Chapter XXIX: Part II: Embryology of Insects (3)

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.

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.

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.
]

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.

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.

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.

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_).

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.)

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.

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.

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.

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).

FIG. 555.—The three pairs of hatching spines (_hsp_) on the late
embryo of Doryphora.—After Wheeler.
]

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.
]

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
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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