a fluid condition. It
usually happens that after the complete formation of the primitive
band there results a delimitation of the areas enclosing each
yolk-cell, and this occurrence is called _secondary yolk-division_.
In special cases (Apis, Musca) this occurrence seems not to take
place. The yolk-cells are still, after the complete formation of the
mid-intestine, to be recognized in the yolk-remnants filling the
interior of the same, and gradually become absorbed.
_k._ Farther development of the mesoderm. Formation of the body-cavity
We have seen that by means of an invagination extending throughout the
entire length of the primitive band a layer of cells is produced which
soon spreads out on the inner side of the band and thus forms a second
lower (inner) layer (Fig. 539, _C_). From this inner layer is separated
at the anterior and posterior ends of the primitive band, the endoderm,
which lies in direct contact with the invaginations of the proctodæum
and stomodæum. The remainder, by far the most extensive part of the
inner layer, is the mesoderm.
The mesoderm now becomes divided into two lateral streaks (mesodermal
streaks), by the withdrawal of its cells from the median line (Fig. 539,
_D_). This withdrawal is not, however, always a complete one. In the
free median space thus formed, the yolk often forms the so-called
_median yolk-ridge_. Segmentally arranged cavities soon appear in the
lateral region of the mesoderm (the primitive segmental cavities), and
the bordering mesoderm-cells arrange themselves in the form of an
epithelium, and constitute the wall of the primitive segments or
cœlom-sac. (Korschelt and Heider).
The primitive segmental cavities in general arise through a split in
the mesoderm. In Phyllodromia, according to Heymons, the primitive
segments are very extensive. The mesoderm, at the time of the
formation of the rudiments of the appendages, is raised with the
ectoderm from the surface of the yolk, and in this way there arise
in each segment cavities, which, since they are surrounded by
mesodermal elements, become the closed cœlom-sacs (Fig. 540, _c_,
_c′_, _c″_).
The cœlom-sacs differ in different groups. They are largest in
Orthoptera (Phyllodromia), where they take up almost all the cell
material of the mesoderm in their formation, and exhibit certain
conditions recalling those of Peripatus. The very large primitive
segmental cavities, which in Orthoptera also extend into the
rudiments of the appendages (Fig. 540, _B_, _ex_), in their later
stages are, through the formation of a constriction, divided into a
dorsal and a ventral half (Fig. 540, _B_, _c′_, _c″_). The ventral
portions of these cavities extending into the extremities soon
disappear, while the cells of their walls lose their epithelial
nature, and group themselves irregularly into a sort of mesenchym.
In this tissue, then, arises, partly through a separation among its
cells, partly through the elevation of the same from the upper
surface of the yolk, _the definite body-cavity_. The dorsal portions
of the primitive segmental cavities remain unchanged a longer time
in order to play a rôle in the formation of the intestinal muscular
layer, of the heart, pericardial septum, and sexual organs.
[Illustration:
FIG. 539.—Cross-section through the primitive streak of Hydrophilus in
six successive stages: _A_, gastrula-stage (compare Fig. 515, _A_,
corresponding to the point _a_). _B_, cross-section through stage,
Fig. 515, _D_, in the most anterior section of the primitive band,
where the same is not completely overgrown by the amnion-folds. _C_,
cross-section through the trunk-segment of stage, Fig. 515, _E_.
_D_, _E_, _F_, cross-sections through later stages: _am_, amnion;
_b_, lower (inner) layer; _d_, yolk; _dz_, yolk-cells; _ec_,
ectoderm; _en_, entoderm; _l_, definite body-cavity; _pr_, primitive
groove (= neural groove); _pw_, primitive roll, or strip, of the
ventral nerve-cord; _r_, blastopore; _sp_, fissure in the mesoderm
(remains of the cavity of the primitive intestine); _se_, serosa;
_s_, lateral cord of the rudiment of the nervous cord; _spm_,
splanchnic layer of the mesoderm; _tr_, rudiment of a trachea (in
_E_ appearing as an invagination of the ectoderm) in _F_ in
cross-section; _us_, primitive segment (= cœlomic sac).—After
Heider, from Lang.
]
[Illustration:
FIG. 540.—Cross-sections through the abdominal part of three
successive stages of evolution of _Phyllodromia germanica_: _am_,
amnion; _bg_, rudiment of the ventral nervous chord; _c_, cœlomic
cavity; _c′_, dorsal, and _c″_, ventral, section of the cœlomic sac;
_cz_, cells of the walls of the primitive segment, which are joined
to the genital rudiments; _gz_, genital cells; _dw_, dorsal wall of
the cœlomic sac; _d_, yolk; _ec_, ectoderm; _ep_, epithelium-cells;
_ex_, rudiment of the abdominal appendages; _f_, germ of the
fat-body; _lw_, lateral wall of the cœlomic sac; _m_, mesoderm
cells, which take no part in the formation of the cœlomic sac; _mw_,
median wall of the cœlomic sac; _so_, somatic mesoderm layer; _vm_,
ventral longitudinal muscle.—After Heymons, from Korschelt and
Heider.
]
In the highest groups of insects (Coleoptera, Lepidoptera, and
Hymenoptera) the primitive segments are not so extensively developed
(Fig. 539, _D-F_, _us_). They here form only relatively small sacs
situated in the lateral parts of the primitive band which correspond
to the dorsal section of the cœlom-sacs of Orthoptera. The ventral
part is here from the very outset replaced by a mesenchym. As a
result in these forms also no cœlomic diverticula occur in the
rudiments of the extremities.
The definite body-cavity of insects arises entirely independent of
the cœlom cavities, and in fact, as Bütschli showed, through the
separation of the primitive band from the yolk (Fig. 539, _F_, _l_).
It appears bounded on the one hand by the surface of the yolk, on
the other side by the irregularly arranged mesenchym cells.
Originally we can in cross-sections distinguish three separate
cavities of the definite body-cavity (in Hydrophilus according to
Heider), a median and two larger paired lateral ones which later
fuse with each other and with wide lacunæ (_e.g._ in the appendages)
arising by the separation of the mesenchym cells. We refer the
compartments of the definite body-cavity, as in Peripatus, to the
primary body-cavity or segmentation-cavity. They are only lacunæ in
the area of the mesenchym, and throughout bear the character of a
pseudocœl.
In later stages of embryonic development the cœlom-sacs and the
definite body-cavity enter into communication with one another (Fig.
523, _A_, _us_, _lh_). (Korschelt and Heider.)
Then the hinder cœlom-sacs unite through the degeneration of the
transverse dissepiments which separate them. After this a fissure
opens in the median wall of the cœlomic sac, through which its
cavity unites with the definite body-cavity. In the subsequent
changes which the wall of the cœlom-sacs undergoes, these can be
recognised no longer as separate divisions of the whole body-cavity.
_l._ Formation of organs
=The nervous system.=—As we have already seen (p. 554), the rudiments of
the ventral nervous cord arise, after the gastrula invagination is
completed, as two ectodermal thickenings situated on each side of the
median line, the so-called primitive rolls or strips (Fig. 528, _s_),
which extend from the centre of the procephalic lobes of the head to the
last segment, enclosing between them the single median “primitive
groove” (Fig. 539, _C_, _pr_, and _pw_).
Soon after the appearance of the primitive strips, the first traces of
segmentation may be detected. The ventral cord is from the first in
direct connection and continuous with the brain. From the segmental
expansions of the primitive strip arise the ventral nervous ganglia, and
from the intersegmental constrictions are developed the paired
longitudinal commissures.
Transverse sections of the ectoderm in the region of the primitive
strips (Figs. 539, _C_, and 517) show several layers of cells. Of these
cellular layers the deeper ones afterwards, by a kind of delamination,
separate from the superficial ones and form the “lateral cords,” _i.e._
the germs of the longitudinal cords of the ventral ganglionic cord.
Meanwhile the primitive groove (_pr_) deepens and forms an invagination
extending between the lateral cords. The cells at the bottom of this
invagination form the so-called “median cord,” and give rise to the
transverse commissures connecting the ganglia.
[Illustration:
FIG. 541.—Transverse section through the rudiment of the ventral
nervous cord of Xiphidium: _f_, fibrous mass; _m_, neuroblast cells
of the median cord; _n_{1}-n_{4}_, neuroblasts of the lateral cord;
_z_, pillar of ganglion-cells arising from the neuroblasts.—After
Wheeler.
]
Wheeler has detected in the rudiment of the ventral cord of several
Orthoptera, on the upper surface of the lateral cords, four large
cells which he calls _neuroblasts_ (Figure 541, _n_{1}-n_{4}_), from
which cells arise by budding and become arranged in vertically
arranged layers or pillars (_z_). Graber has observed them in
Stenobothrus and Viallanes in Mantis. These neuroblasts are only
present in the inter-ganglionic region, and soon move back to the
hinder side of the transverse commissures.
At first there is a pair of ganglia to each of the 16 trunk-segments of
the embryo, but afterwards these become more or less fused together;
thus those of the three gnathal segments unite to form the subœsophageal
ganglion of the adult, and the last abdominal ganglia are fused together
and move a little anteriorly (see also pp. 227, 228).
=Development of the brain.=—The supraœsophageal ganglion is due to the
spreading out of the procephalic lobes. The rudiment of the brain is due
to a thickening of the ectoderm on the sides of the mouth and of the
forehead, this expansion of germinal brain-cells being the direct
continuation of the primitive rolls or strips, and which finally becomes
differentiated into the protocerebrum, deutocerebrum, and tritocerebrum,
as stated on p. 228.
The ganglion opticum, now regarded as a part of the compound eye, arises
as an ectodermal thickening on each side of the rudimentary brain. The
optic ganglion belongs exclusively to the foremost division of the brain
(see also p. 227).
=Development of the eyes.=—Compound eyes do not appear until the
beginning of pupal life, the single eye (ocellus) being the primitive
organ of vision. The ocellus of Acilius, according to Patten, arises as
a pit or depression of the ectoderm (Fig. 542). The long hypodermal
cells which form the walls of this pit or hollow are arranged in a
single layer, and bear at their free ends a striated cuticular edge
(_c_), while from their inner or basal end arise the fibres destined to
form the common optic nerve.
At a later stage (Fig. 542, _B_), the eye-pit is closed over, the edges
growing over and covering the deeper part of the eye. In this way there
arises out of the pit-like rudiment a two-layered optic cup. The outer
or superficial layer (_l_) becomes in its central part the crystalline
lens, while the peripheral parts form the iris. From the cuticular
striated border of these cells arise the chitinous or corneal lens. On
its outer edge the superficial layer of the eye passes gradually into
the unmodified hypodermis (_h_).
[Illustration:
FIG. 542.—Two stages of development of the 5th of the six ocelli of
larva of Acilius: _c_, cuticular striated band; _cl_, germ destined
to form the corneal lens; _h_, hypodermis; _l_, crystalline-lens
layer; _n_, optic nerve; _r_, retinal germ; _sp_, vertical fissure
of the retina; _x_, the retina-cells bordering this fissure.
]
[Illustration:
FIG. 543.—Two later stages of development of the same eye as in Fig.
542: _i_, iris; _m_, middle inverted layer of the eye; _r_, retina;
_sp_, vertical fissure of the retina; _st_, rods; other letters as
in Fig. 542.—This and Fig. 542 after Patten, from Korschelt and
Heider.
]
The inner, deeper layer of the eye, which forms the contracted
cup-shaped portion, appears to be the rudimentary retina (_r_). From its
cuticular rod-like or fibrous edge arise the visual rods. There soon
arise certain peculiarities characteristic of the eye of Acilius, _i.e._
the fissure (_sp_) bordered by the horizontally situated rods of the
large retina-cells (_x_).
In the farther developed eye (Fig. 543) there is a flattening of the
cup-shaped inner edge, by which the bottom of the eye is levelled and
the little rods belonging to it stand up vertically (Fig. 543, _B_,
_st_). Then the cells belonging to the edge of the retinal cup (_m_) are
turned in, forming an inverted layer constituting the germs of a third
layer interpolated between the two chief layers of the eye. (Korschelt
and Heider, from Patten.) Patten concludes that the structure of the
retina in the larval ocelli of insects is much like that of myriopods,
and that the whole eye is constructed on the same plan as that of
Peripatus and most molluscs.
=Intestinal canal and glands.=—The intestinal or digestive canal is
primitively divided, as already stated on p. 299, into three sections,
of which the anterior and posterior are called respectively the
stomodæum and proctodæum, and are invaginations of the ectoderm, forming
sacs whose blind ends face the future site of the mid-intestine. The
fore-intestine (stomodæum) in most cases arises earlier than the
proctodæum. Its muscles are derived from the mesoderm. From the
stomodæum arises at an early date an unpaired dorsal invagination out of
which develops the ganglion frontale and the pharyngeal nerve.
The absorption of the ends of the blind sacs of the fore and hind
intestine, and opening up of the passage into the mid-intestine,
occur rather early in embryonic life. In the wasps and bees, as well
as the larva of the ant-lion, the mid-intestine remains closed at
the end, not communicating with the proctodæum, which has an
exclusively excretory function (Fig. 497).
The mid-intestine arises from two originally separate rudiments, _i.e._
the fore and hind endodermal rudiments, which at the outset stand in the
most intimate relation with the invagination of the fore and hind
intestine. Originating as a simple collection of cells, so closely
adjoining these invaginations that Voeltzkow, Patten, and Graber derived
them directly through outgrowths of them, they become extended by
advancing cell-multiplication until they assume a U-shaped form. The
legs of the U-shaped rudiment are in the anterior endodermal mass,
directed backwards; those in the posterior mass, on the other hand, are
directed anteriorly. These legs grow towards each other until they
become fused together, forming two paired endodermal streaks, which pass
under the primitive band along its whole length, and are fused with it
at the fore and hind ends. In these places they stand in intimate union
with the proctodeal and stomodeal invaginations.
The paired endodermal streaks belong to the lateral portions of the
primitive band. As a rule, they lie directly under the row of cœlom-sacs
(Fig. 539, _F_). The dorsal wall of the primitive segments stands
consequently in intimate contact with the endodermal streaks. On this
wall of the primitive segments an active cell-growth takes place, and
the cell-material produced in this way, which separates from the dorsal
wall of the primitive segments, forms the outer or splanchnic layer of
the rudiment of the mid-intestine (_spm_, Figs. 539, _F_, 544, _sp_).
What remains of the dorsal wall of the cœlom-sacs after this separation
joins the genital rudiments and gives rise to the so-called terminal
thread-plate (Fig. 544, _ef_). The endodermal streaks, with the
splanchnic layer lying next to them, may now be considered as the
rudiments of the mid-intestine (Fig. 530, _m_, etc.). These are
noticeable in the following stages by their considerable lateral growth;
they spread out over the upper surface of the yolk, around which they
finally entirely grow (Figs. 539, _C-F_, 544, 545). This growth around
the yolk goes on in most cases in such a way as to unite the two
mid-intestinal streaks in the region of the ventral median line with
each other. Then afterwards their union on the dorsal side takes place
(Figs. 539, _F_, 545). The yolk thus passes completely into the interior
of the mid-intestine, and with it the remains of the dorsal tube or
dorsal organ, when such an one is present.
[Illustration:
FIG. 544.—Cross-section through the abdominal region of a somewhat
older primitive band of _Phyllodromia germanica_: _bg_, rudiment of
the nerve-cord; _c_, remains of the cœlomic cavity; _cz_, rudiment
of the genital efferent passage; _ec_, ectoderm; _en_, endoderm;
_ef_, terminal cord-plate; _fk_, fat-body tissue; _gz_, genital
cells; _h_, rudiment of the heart; _p_, rudiment of the pericardial
cavity; _ps_, rudiment of the pericardial septum; _so_, somatic
mesoderm layer; _sp_, splanchnic mesoderm layer.
]
=The salivary glands.=—These segmentally arranged glands, which open by
pairs into the three gnathal segments of the head, arise as ectodermal
invaginations originally opening not into the stomodæum, but outwards on
the surface of the body; hence Korschelt and Heider suggest that they
were originally dermal glands, whose mouths became drawn into the buccal
cavity.
[Illustration:
FIG. 545.—Cross-section through the abdominal region of an embryo of
cockroach (_P. germanica_) after the yolk has been completely
enclosed by the primitive band and the closure of the back; _s_,
tracheal stigma; other letters as in Figs. 540, 544.—This and Fig.
544 after Heymons, from Korschelt and Heider.
]
[Illustration:
FIG. 546.—Embryo of Doryphora shortly after the appearance of the
appendages, unrolled and isolated: _o_, stomodæum; _lb_, labrum;
_b^1–b^3_, three brain segments; _og^1–og^3_, three segments of the
optic ganglion; _op^1–op^3_, three segments of the optic plate;
_f^1–f^5_, five pairs of invaginations which form the tentorium,
etc.; _t^7–t^{20}_, tracheal invaginations; the two last pairs
(_t^{19}-t^{20}_) either disappear or form the openings of the
sexual ducts; _at_, antennæ; _md_, mandibles; _mx^1–mx^2_, maxillæ:
_p^1–p^3_, legs; _c_, commissure connecting the two ganglionic
thickenings (_g^4_) of the premandibular segment; _gl_, ganglia;
_mst_, middlecord thickenings; _mpg^1–mpg^3_, rudiments of three
pairs of urinary tubes; _a_, proctodæum.—After Wheeler.
]
For their serial arrangement, see p. 337. Korschelt and Heider state
that they would be inclined to homologize the salivary glands of
insects with those glands of myriopods opening into the
mouth-cavity, were it not that these glands in myriopods opening
into the mouth are in reality transformed nephridia originating from
the mesoderm, while the salivary glands of insects are clearly
ectodermal structures. We must, therefore, they add, leave to later
researches the question of the homology of these organs, also of
their relations to the similar glands of Peripatus.
[Illustration:
FIG. 547.—Section of proctodæum of embryo locust, showing origin of
urinary tubes (_ur.t_): _ep_, epithelial or glandular layer; _m_,
cells of outer or muscular layer; _a_, section of a tube.
]
=The urinary tubes.=—These excretory vessels arise as paired
evaginations of the hind intestine or proctodæum. They are ectodermal
structures arising as lateral diverticula of the intestinal cavity (Fig.
546). Figure 547 represents their mode of origin at the anterior end of
the proctodæum of a locust. It will be seen that there are 10 primary
tubes. There are 150 such tubes in locusts, or 10 groups of 15 each. The
15 secondary tubes probably arise from the primary ones in the manner
described by Hatschek for Lepidoptera (see his Taf. III, Fig. 7).
While the Malpighian tubes usually first arise as diverticula of the
proctodæum, in the Hymenoptera (Apis and Chalicodoma) they appear,
even before the completion of the proctodæum, as invaginations of
the ectoderm which at first open out on the outer surface of the
primitive band. They seem, then, in some degree, to be similar to
the tracheal rudiments, which perhaps is the reason why they have
been homologized with them, a view which we do not share, and in
which Carrière does not concur. They afterwards pass, with the
growing proctodæum, into the interior of the embryo. (Korschelt and
Heider.)
=The heart.=—The dorsal vessel is first indicated, according to
Korotneff, by a long string or row of cells (_cardioblasts_), which on
each side border the mesodermal layer of the primitive band (Figs. 544,
_h_, 548, _h_). In the advancing growth of the primitive band around the
yolk, this rudiment steadily passes up more towards the dorsal side. It
is in connection with the wall of the primitive segment (Figs. 544 and
548), and represents the point at which the dorsal wall of the cœlom-sac
passes into the lateral wall. According to Korotneff, the cardioblasts
arise directly through a migration out from the wall of the primitive
segment.
In Gryllotalpa the formation of the dorsal organ, which, as Korotneff
states, is in this insect nothing else than a stopper which fills up the
dorsal gap of the body-wall of the embryo, is effected by the rupture of
the embryonal membranes. The serosa is drawn together to form a thick
plate (Fig. 523, _A_, _rp_), and the much degenerated amnion-folds
(_am_) which are laterally attached to it have moved from the edges of
the primitive streak (_*x-*y_) far towards the dorsal side (see Fig.
539, _C_, which represents a similar stage). The distance between the
rudiment of the amnion-fold and the lateral edge of the primitive band
(_*x_, _*y_) is occupied by an epithelial lamella (_l_), in which we
recognize the earlier amnion. This lamella does not lie directly on the
yolk, but is separated from it by a spacious blood-lacuna (_A_, _bs_),
in which can be seen numerous blood-corpuscles which have migrated in
from the mesoderm of the primitive band. The cardioblasts which have
arisen from the wall of the primitive segment (_us_) are on each side
arranged into the form of a furrow (_gr_), which bounds the blood sinus
below.
[Illustration:
FIG. 548.—Cross-section through the abdominal part of an older
primitive band of _P. germanica_ when beginning to grow around the
yolk: _vm_, ventral longitudinal muscle; other lettering as in Fig.
545.—After Heymons, from Korschelt and Heider.
]
By the continuous growth of the primitive band around the yolk, after
the resulting invagination and degeneration of the dorsal plate, the two
blood-lacunæ unite together on the dorsal side into a single one (_B_,
_bs_). These constitute the first cavity of the heart. The vascular
furrows (_gr_) come in contact with each other and grow together, and
the wall of the heart is thus formed. Ayers states that in Œcanthus the
heart is formed in the head region only after the yolk-sac has passed
entirely within the body. The venous ostia arise by two paired
invaginations of the lateral walls, forming a split at their bottom.
The rudiment of the heart stands, as we have seen, in intimate union
with the primitive segments. Out of the lateral walls of these segments,
after giving off the elements of the somatic mesoderm, arises an
epithelial plate which becomes the rudiment of the pericardial septum or
dorsal diaphragm (Figs. 523, _A-C_, _dd_, 544–545, _ps_). As soon as the
two halves of the rudiments of the heart have united with each other in
the dorsal middle line, the two halves of the pericardial septum unite
with each other and form the wall to the pericardial cavity and shut it
off from the rest of the body-cavity. For a long time the pericardial
septum remains in union with the wall of the heart. Afterwards, however,
it separates from it (Fig. 523, _C_, _dd_). (Korschelt and Heider.)
The statements of other authors (Ayers, Grassi, Patten, Tichomeroff,
Carrière, Heider, Heymons, etc.) as to the mode of origin of the
heart in insects of other orders are all similar to the type
described in Gryllotalpa. The difference consists mostly in the fact
that the two large blood-lacunæ are wanting or only exist to a
slight extent. It results that the rudiment of the cavity of the
heart in the earlier stages is of slight extent and often scarcely
recognizable.
In Œcanthus (Ayers) and in Gryllotalpa, the hinder section of the
heart is the first to develop, the development advancing from behind
forward.
=The blood-corpuscles.=—Blood-cells are said by Korotneff to be, in
Gryllotalpa, at an early period present almost everywhere between the
yolk and mesoderm; they are derived, as he states, from the cells of the
somatic mesoderm layer, which has lost its connection with the other
parts of the mesoderm, and fall into the body-cavity. Ayers states that
the blood-corpuscles arise from serosa nuclei which have passed into the
body-cavity, where they become more vesicular, and ultimately all of the
nuclear substance goes to form from one to three spherical bodies, which
are surrounded by the common membrane.
“These bodies are blood-corpuscles and are free nucleoli immediately on
the rupturing of the vesicle which surrounds them.” (Ayers, Pl. 22,
Figs. 1, 3, p. 250.) More recently, Schaeffer has observed in
caterpillars certain cell-complexes associated with the fat-body which
he has called blood-forming masses.
=Musculature, connective tissue, fat-body.=—The muscles of various parts
of the body, as well as the connective tissue, arise by histological
differentiation from the somatic layer of the mesoderm (Fig. 523, _so_).
The fat-body originates from the same source, as shown by the researches
of Kowalevsky, Grassi, and of Carrière. In Hydrophilus a dorsal band of
the fat-body passes over the digestive canal arising by direct
transformation of the wall of the cœlom-sacs. But also the other
portions of the fat-body, as the fat-body lobes accompanying the
tracheal system, are of undoubted mesodermal origin. Heymons’
observations on the cockroach (Phyllodromia) agree with the foregoing
view. In this insect at a very early period certain cells in the wall of
the cœlom-sacs undergo a change, and may be recognized as the rudiments
of what are afterwards fat-body tissues (Fig. 540, _B_ and _C_, _f_).
=The reproductive organs.=—Our knowledge of the mode of development of
the genital organs is in a less satisfactory state than that of the
other organs. It is now known that the rudiments of the sexual glands
belong to the mesoderm, and are developed from the wall of the
cœlom-sacs. In the cockroach (Phyllodromia), the most generalized of the
winged insects, as Heymons has shown, in the earlier stages of the
embryo separate genital cells are already distinguished by their
histologically different characters from the other mesodermal cells. The
genital cells are larger and show a feebly stained nucleus with a clear
nucleolus. These genital cells, which are transformed normal mesodermal
cells, lie originally within the mesoderm layer or on the surface of
this layer turned towards the yolk, on the edge of the segments. After
the complete formation of the cœlom-sacs we find them (Fig. 549, _gz_)
in the dissepiments which separate the successive cœlom-sacs from one
another. Here new genital cells are constantly formed through the
transformation of mesoderm cells. The development of the genital cells
takes place in the 2d to the 7th abdominal segments.
Afterwards the genital cells pass into the interior of the cœlom-sacs,
and soon pass to the dorsal wall of the same (Fig. 540, _A_, _gz_) and
enter between the cells of this wall. The cœlom-sacs (_c_) show in
cross-section in this stage a triangular outline, so that we can
distinguish a dorsal, lateral, and median wall. The dorsal wall lies
next to the surface of the yolk, and afterwards gives rise by separation
or splitting to the splanchnic mesoderm (Fig. 544, _sp_), while from its
remains the terminal thread-plate (_ef_) originates. The lateral wall,
which is turned towards the ectoderm of the primitive band, is
intimately concerned in the formation of the somatic layer (Fig. 540,
_C_, _so_) of the mesoderm. Out of what remains arises the pericardial
septum (Fig. 544, _ps_).
When the genital cells have entered into the dorsal wall of the
primitive segments, they are already so numerous that they form a
continuous series extending from before backward. The genital rudiment
consists, then, of a string of cells lying on each side in the dorsal
wall of the primitive segments, which extend from the 2d to the 7th
abdominal segments. In the formation of these strings or rows of cells
not only are the genital cells concerned, but also still
undifferentiated mesoderm cells (Fig. 540, _B_, _C_), which originate
from the dorsal wall of the cœlom-sacs and lie next to the genital
cells. Some of these last tend to envelop the genital cells. We
designate them the epithelial cells of the genital rudiments (_ep_),
while others form a cellular cord which takes a position medial and
ventral to the genital cells.
[Illustration:
FIG. 549.—Sagittal (longitudinal) section through the abdominal part
of a primitive band of cockroach (_Phyllodromia germanica_) after
the end of the formation of the primitive segments: 1–7, 1st to 7th
abdominal segments; from the 8th abdominal segment (8) to the last
segment (_es_) extends the inturned ventral part of the primitive
band; _am_, amnion; _c_, cœlom-sac; _d_, yolk; _gz_, genital cells,
lying partly in the dissepiments, partly in the wall or in the
cavity of the primitive segments.
]
From the genital cells in the female arise only the egg-cells (and the
nutritive cells in those forms which have such). The follicular
epithelium of the egg-tube, on the other hand, also the corresponding
cells of the terminal chamber, originate from the epithelial cells.
Phyllodromia and Orthoptera in general, to which this description
applies, show in this respect tolerably simple relations, since the
germinal or terminal compartment of the ovary in them is composed of
relatively few cells. In most other insects, and especially those which
have a great number of food-cells in the ovary, the germinal chamber
(Keimfach) is extraordinarily large.
The ventral cellular cord (_cz_) develops into the proximal part of the
oviduct, which widens out and receives the single egg-tubes.
The cœlom-sacs in the farther course of their development, through the
retrograde development of the parts extending into the appendages,
through the development of the fat-bodies and through the delamination
of the somatic and the splanchnic mesoderm layer, become greatly
diminished in size. Finally, there remains left of them only a rather
small cavity (_c_), which is bordered on the side by the rudiment of the
pericardial septum (_ps_) and within by the terminal thread-plate
(_ef_). The dorsally situated point where these two lamellæ pass into
each other seems to stand in intimate connection with the cells of the
rudiment of the heart (_h_). The cord-like genital rudiment hangs from
the terminal thread-plate as from a mesentery (Fig. 549, _gz_).
[Illustration:
FIG. 550.—Longitudinal section through the female genital rudiments of
_P. germanica_. _A_, with beginning, _B_, with farther advanced
growth of the ovarian tubes: _cz_, rudiment of the genital efferent
passage; _ef_, terminal threads; _ep_, nucleus of the epithelial
cells; _gz_, genital cells.—After Heymons, from Korschelt and
Heider.
]
Together with the growth of the primitive band around the yolk, and the
formation of the back, the paired rudiments of the heart gradually
extend to the neighborhood of the dorsal median line, followed by the
genital rudiments which are connected with them by the terminal
thread-plates. The genital rudiments advance thus to the dorsal side of
the developing mid-intestine (Fig. 545, _gz_).
The terminal thread-plate (_ef_) is at first a simple epithelial plate.
Soon, however, follows an arrangement of its cells whereby they appear
to be arranged in vertical rows, each one of which corresponds to a
developing ovarian tube. In this way the terminal thread-plate separates
into the separate terminal threads of the ovarian tubes (Fig. 550,
_ef_). In this process of division the uppermost dorsal edge of the
terminal thread-plate takes no part. From it afterwards grows a thread
which extends anteriorly, which becomes the common terminal thread of
all the ovarian tubes, the so-called Müller’s thread. This is originally
united with the pericardial septum, but seems in later stages to have no
longer an intimate connection with it.
The formation of the single ovarian tubes, which in Phyllodromia number
about 20, is accomplished by the extension of indentations from the
dorsal side towards the ventral side of the ovarian rudiment (Fig. 550).
At the same time the epithelial cells (_ep_), which were originally
situated in part between the genital cells, become arranged in the form
of an epithelium on the surface of the ovarian tubes, which soon forms
on its outer surface a structureless cuticular _tunica propria_. The
outer peritoneal membrane of the ovary becomes formed of the cells of
the surrounding tissue of the fat-body.
The genital rudiment originally extends, as already stated, from the 2d
to the 7th abdominal segment. In the last, however, the genital cells at
first occur only sparingly, and afterwards completely disappear, so that
here the genital cord appears composed of epithelial cells only. This
part is the rudiment of the oviduct proper, and forms a direct
continuation of the above-mentioned cell-cord which is situated
ventralward from the genital cells, from which, as we have seen, the
proximal cup-shaped section of the oviduct is formed. The hinder section
of the oviduct turns down ventrally in order to unite at the boundary
between the 7th and 8th abdominal segments with the hypodermis. The
rudiment of the oviduct originally forms a solid strand of cells.
Afterwards a cavity is formed by the separation of the cells.
In later stages there is a considerable shortening of the genital
rudiment, so that it occupies a smaller number of abdominal segments
than at first. At the same time the single ovarian tubes pass out of
their originally vertical position into one more horizontal.
The paired connections of the rudiments of the oviducts with the
hypodermis of the intersegmental furrow between the 7th and 8th
abdominal segments reminds us of the conditions in the Ephemeridæ. This
is the primitive condition in insects. In the female of Phyllodromia
there is developed during larval life, from an ectodermal invagination,
an unpaired terminal section of the genital passage, which becomes the
genital pouch in which the egg-case (oötheca) is held. This genital
pouch is formed, as Haase has already proved, by the withdrawal of the
chitinous ventral plate of the 8th and 9th abdominal segment by
invagination into the interior of the body.
The development of the efferent passages has been investigated by
Nusbaum in the cockroach (Periplaneta) and in the Pediculina. He found
that only the vasa deferentia and the oviducts arise from the hinder
cord of the germs of the sexual glands, that is, out of the mesodermal
rudiments, while the other parts of the sexual efferent apparatus
(uterus, vagina, receptaculum seminis, ejaculatory duct, penis, and all
the accessory glands) develop from the integumental epithelium and are
of ectodermal origin. In fact, the unpaired parts (uterus, penis,
receptaculum seminis, unpaired glands) have developed from paired
rudiments, being outgrowths of the hypodermis. The hinder portions of
the rudiments of the sexual glands approach these hypodermal growths and
fuse with them. Through a median fusion of the paired hypodermal growths
arise the germs of the unpaired organs. These observations are in
complete agreement with the results at which Palmén arrived by
anatomical investigation (see p. 492).
From the agreement of the position of the sexual openings in
Phyllodromia with the conditions observed in the Ephemeridæ, with which
the Perlidæ also agree, we conclude that in the entire group of insects
an opening between the 7th and 8th abdominal segments is the primitive
condition, and that only by a secondary shifting has a more posterior
position of the opening (in many forms) been brought about. In this
category we must certainly include the Thysanura, in which the sexual
opening is single and situated between the 8th and 9th abdominal
segments.
=Development of the male germinal glands.=—These rudiments arise in
exactly the same manner as those of the female. Sexual differentiation
takes place in the later embryonic stages. We then notice that in the
male four masses of genital cells become surrounded by epithelial cells.
These masses, which form the germs of the four testicular follicles of
Phyllodromia, stand in intimate union with the rudiment of the vas
deferens, and in the later stages move in connection with the latter,
away from and behind the original genital rudiment. There remains, then,
with the terminal thread-plate a remnant of the genital rudiment, which,
according to Heymons, forms the female part of the original
hermaphroditic genital rudiment, and in special cases may develop even
into rudimentary egg-tubes and eggs. The rudimentary organ arising out
of this genital rudiment may also be demonstrated in the adult male of
Phyllodromia.
In the female the oviduct arises directly out of the originally
established efferent passage. In the male, on the contrary, it is not,
along its whole length, transformed into the vas deferens, but its
distal terminal portion degenerates and is replaced by a newly formed
terminal portion of the vas deferens, which then unites with the
ectodermal ductus ejaculatorius. (Korschelt and Heider.)
On reviewing the facts as to the origin of the sexual organs, as in
Phyllodromia,[84] as just described, it will be seen that they
afford proof that in the derivation of the genital cells from the
epithelial cells of the cœlom-sacs, there is a direct agreement with
the annelids. In the later development of the paired genital glands,
and of an efferent passage standing in direct union with the glands
themselves, there is a certain agreement with the conditions in
Peripatus. In the first place, the dorsal position of the genital
glands is the same in the two groups. On the other hand, the genital
glands of Peripatus, according to Sedgwick, are formed by direct
fusion of the successive cœlom-sacs (and a similar point of view has
been taken by Heathcote for the myriopods), hence it results that in
Peripatus the genital cavities arise out of the cœlom-cavities. In
the insects, on the other hand, the genital rudiment lies, to be
sure, in the wall of the cœlom-sac, but the genital cavity (lumen of
the oviducts) in them arises separately from the cœlom-sacs, while
the cœlom-cavities finally become a small part of the definite
body-cavity. We must consider the conditions in Peripatus and the
myriopods as the more primitive, directly pointing to the annelids;
on the other hand, those of the insects as derived and secondary.
If we attempt to homologize the sexual efferent passages of insects
with those of Peripatus, we are compelled to refer them to a
modified pair of nephridia, and the origin of the latter (Peripatus)
from the mesoderm agrees with that of insects. In general, however,
in the development of the sexual outlets of insects, there are no
characters which can be regarded as favorable to such a view. We
must here accept the fact that the mode of development is secondary.
Mention should be specially made of the fact we owe to Heymons, that
in the genital rudiment of Phyllodromia the genital cells and
epithelial cells can be distinguished from each other from the very
beginning. This fact does not favor the generally accepted view that
the follicle-cells and egg-cells arise through a later
differentiation from one and the same kind of cell. From their first
origin, indeed, in Phyllodromia, both kinds of cells may be referred
to the same source.
The mode of origin of the genital rudiments in Diptera and Aphides
deserve special mention. In these groups the sexual germs are
present in very early stages of life. This certainly in part is the
result of the parthenogenetic and pædogenetic mode of reproduction
in the two groups, which leads to an early differentiation of the
sexual germs.
In the Diptera the first germs of the genital glands are represented
by the polar cells (Fig. 551, _pz_). In the asexual developing eggs
of the oviparous Cecidomyia larva, before the formation of the
blastoderm, there separates from the hinder pole (_D_) a rather
large cell rich in granules, which soon divides into two and
afterwards four polar cells. After the completion of the blastoderm
these polar cells then pass in among the blastoderm cells (_G_) and
into the interior of the embryo, where they are in later stages
symmetrically arranged in two groups, and, enveloped by the cells of
surrounding tissues, transformed into the genital rudiments.
(Metschnikoff.)
In Chironomus (Fig. 552, _p_), according to Balbiani, two polar
cells almost simultaneously separate from the hinder pole of the
egg, which, by division, form a group of four and eight cells.
Exactly as in the case in Cecidomyia, these cells are taken within
the embryo, where they lie divided into two groups on each side of
the proctodæum. In all the young, freshly hatched larvæ; these two
spindle-shaped groups, whose cells soon increase in number, may be
seen situated dorsally on the side of the heart, enveloped by a
clear cellular membrane which ends before and behind in a
ligament-like terminal thread. The anterior terminal thread is the
rudiment of the so-called Müller’s thread. The thread at the
posterior end is the rudiment of the paired efferent passage of the
genital glands. Through a division of the cells lying in the
interior of the rudiments of the ovaries, there results the
formation of a rosette-shaped group of cells which corresponds to
the contents of an ovarian tube. With this view of Balbiani the
later observations of Ritter agree.
As in the Diptera, so in the Aphides, the first germs of the genital
organs are differentiated very early in life. In the early stage in
which through an invagination from the hinder pole of the egg the
first rudiment of the amnion-cavity is formed, a group of cells
becomes separated from the wall of this invagination before the
formation of the lower layer, which at this time lies as an unpaired
roundish mass within the embryo. This group of cells, according to
Balbiani and Witlaczil, has arisen by division of a single cell.
Afterwards it becomes horseshoe-shaped and divides into a number of
roundish masses of cells, which are arranged in similar numbers on
each side of the median plane of the body, and form the rudiments of
the terminal fan (Endfächer). They are covered by an epithelial
envelope which passes anteriorly into the terminal threads,
posteriorly into the efferent passage. The origin of this epithelial
case is unknown. The efferent passages of the separate ovarian tubes
are united into a common oviduct, and this fuses with an unpaired
ectodermal invagination lying under the hind intestine from which
the accessory sexual organs are formed. (Korschelt and Heider from
Metschnikoff, Witlaczil, Will.)
[Illustration:
FIG. 551.—First developmental stages of the parthenogenetic eggs of
the larva of Cecidomyia: _b_, peripheral protoplasmic layer
(Keimhautblastem); _bl_, blastoderm; _d_, central yolk; _f_,
division-nuclei; _n_, nutritive cell (“corpus luteum”) about to
break up; _pz_, polar cells.—After Metschnikoff, from Korschelt
and Heider.
]
In the Hymenoptera Ganin has observed in the embryo of Platygaster
the rudiments of the sexual glands in the form of two rounded masses
situated near the posterior intestine and apparently derived from
the same blastems or buds as the latter.
Uljanin studied these organs in the larva of the honey-bee. They are
two reniform bodies in the middle of which will soon appear the
ovarian tubes. They also give birth to the internal parts of the
excretory ducts, while the external part of the genital tube, as
also the accessory glands which are connected with it, are derived
by an invagination of the hypodermis at the surface of the
penultimate segment.
Dohrn observed in the larva of ants the rudiments of the ovaries in
the form of two pyriform masses, each with eight prolongations which
he regarded as young ovarian tubes.
[Illustration:
FIG. 552.—Three longitudinal sections through the embryo of
Chironomus. In _A_, the blastoderm (_bl_) is beginning to form,
the polar cells (_p_) outside of it; in _B_, the polar cells have
pressed in between the blastoderm cells; in _C_, they lie in the
interior of the embryo: _b_, protoplasmic layer (Keimhautblast);
_d_, yolk; _k_, nucleus of the forming blastoderm.—After Ritter,
from Korschelt and Heider.
]
In Encyrtus Bugnion observed the rudiments of the sexual glands in
the middle of the larval period; they were rounded and with no
apparent connection with the neighboring organs. Afterwards these
rudiments elongated, approached nearer to the ventral surface, and
placed themselves in relation with some small cell-groups which
appeared under the rectum, and seemed destined to form the efferent
canal (vas deferens) and accessory glands of the genital organs. He
thought the sex could be recognized in the second half of larval
life, the male gland being distinguished by its rounded shape and
smaller size; the ovary by its oval form and larger size. In larvæ
ready to be transformed the testis formed a cellular mass enveloped
by a cuticle, and at its hinder end prolonged into an epithelial
cord, which is undoubtedly the vas deferens. The ovary had a similar
envelope, and from its cellular mass arose epithelial cords which
were destined to become the ovarian tubes.
_m._ Length of embryonic life
The duration of embryonic life varies greatly in different insects. The
embryo of the blow-fly is fully developed in less than 24 hours, that of
the house-fly in 24 hours. In the locusts and tree-cricket the embryos
begin to develop at the end of the summer, continuing to grow until the
cool weather of autumn, when growth is arrested, the later stages being
finished in the latter part of the spring. It is so, likewise, with the
embryos of many moths and other insects.
_n._ The process of hatching
This has been observed only in a few cases, and careful observations as
to the exact manner in which the embryo breaks the egg-shell and frees
itself from the amnion are much needed. Also the rapid changes of form
from that of the embryo within the egg-shell, and that which it
immediately assumes after breaking forth from the shell and membranes,
have yet to be observed; for these will undoubtedly be found to have
special phylogenetic significance. Indeed, the phylogenetic importance
of the latest embryonic changes in insects just entering on the nymph or
the larval stages is very great, though little attention has as yet been
bestowed upon the matter.
As regards the changes at the time of hatching, Wheeler tells us that
the cockroach (Phyllodromia), shortly after leaving its narrow place in
the egg-capsule, undergoes a peculiar change in shape. Before hatching,
and when confined in the egg-shell, the body is about one-third as wide
as thick; but soon after breaking out of the chorion its body is much
flattened, its dorso-ventral diameter being only about a third as great
as its greatest breadth. This shows that the flattened shape of the body
of cockroaches, which adapts them for their life under bark and stones,
is a very late inheritance, and that these insects have descended from
those with more cylindrical bodies. The end of the body, also, which in
the egg is bent underneath the abdomen, is, after hatching, bent
dorsally, as indicated by the anal stylets, which now point directly
upwards and outwards. The spines and claws are developed shortly before
hatching. In the Locustidæ (Xiphidium, etc.) Wheeler has observed that
the pleuropodia, or 1st pair of abdominal temporary embryonic
appendages, are shed during hatching. All the other embryonic appendages
have also disappeared, except those which persist and have rapidly
become modified to form the cercopods, or the ovipositor.
In locusts, as we have observed[85] in the case of _Melanoplus spretus_,
the egg-shell bursts open at the head end, when the nymph, immediately
after extricating itself from the egg, casts off a thin pellicle (the
amnion), as we have also noticed in the case of the larvæ of the flea,
currant saw-fly, and other insects. Before the amnion is cast off, the
young nymph is almost motionless, but by slight movements of the body
draws itself, in about five minutes, out of the amnion. The exact
process of extraction is as follows: While it lies motionless, it puffs
out the thin, loose skin connecting the back of the head with the front
edge of the prothorax. The distention of this part probably ruptures the
skin, which slips over the head, the body meanwhile curved over until
the skin is drawn back from the head; when the latter is thrown back, it
withdraws its antennæ and legs, and the skin is in a second of time
pushed back to near the end of the abdomen; finally, it draws its hind
tarsi out of the skin, and in a moment or two more the young locust
frees itself, kicks away the cast skin, which resembles a little white
crumpled pellet, and which has also been compared to a diminutive
mushroom, and walks actively off,—sometimes, however, with the cast skin
adhering to the end of the abdomen. Before the shedding of the amnion
the body and legs are soft and flabby; immediately after, it walks
firmly on its legs. All the eggs hatched—at least one or more
hundreds—at about the same time, _i.e._ before 11 A.M.
[Illustration:
FIG. 553.—Locust just before the amnion is cast, enlarged.—Emerton
_del._
]
The nymph of _Stagmomantis carolina_ also sheds an amnion-skin, like
that of the locust; but the embryo before casting it off is much
elongated, and probably, like the European _Mantis religiosa_, the
curious elongated embryos have the same singular habit of suspending
themselves by threads, as shown in Fig. 554.
The account by Pagenstecher of the first ecdysis of the European
Mantis was so extraordinary that we asked Professor Cockerell to
collect the eggs of our Stagmomantis in New Mexico and send them to
us. This he has kindly done, writing that he can “hardly recognize a
true moult, since all that is cast off is the egg-membrane. In
short, Pagenstecher’s account must be not a little fanciful, unless
our insect differs very much in its development from _Mantis
religiosa_. The main change is that after leaving the egg the thorax
enormously elongates, producing a bulging out, and thrusting the
head forward.” Our observations on the alcoholic specimens fully
corroborate Cockerell’s conclusions. Pagenstecher’s figure of the
embryo appears to be inaccurate. Sharp states that the hatching
nymphs remain suspended for some days until the “first change of
skin is effected.” This so-called “skin” is evidently the amnion.
The 17–year Cicada, after hatching, is enveloped by the amnion, from
which it soon extricates itself, and then drops deliberately to the
ground, “its specific gravity being so insignificant that it falls
through the air as gently and as softly as does a feather.” (Riley.)
Other insects, as caterpillars, have room enough to turn around within
their shell and to eat their way through the walls of the chorion.
The meat-fly, as we have observed, hatches in the following manner. The
embryo moves to and fro, the body twisting until the exochorion is
ruptured; the egg-shell splits longitudinally, and in one or two seconds
the larva pushes its way out through the anterior end, and in a second
or two more extricates itself from the shell. The latter scarcely
changes its form, and the larva slips out, leaving the amnion within.
[Illustration:
FIG. 554.—Egg-case of Mantis with young escaping: _A_, the case with
young in their position of suspension. _B_, cerci magnified, showing
the suspensory threads.—After Brongniart, from Sharp.
]
In the case of a fossorial wasp, _Specius speciosus_, which carries
Cicadæ into its burrow, laying an elongated egg on the body under the
median thigh of its victim, the larva on hatching, Riley states, “does
not emerge from the skin of the egg, but merely protrudes its head and
begins at once to draw nourishment from between the sternal sutures of
the Cicada.”
=The hatching spines.=—Animals belonging to quite distinct classes are
provided late in embryonic life with hard knobs or spines, which are
temporary structures for the purpose of breaking or cutting open the
egg-shell, when it is too thick and solid to be ruptured by the
movements of the embryo. The embryos of certain lizards, turtles, the
blind worm and some snakes, of the crocodile, and even birds, as well as
the duckbill and Echidna, are provided with them, always occurring, so
far as we are aware, on the end of the upper jaw. In the Arthropoda
similar structures have thus far only been met with in myriopods and
insects, though an analogous structure on the cephalothorax of the
embryo of phalangids has been observed by Balbiani. Metschnikoff
describes and figures a low conical spine serving this purpose situated
on the embryonal cuticle over the head of the advanced embryo of
Strongylosoma, and one on the 3d pair of mouth-parts of Geophilus.
In the winged insects, the embryo of Forficula is said by Heymons to
bear a single spine between the eyes, which serves as an egg-tooth. The
embryo of the Hemerobiidæ, according to Hagen, “opens the egg with an
egg-burster like a saw.” (Proc. Bost. Soc. Nat. Hist., xv, p. 247.)
Riley states that the egg-burster, or _ruptor ovi_, as he calls it, of
_Corydalus cornutus_, has “the form of the common immature mushroom,”
and he adds that it is a part of the amnion, being “easily perceived on
the end of the vacated shell.” Wheeler has observed three pairs of
broad-based chitinous “hatching spines” used by Doryphora in rupturing
its embryonic envelopes, and which are secreted by pyramidal thickenings
of the hypodermis (Figs. 555, 556).
[Illustration:
FIG. 555.—The three pairs of hatching spines (_hsp_) on the late
embryo of Doryphora.—After Wheeler.
]
[Illustration:
FIG. 556.—Rudiment of the hatching spine: _eb_, being a thickening of
the ectoderm (_ec_) in embryo Doryphora after formation of the
heart; _s_, serosa.—After Wheeler.
]
[Illustration:
FIG. 557.—Head of freshly hatched larva of _Pulex canis_: _eb_,
hatching spine; _ant_, antennæ; _md_, mandible; _mx_, maxilla;
_mx′_, 2d maxilla; _lbr_, labrum.
]
The hatching spine of _Pulex canis_ (Fig. 557) is a thin vertical plate,
like the edge of a knife, situated in the median line of the head very
near the posterior end, and is somewhat cultriform, the upper edge
slightly hollow, and turned up a little at the anterior end. Though we
did not see it working, it is situated at just the point on the head
where it would come in contact with the egg-shell, and it was evident
that the larva, by moving its head back and forth, would produce a
slight split in the chorion and cause it to burst asunder. Later on in
larval life it disappears, probably at the first moult.
LITERATURE ON EMBRYOLOGY
=Koelliker, Albert.= Observationes de prima insectorum genesi, etc.
Turici, 1842, pp. 29, 3 Pls.
=Rathke, H.= Zur Entwickelungsgeschichte der Maulwurfsgrille
(_Gryllotalpa vulgaris_). (Müller’s Archiv, 1844, ii, p. 27, Figs.
1–5.)
=Zaddach, G.= Untersuchungen über die Entwicklung und den Bau der
Gliederthiere. I. Die Entwicklung des Phryganideneies. Berlin, 1854,
pp. 138, 5 Taf.
—— Ueber die Entwicklung der Insekten. (Schrift, d. k. phys.-oekon.
Gesell. Königsberg, viii Jahrg., 1867, Sitzb., p. 16.)
=Leuckart, Rudolph.= Die Fortpflanzung und Entwicklung der Pupiparen.
Nach Beobachtungen an _Melophagus ovinus_. (Abhandl. Naturf. Gesell.
Halle, iv, pp. 1–82, 3 Taf.) Halle, 1858.
=Huxley, T. H.= On the organic reproduction and morphology of Aphis.
Pt. I, 1858; Pt. II, 1858. (Trans. Linn. Soc., xxii, pp. 193–219,
221–236, 5 Pls.)
=Weismann, A.= Die Entwicklung der Dipteren im Ei, nach Beobachtungen
an Chironomus sp., _Musca vomitoria_ und _Pulex canis_. (Zeitschr.
f. wiss. Zool., xiii, 1863, pp. i-xvi, 1–263, 14 Taf.)
—— Zur Embryologie der Insecten. (Arch. f. Anat. u. Physiol., 1864.)
—— Beiträge zur Kenntnis der ersten Entwicklungsvorgänge im
Insectenei. In: Beiträge zur Anatomie und Embryologie, etc.
(Festschrift für J. Henle. Bonn, 1882.)
=Kupffer, C.= Ueber das Faltenblatt an den Embryonen der Gattung
Chironomus. (Arch. Micr. Anat., ii, 1866.)
=Metschnikoff, E.= Embryologische Studien an Insecten. (Zeitschr. f.
wiss. Zool., xvi, 1866, pp. 389–500, 10 Taf.)
=Kupffer, Carl.= De embryogenesi apud Chironomos Observationes, etc.
Kiliæ, 1867, pp. 16, 1 Pl.
=Packard, A. S.= On the development of a dragon-fly (Diplax)
[Æschna?]. (Proc. Bost. Soc. Nat. Hist., xi, 1868, pp. 366–372, 8
Figs.)
—— Embryology of Isotoma, a genus of Poduridæ. (Proc. Bost. Soc. Nat.
Hist., xiv, 1870, pp. 13–15, 4 Figs.)
—— Embryological Studies on Diplax [Æschna?], Perithemis, and the
thysanurous genus Isotoma. (Mem. Peabody Academy of Science, Salem,
i, 1871, pp. 1–21, 3 Pls.)
—— Embryological studies on hexapodous insects. (Ibid., 1872, pp.
1–17, 3 Pls.)
—— The embryological development of the locust. (Chap. X, Third Report
U. S. Ent. Commission, Washington, 1883, pp. 263–282, 7 Pls.)
=Brandt, A.= Beiträge zur Entwicklungsgeschichte der Libelluliden und
Hemipteren. (Mém. Acad. St. Pétersbourg (7), xiii, 1869, pp. 1–33, 3
Taf.)
—— Ueber das Ei und seine Bildungsstätte. (Leipzig, 1878, pp. 196, 4
Taf.)
—— Commentare zur Keimbläschentheorie des Eies. I. Die
Blastodermelemente und Dotterballen der Insecten. (Arch. f. Micr.
Anat., 1880, xvii.)
=Melnikow, N.= Beiträge zur Embryonalentwicklung der Insecten. (Arch.
f. Naturg., xxxv, 1869, pp. 137–189, 4 Taf.)
=Bütschli, O.= Zur Entwicklungsgeschichte der Biene. (Zeitschr. f.
wiss. Zool., xx, 1870, pp. 519–564, 4 Taf.)
—— Bemerkungen über die Entwicklungsgeschichte von Musca. (Morph.
Jahrb., xiv, 1888, pp. 170–174, 3 Figs.)
=Kowalevsky, A.= Embryologische Studien an Würmern und Arthropoden.
(Mém. Acad. St. Pétersbourg (7), xvi, 1871, pp. 1–70, 12 Taf.)
—— Zur embryonalen Entwicklung der Musciden. (Biol. Centralbl., vi,
1886, pp. 49–54.)
=Müller, Fritz.= Beiträge zur Kenntniss der Termiten. (Jena. Zeitschr.
Wissens., ix, 1875, pp. 241–263, 4 Taf.)
=Oulganine, W. N.= (also spelled Uljanin). Sur le développement des
Podurelles. (Arch. Zool. Expér., iv, 1875, pp. xxxix-xl, und v,
1876, pp. xvii-xix.)
—— Beobachtungen über die Entwicklung der Poduren. (Russian.) (Nachr.
k. Gesellsch. Freunde Naturw., Anthrop. und Ethnogr., xvi, 1875.)
=Dohrn, A.= Notizen zur Kenntniss der Insectenentwicklung. (Zeitschr.
f. wiss. Zool., xxvi, 1876, pp. 112–138.)
=Hatschek, B.= Beiträge zur Entwicklungsgeschichte der Lepidopteren.
(Jena. Zeitschr. f. Naturw., xi, 1877, pp. 38, 3 Taf., 2 Figs.)
=Bobretzky, N.= Ueber die Bildung des Blastoderms und der Keimblätter
bei Insecten. (Zeitschr. f. wiss. Zool., xxxi, 1878, pp. 195–215, 1
Taf.)
=Graber, Vitus.= Vorläufige Ergebnisse einer grösseren Arbeit über
vergl. Embryologie der Insecten. (Arch. f. Micr. Anat., xv, 1878,
pp. 630–640, 1 Fig.)
—— Ueber die Polypodie bei Insectenembryonen. (Morph. Jahrb., xiii,
1888, pp. 586–615, 2 Taf.)
—— Ueber die primäre Segmentirung des Keimstreifs der Insecten.
(Morph. Jahrb., xiv, 1888, pp. 345–368, 2 Taf., 4 Figs.)
—— Vergleichende Studien über die Keimhüllen und die Rückenbildung der
Insecten. (Denkschr. Acad. Wiss. Wien., lv, 1888.)
—— Vergleichende Studien über die Embryologie der Insecten und insbes.
der Musciden. (Denkschr. Acad. Wiss. Wien., lvi, 1889.)
—— Ueber den Bau und die phylogenetische Bedeutung der embryonalen
Bauchanhänge der Insekten. (Biol. Centralbl., ix, 1889, pp.
355–363.)
—— Vergleichende Studien am Keimstreif der Insecten. (Denkschr. Acad.
Wiss. Wien., lvii, 1890.)
—— Üeber die embryonale Anlage des Blut- und Fettgewebes der Insecten.
(Biol. Centralbl., xi, 1891, pp. 212–224.)
—— Zur Embryologie der Insekten. (Zool. Anzeiger, xiv, 1891, pp.
286–291.)
—— Über die morphologische Bedeutung der ventralen Abdominalanhänge
der Insekten-Embryonen. (Morph. Jahrb., xvii, 1892, pp. 467–482.)
=Barrois, J.= Développement des Podurelles. (Assoc. Franc. p.
l’Avance. des Sc., 7^e Sess., 1879.)
=Kadyi, H.= Beitrag zur Kenntnis der Vorgänge beim Eierlegen der
_Blatta orientalis_. (Zool. Anzeiger, 1879, ii, pp. 632–636.)
=Tichomiroff, A.= Ueber die Entwicklungsgeschichte des Seidenwurms.
(Zool. Anzeiger, ii Jahrg., 1879, pp. 64–67.)
—— Zur Entwicklungsgeschichte des Seidenspinners (_Bombyx mori_) im
Ei. (Arb. Laborat. Zool. Mus. Moskau, i, 1882, pp. vii, v, 1–80, 3
Taf., 48 Figs.) (Russian.)
—— Ueber die Entwicklung der =Calandra granaria=. (Biol. Centralbl.,
x, 1890, p. 424.)
=Balfour, Francis M.= A treatise on comparative embryology. i, ii.
London, 1880. 2d edit., London, 1885.
=Hertwig, O. und R.= Die Coelomtheorie. Versuch einer Erklärung des
mittleren Keimblattes. Jena, 1881. (Jena. Zeitschr., xv, pp. 150, 3
Taf.)
=Selvatico, D. S.= Sullo sviluppo embrionale dei Bombicini. (Boll.
Bachicoltura, viii, 1881.)
=Lemoine, V.= Recherches sur le développement des Podurelles. (Assoc.
Franç. pour l’Avanc. d. Sc. Congrès de la Rochelle, 1882.)
=Balbiani, E. G.= Sur la signification des cellules polaires des
Insectes. (Compt. rend. Ac. Sc. Paris, xcv, 1882.)
—— Contribution à l’étude de la formation des organes sexuels chez les
Insectes. (Recueil Zool. Suisse, ii, 1885.)
=Korotneff, A.= Entwicklung des Herzens bei Gryllotalpa. (Zool.
Anzeiger, vi Jahrg., 1883, pp. 687–690, 2 Figs.)
—— Die Embryologie der Gryllotalpa. (Zeitschr. f. wiss. Zool., xli,
1885, pp. 570–604, 3 Taf.)
=Nusbaum, J.= Vorl. Mittheilung über die Chorda der Arthropoden.
(Zool. Anzeiger, vi Jahrg., 1883, pp. 291–295, 3 Figs.)
—— Die Entwicklung der Keimblätter bei _Meloë proscarabæus_. (Biol.
Centralbl., viii, 1888, pp. 449–452, 2 Figs.)
—— Zur Frage der Segmentirung des Keimstreifs und der Bauchanhänge der
Insectenembryonen. (Biol. Centralbl., ix, 1889, pp. 516–522, 1 Fig.)
—— Zur Frage der Rückenbildung bei den Insectenembryonen. (Biol.
Centralbl., x, 1890, pp. 110–114.)
—— Ueber die Entwicklungsgeschichte der Ausführungsgänge der
Sexualdrüsen bei den Insecten. (In Polish, with German _résumé_, pp.
39–42.) (“Kosmos” Lemberg, 1884, ix Jahrg.)
—— Zur Embryologie des _Meloë proscarabæus_, Marscham. (In Polish,
with Latin explanation of plates.) (“Kosmos” Lemberg, 1891.)
=Schneider, A.= Ueber die Entwicklung der Geschlechtsorgane der
Insecten. (Zool. Beiträge, herausg. v. A. Schneider, i, 1883.)
=Will, L.= Zur Bildung des Eies und des Blastoderms bei den viviparen
Aphiden. (Arb. Zool. Zoot. Inst. Würzburg, vi, 1883, pp. 217–258, 1
Taf.)
—— Entwicklungsgeschichte der viviparen Aphiden. (Spengel’s Zool.
Jahrbücher. Abth. f. Anat. und Ont., iii, 1888, pp. 201–286, 5 Taf.)
=Patten, W.= The development of Phryganids, with a preliminary note on
the development of _Blatta germanica_. (Quart. Journ. Micr. Sc.,
xxiv, 1884, pp. 54, 3 Pls.)
—— Studies on the eyes of Arthropods. I. Development of the eyes of
Vespa, with observations on the ocelli of some insects. (Journ. of
Morphol., Boston, i, pp. 193–226, 1 Pl.)
—— Studies on the eyes of Arthropods. II. Eyes of Acilius. (Journ. of
Morphol., Boston, ii, 1888, pp. 97–190, 7 Pl., 4 Figs.)
—— Eyes of molluscs and Arthropods. (Mitth. Zool. Station Neapel., vi,
1888, pp. 542–756, 5 Pls.)
=Grassi, B.= Interno allo sviluppo delle api nell’ uovo. (Atti Acad.
Gioenia. Scienc. Nat. Catania (3), xviii, 1884.)
—— Breve nota intorno allo sviluppo degli Japyx. Catania, 1884, also
in I progenitori degli Insetti e dei Miriopodi. 1. L’ Japyx e la
Campodea. (Atti Acad. Gioenia Sc. Nat. Catania (3), xix, 1885.)
=Ayers, H.= On the development of _Œcanthus niveus_ and its parasite
Teleas. (Mem. Boston Soc. Nat. Hist., iii, 1884, pp. 225–281, 8
Pls.)
=Witlaczil, Em.= Entwicklungsgeschichte der Aphiden. (Zeitschr. f.
wiss. Zool., xl, 1884, pp. 559–696, 9 Taf.)
=Hallez, P.= Orientation de l’embryon et formation du cocon chez la
_Periplaneta orientalis_. (Compt. rend. Ac. Sc. Paris, ci, 1885, pp.
444–446.)
—— Sur la loi de l’orientation de l’embryon chez les Insectes. (Compt.
rend. Ac. Sc. Paris, ciii, 1886, pp. 606–608.)
=Heider, K.= Ueber die Anlage der Keimblätter von _Hydrophilus
piceus_. (Abh. k. Acad. Wiss. Berlin, 1885.)
—— Die Embryonalentwicklung von _Hydrophilus piceus_ L. (I. Theil.
Jena, 1889, pp. 1–98, 13 Taf., 9 Figs.)
=Carrière, J.= Kurze Mittheilungen aus fortgesetzten Untersuchungen
über die Sehorgane. 7, Die Entwicklung und die verschiedenen Arten
der Ocellen. (Zool. Anzeiger, ix Jahrg, 1886, pp. 141–147, 479–481,
496–500.)
—— Die Entwicklung der Mauerbiene (_Chalicodoma muraria_ Fabr.) im Ei.
(Arch. f. Micr. Anat., xxxv, 1890, pp. 141–165, 1 Taf.)
—— Die Drüsen am ersten Hinterleibsringe der Insectenembryonen. (Biol.
Centralbl., xi, 1891, pp. 110–127, 3 Figs.)
=Miall, L. C., and Denny, A.= The structure and life-history of the
cockroach (_Periplaneta orientalis_). London, 1886. (The section on
embryology by J. Nusbaum.)
=Ryder, J.= The development of _Anurida maritima_ Guerin. (Amer.
Naturalist, xx, 1886, pp. 299–302, 1 Pl)
=Stuhlmann, F.= Die Reifung des Arthropodeneies nach Beobachtungen an
Insecten, Spinnen, Myriopoden und Peripatus. (Ber. Freib.
Naturf.-Gesellsch., i, 1886.)
=Blochmann, F.= Ueber die Richtungskörper bei Insecteneiern. (Morph.
Jahrbuch, xii, 1887, pp. 544–574, 2 Taf.)
=Bruce, Adam Todd.= Observations on the embryology of insects and
arachnids. A memorial volume. Baltimore, 1887, 4º, pp. 31, 6 Pls.
=Weismann, A., und Ischikawa, Ch.= Ueber die Bildung der
Richtungskörper bei thierischen Eiern. (Ber. Naturf. Ges. Freiburg,
iii, 1887.)
=Cholodkowsky, N.= Ueber die Bildung des Entoderms bei _Blatta
germanica_. (Zool. Anzeiger, xi Jahrg., 1888, pp. 163–166, 2 Figs.)
—— Studien zur Entwicklungsgeschichte der Insecten. (Zeitschr. f.
wiss. Zool., xlviii, 1889, pp. 89–100, 1 Taf.)
—— Zur Embryologie von _Blatta germanica_. (Zool. Anzeiger, xiii
Jahrg., 1890, pp. 137–138.)
—— Zur Embryologie der Hausschabe (_Blatta germanica_). (Biol.
Centralbl., x, 1890, p. 425.)
—— Ueber die Entwicklung des centralen Nervensystems bei _Blatta
germanica_. (Zool. Anzeiger, xiv Jahrg., 1891, pp. 115–116.)
—— Zur Embryologie der Insecten. (Ibid., 1891, pp. 465–466.)
—— Die Embryonalentwicklung von Phyllodromia (_Blatta germanica_).
(Mém. Acad. St. Pétersbourg (7), xxxviii, 1891.)
=Henking, H.= Die ersten Entwicklungsvorgänge im Fliegenei und freie
Kernbildung. (Zeitschr. f. wiss. Zool., xlvi, 1888, pp. 289–336, 4
Taf., 3 Figs.)
—— Ueber die Bildung von Richtungskörpern in den Eiern der Insekten
und deren Schicksal. (Nachr. Ges. Wiss. Göttingen, 1888.)
=Platner, G.= Die erste Entwicklung befruchteter und
parthenogenetischer Eier von _Liparis dispar_. (Biol. Centralbl.,
viii, 1888, pp. 521–524.)
=Schmidt, F.= Die Bildung des Blastoderms und des Keimstreifs der
Musciden. (Sitz. Naturf. Ges., Dorpat, viii, 1889.)
=Viallanes, H.= Sur quelques points de l’histoire du développement
embryonnaire de la Mante religieuse. (Rec. Biol. du Nord de la
France, ii, 1889–1890.)
=Voeltzkow, A.= Entwicklung im Ei von _Musca vomitoria_. (Arb. Zool.
Zoot. Inst. Würzburg, ix, 1889, pp. 1–48, 4 Taf.)
—— _Melolontha vulgaris_, ein Beitrag zur Entwicklung im Ei bei
Insecten. (Arb. Zool. Zoot. Inst. Würzburg, ix, 1889, pp. 49–64, 1
Taf.)
=Wheeler, W. M.= Ueber drüsenartige Gebilde im ersten Abdominalsegment
der Hemipterenembryonen. (Zool. Anzieger, xii, 1889, pp. 500–504, 2
Figs.)
—— The embryology of _Blatta germanica_ and _Doryphora decemlineata_.
(Journ. of Morph., Boston, iii, 1889, pp. 291–374, 7 Pls., 16 Figs.)
—— On the appendages of the first abdominal segment of the embryo
cockroach (_Blatta germanica_). (Proceed. Wis. Acad. Science, Arts,
and Letters, viii, 1890, pp. 87–140, 3 Pls.)
—— Ueber ein eigenthümliches Organ im Locustidenembryo (_Xiphidium
ensiferum_). (Zool. Anzieger, xiii, 1890, pp. 475–480.)
—— Neuroblasts in the Arthropod embryo. (Journ. of Morphol., iv, 1891,
pp. 337–343, 1 Fig.)
—— A contribution to insect embryology. In. Diss. (Journ. Morph.,
Boston, viii, 1893, pp. 1–160, 6 Pls., 7 Figs.)
=Heymons, R.= Ueber die hermaphroditische Anlage der Sexualdrüsen beim
Männchen von _Phyllodromia_ (Blatta) _germanica_. (Zool. Anzeiger,
xiii Jahrg., 1890, pp. 451–457, 3 Figs.)
—— Die Entstehung der Geschlechtsdrüsen von _Phyllodromia_ (Blatta)
_germanica_ L. In. Diss. Berlin, 1891.
—— Die Embryonalentwickelung von Dermapteren und Orthopteren unter
besonderer Berücksichtigung der Keimblätterbildung. Monographisch
bearbeiteit, 12 Lith. Taf. und 33 Figs. Jena, 1895, 4º, pp. 136.
—— Ueber die Fortpflanzung und Entwickelungsgeschichte der _Ephemera
vulgata_ L. (Sitzungsb. Gesell. Naturf. Freunde, Berlin, 1896, pp.
81–96.)
—— Entwicklungsgeschichtliche Untersuchungen an _Lepisma saccharina_
L. (Zeitschr. wiss. Zool., lxii, 1897, pp. 588–631, 2 Taf.)
=Ritter, R.= Die Entwicklung der Geschlechtsorgane und des Darmes bei
Chironomus. (Zeitschr. f. wiss. Zool., 1, 1890, pp. 408–427, 1 Taf.)
=Tichomirowa, O. S.= Zur Embryologie von Chrysopa. (Biol. Centralbl.,
x, 1890, p. 423.)
=Pedaschenko, D.= Sur la formation de la bandelette germinative chez
_Notonecta glauca_. (In Russian.) (Revue Sc. Natural. St.
Pétersbourg, i, 1891.)
=Korschelt, E., and Heider, K.= Lehrbuch der vergleichenden
Entwicklungsgeschichte der wirbellosen Thiere. (Spec. Th. Heft ii,
Jena, 1891, many Figs.)
With the writings of Lang (Comp. Anatomy).
END OF PART II