feel fairly confident that it is not so in the stages I have
investigated.
There is a section of the body cavity in the limbs as in embryo
Myriapods, Spiders, etc.
In the procephalic lobe there is a well-developed section of the body
cavity, which lies dorsal to and in front of the rudiment of the
supra-oesophageal ganglia.
The alimentary tract is formed of a mesenteron (fig. 172), a
stomodæum, and proctodæum. The wall of the mesenteron is formed, in
the stages investigated by me, of a single layer of cells with yolk
particles, and encloses a lumen free from yolk. The forward extension
of the mesenteron is remarkable.
The stomodæum in the earlier stage is a simple pit, which meets but
does not open into the mesenteron. In the later stage the external
opening of the pit is complicated by the structures already described.
The proctodæum is a moderately deep pit near the hinder end of the
body.
The existence of a tracheal system[162] is in itself almost sufficient
to demonstrate the affinities of Peripatus with the Tracheata, in
spite of the presence of nephridia. The embryological characters of
the procephalic lobes, of the limbs and claws, place however this
conclusion beyond the reach of scepticism. If the reader will compare
the figure of Peripatus with that of an embryo Scorpion (fig. 196 A)
or Spider (fig. 200 C) or better still with Metschnikoff's figure of
Geophilus (No. 399) Pl. XXI. fig. II, he will be satisfied on this
point.
[162] The specimens shewing tracheæ which Moseley has placed in
my hands are quite sufficient to leave no doubt whatever in my
mind as to the general accuracy of his description of the
tracheal system.
The homologies of the anterior appendages are not very easy to
determine; but since there does not appear to me to be sufficient
evidence to shew that any of the anterior appendages have become
aborted, the first post-oral appendages embedded in the lips may
provisionally be regarded as equivalent to the mandibles, and the oral
papillæ to the first pair of maxillæ, etc. Moseley is somewhat
doubtful about the homologies of the appendages, and hesitates between
considering the oral papillæ as equivalent to the second pair of
maxillæ (on account of their containing the openings of the mucous
glands, which he compares with the spinning glands of caterpillars),
or to the poison claws (fourth post-oral appendages) of the Chilopoda
(on account of the poison glands which he thinks may be homologous
with the mucous glands).
The arguments for either of these views do not appear to me
conclusive. There are glands opening into various anterior appendages
in the Tracheata, such as the poison glands in the Cheliceræ
(mandibles) of Spiders, and there is some evidence in Insects for the
existence of a gland belonging to the first pair of maxillæ, which
might be compared with the mucous gland of Peripatus. For reasons
already stated I do not regard the processes of the cephalic lobes,
which form the lips, as a pair of true appendages.
BIBLIOGRAPHY.
(396) H. N. Moseley. "On the Structure and Development of Peripatus
capensis." _Phil. Trans._ Vol. 164, 1874.
MYRIAPODA[163].
[163] The classification of the Myriapoda employed in the present
section is:
I. Chilognatha. (Millipedes.)
II. Chilopoda. (Centipedes.)
Chilognatha. The first stages in the development of the Chilognatha
have been investigated by Metschnikoff and Stecker, but their accounts
are so contradictory as hardly to admit of reconciliation.
According to Metschnikoff, by whom the following four species have
been investigated, viz., Strongylosoma Guerinii, Polydesmus
complanatus, Polyxenus lagurus, and Julus Moneletei, the segmentation
is at first regular and complete, but, when the segments are still
fairly large, the regular segmentation is supplemented by the
appearance of a number of small cells at various points on the
surface, which in time give rise to a continuous blastoderm.
The blastoderm becomes thickened on the ventral surface, and so forms
a ventral plate[164].
[164] Stecker's (No. 400) observations were made on the eggs of
Julus fasciatus, Julus foetidus, Craspedosoma marmoratum,
Polydesmus complanatus, and Strongylosoma pallipes, and though
carried on by means of sections, still leave some points very
obscure, and do not appear to me deserving of much confidence.
The two species of Julus and Craspedosoma undergo, according to
Stecker, a nearly identical development. The egg before
segmentation is constituted of two substances, a central
protoplasmic, and a peripheral deutoplastic. It first divides
into two equal segments, and coincidentally with their formation
part of the central protoplasm travels to the surface as two
clear fluid segments. The ovum is thus composed of two yolk
segments to two protoplasmic segments. The two former next divide
into four, with the production of two fresh protoplasmic
segments. The four protoplasmic segments now constitute the upper
or animal pole of the egg, and occupy the position of the future
ventral plate. The yolk segments form the lower pole, which is
however _dorsal_ in relation to the future animal. The
protoplasmic segments increase in number by a regular division,
and arrange themselves in three rows, of which the two outermost
rapidly grow over the yolk segments. A large segmentation cavity
is stated to be present in the interior of the ovum.
It would appear from Stecker's description that the yolk segments
(hypoblast) next become regularly invaginated, so as to enclose a
gastric cavity, opening externally by a blastopore; but it is
difficult to believe that a typical gastrula, such as that
represented by Stecker, really comes into the cycle of
development of the Chilognatha.
The mesoblast is stated to be derived mainly from the epiblast.
This layer in the region of the future ventral plate becomes
reduced to two rows of cells, and the inner of these by the
division of its constituent elements gives rise to the mesoblast.
The development of Polydesmus and Strongylosoma is not very
different from that of Julus. The protoplasm at the upper pole
occupies from the first a superficial position. Segmentation
commences at the lower pole, where the food-yolk is mainly
present! The gastrula is stated to be similar to that of Julus.
The mesoblast is formed in Polydesmus as a layer of cells split
off from the epiblast, but in Strongylosoma as an outgrowth from
the lips of the blastopore. Stecker, in spite of the statements
in his paper as to the origin of the mesoblast from the epiblast,
sums up at the end to the effect that both the primary layers
have a share in the formation of the mesoblast, which originates
by a process of endogenous cell division!
It may be noted that the closure of the blastopore takes place,
according to Stecker, on the dorsal side of the embryo.
The most important sources of information for the general embryology
of the Chilognatha are the papers of Newport (No. 397) and
Metschnikoff (No. 398). The development of Strongylosoma may be taken
as fairly typical for the group; and the subsequent statements, unless
the reverse is stated, apply to the species of Strongylosoma
investigated by Metschnikoff.
[FIG. 173. THREE STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA
GUERINII. (After Metschnikoff.)
A. Embryo on eleventh day with commencing ventral flexure (_x_)
B. Embryo with three pairs of post-oral appendages.
C. Embryo with five pairs of post-oral appendages.
_gs._ ventral plate; _at._ antennæ; 1-5 post-oral appendages; _x._
point of flexure of the ventral plate.]
After the segmentation and formation of the layers the first
observable structure is a transverse furrow in the thickening of the
epiblast on the ventral surface of the embryo. This furrow rapidly
deepens, and gives rise to a ventral flexure of the embryo (fig. 173
A, _x_), which is much later in making its appearance in Julus than in
Strongylosoma and Polyxenus. A pair of appendages, which become the
antennæ, makes its appearance shortly after the formation of the
transverse furrow, and there soon follow in order the next three pairs
of appendages. All these parts are formed in the infolded portion of
the ventral thickening of the blastoderm (fig. 173 B). The ventral
thickening has in the meantime become marked by a longitudinal furrow,
but whether this is connected with the formation of the nervous
system, or is equivalent to the mesoblastic furrow in Insects, and
connected with the formation of the mesoblast, has not been made out.
Shortly after the appearance of the three pairs of appendages behind
the antennæ two further pairs become added, and at the same time oral
and anal invaginations become formed (fig. 173 C). In front of the
oral opening an unpaired upper lip is developed. The præ-oral part of
the ventral plate develops into the bilobed procephalic lobes, the
epiblast of which is mainly employed in the formation of the
supra-oesophageal ganglia. The next important change which takes place
is the segmentation of the body of the embryo (fig. 174 A), the most
essential feature in which is the division of the mesoblast into
somites. Segments are formed in order from before backwards, and soon
extend to the region behind the appendages. On the appearance of
segmentation the appendages commence to assume their permanent form.
The two anterior pairs of post-oral appendages become jaws; and the
part of the embryo which carries them and the antennæ is marked off
from the trunk as the head. The three following pairs of appendages
grow in length and assume a form suited for locomotion. Behind the
three existing pairs of limbs there are developed three fresh pairs,
of _which the two anterior belong to a single primitive segment_.
While the above changes take place in the appendages the embryo
undergoes an ecdysis, which gives rise to a cuticular membrane within
the single egg membrane (chorion, _Metschnikoff_). On this cuticle a
tooth-like process is developed, the function of which is to assist in
the hatching of the embryo (fig. 174 A).
In Polyxenus a cuticular membrane is present as in Strongylosoma, but
it is not provided with a tooth-like process. In the same form
amoeboid cells separate themselves from the blastoderm at an early
period. These cells have been compared to the embryonic envelopes of
Insects described below.
In Julus _two_ cuticular membranes are present at the time of
hatching: the inner one is very strongly developed and encloses the
embryo after hatching. After leaving the chorion the embryo Julus
remains connected with it by a structureless membrane which is
probably the outer of the two cuticular membranes.
[FIG. 174. TWO STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUERINII.
(After Metschnikoff.)
A. A seventeen days' embryo, already segmented.
B. A just hatched larva.]
At the time when the embryo of Strongylosoma is hatched (fig. 174 B)
nine post-cephalic segments appear to be present. Of these segments
the second is apparently (from Metschnikoff's figure, 174 B) without a
pair of appendages; the third and fourth are each provided with a
single functional pair of limbs; the fifth segment is provided with
two pairs of rudimentary limbs, which are involuted in a single sack
and not visible without preparation, and therefore not shewn in the
figure. The sixth segment is provided with but a single pair of
appendages, though a second pair is subsequently developed on it[165].
[165] Though the superficially hexapodous larva of Strongylosoma
and other Chilognatha has a striking resemblance to some larval
Insects, no real comparison is possible between them, even on the
assumption that the three functional appendages of both are
homologous, because Embryology clearly proves that the hexapodous
Insect type has originated from an ancestor with numerous
appendages by the atrophy of those appendages, and not from an
hexapodous larval form prior to the development of the full
number of adult appendages.
Julus, at the time it leaves the chorion, is imperfectly segmented,
but is provided with antennæ, mandibles, and maxillæ, and seven pairs
of limbs, of which the first three are much more developed than the
remainder. Segmentation soon makes its appearance, and the head
becomes distinct from the trunk, and on each of the three anterior
trunk segments a single pair of limbs is very conspicuous
(Metschnikoff)[166]. Each of the succeeding segments bears eventually
two pairs of appendages. At the time when the inner embryonic cuticle
is cast off, the larva appears to be hexapodous, like the young
Strongylosoma, but there are in reality four pairs of rudimentary
appendages behind the three functional pairs. The latter only appear
on the surface after the first post-embryonic ecdysis. Pauropus
(Lubbock) is hexapodous in a young stage. At the next moult two pairs
of appendages are added, and subsequently one pair at each moult.
[166] Newport states however that a pair of limbs is present on
the first, second, and fourth post-oral segments, but that the
third segment is apodous; and this is undoubtedly the case in the
adult.
There appear to be eight post-oral segments in Julus at the time of
hatching. According to Newport fresh segments are added in
post-embryonic life by successive budding from a blastema between the
penultimate segment and that in front of it. They arise in batches of
six at the successive ecdyses, till the full number is completed. A
functional, though not a real hexapodous condition, appears to be
characteristic of Chilognatha generally at the time of hatching.
The most interesting anatomical feature of the Chilognatha is the
double character of their segments, the feet (except the first three
or four, or more), the circulatory, the respiratory, and the nervous
systems shewing this peculiarity. Newport's and Metschnikoff's
observations have not thrown as much light on the nature of the double
segments as might have been hoped, but it appears probable that they
have _not_ originated from a fusion of two primitively distinct
segments, but from a later imperfect division of each of the primitive
segments into two, and the supply to each of the divisions of a
primitive segment of a complete set of organs.
[FIG. 175. TWO STAGES IN THE DEVELOPMENT OF GEOPHILUS. (After
Metschnikoff.)
A. Side view of embryo at the stage when the segments are beginning
to be formed.
B. Later stage after the appendages have become established.
_at._ antennæ; _an.i._ proctodæum.]
Chilopoda. Up to the present time the development of only one type of
Chilopoda, viz. that of Geophilus, has been worked out. Most forms lay
their eggs, but Scolopendra is viviparous. The segmentation appears to
resemble that in the Chilognatha, and at its close there is present a
blastoderm surrounding a central mass of yolk cells. A ventral
thickening of the blastoderm is soon formed. It becomes divided into
numerous segments, which continue to be formed successively from the
posterior unsegmented part. The antennæ are the first appendages to
appear, and are well developed when eighteen segments have become
visible (fig. 175 A). The post-oral appendages are formed slightly
later, and in order from before backwards. As the embryo grows in
length, and fresh segments continue to be formed, the posterior part
of it becomes bent over so as to face the ventral surface of the
anterior, and it acquires an appearance something like that of many
embryo Crustaceans (fig. 175 B). Between forty and fifty segments are
formed while the embryo is still in the egg. The appendages long
remain unjointed. The fourth post-oral appendage, which becomes the
poison claw, is early marked out by its greater size: on the third
post-oral there is formed a temporary spine to open the egg membrane.
It does not appear, from Metschnikoff's figures of Geophilus, that any
of the anterior segments are without appendages, and it is very
probable that Newport is mistaken in supposing that the embryo has a
segment without appendages behind that with the poison claws, which
coalesces with the segment of the latter. It also appears to me rather
doubtful whether the third pair of post-oral appendages, _i.e._ those
in front of the poison claws, can fairly be considered as forming part
of the basilar plate. The basilar plate is really the segment of the
poison claws, and may fuse more or less completely with the segment in
front and behind it, and the latter is sometimes without a pair of
appendages (Lithobius, Scutigera).
Geophilus, at the time of birth, has a rounded form like that of the
Chilognatha.
The young of Lithobius is born with only six pairs of limbs.
_General observation on the homologies of the appendages of
Myriapoda._
The chief difficulty in this connection is the homology of the third
pair of post-oral appendages.
In adult Chilognatha there is present behind the mandibles a
four-lobed plate, which is usually regarded as representing two pairs
of appendages, viz. the first and second pairs of maxillæ of Insects.
Metschnikoff's observations seem however to shew that this plate
represents but a single pair of appendages, which clearly corresponds
with the first pair of maxillæ in Insects. The pair of appendages
behind this plate is ambulatory, but turned towards the head; it is in
the embryo the foremost of the three functional pairs of legs with
which the larva is born. Is it equivalent to the second pair of
maxillæ of Insects or to the first pair of limbs of Insects? In favour
of the former view is the fact (1) that in embryo Insects the second
pair of maxillæ sometimes resembles the limbs rather than the jaws, so
that it might be supposed that in Chilognatha a primitive ambulatory
condition of the third pair of appendages has been retained; (2) that
the disappearance of a pair of appendages would have to be postulated
if the second alternative is adopted, and that if Insects are
descended from forms related to the Myriapods it is surprising to find
a pair of appendages always present in the former, absent in the
latter. The arguments which can be urged for the opposite view do not
appear to me to have much weight, so that the homology of the
appendages in question with the second pair of maxillæ may be
provisionally assumed.
The third pair of post-oral appendages of the Chilopoda may probably
also be assumed to be equivalent to the second pair of maxillæ; though
they are limb-like and not connected with the head. The subjoined
table shews the probable homologies of the appendages.
+------------------------+---------------------+----------------------+
| | CHILOGNATHA | CHILOPODA |
| | (Strongylosoma | (Scolopendra adult). |
| | at time of birth). | |
+------------------------+---------------------+----------------------+
| Pre-oral region. | Antennæ. | Antennæ. |
+------------------------+---------------------+----------------------+
| 1st Post-oral segment. | Mandibles. | Mandibles. |
+------------------------+------------------- -+----------------------+
| 2nd " " | Maxillæ 1. (Four- | Maxillæ 1. |
| | lobed plate in | (Palp and bilobed |
| | adult, but a simple| median process). |
| | pair of appendages | |
| | in embryo). | |
+------------------------+---------------------+----------------------+
| 3rd " " | 1st pair of | Limb-like appendages |
| (probably equivalent | ambulatory limbs | with basal parts in |
| to segment bearing | | contact. |
| 2nd pair of maxillæ | | |
| in Insects). | | |
+------------------------+---------------------+----------------------+
| 4th " " | (?) Apodous. | Poison claws. |
+------------------------+---------------------+----------------------+
| 5th " " | 2nd pair of | 1st pair of |
| | ambulatory limbs. | ambulatory limbs. |
+------------------------+---------------------+----------------------+
| 6th " " | 3rd " " " | 2nd " " " |
+------------------------+---------------------+----------------------+
| 7th " " | 4th and 5th " " | 3rd " " " |
| | (rudimentary.) | |
+------------------------+---------------------+----------------------+
| 8th " " | 6th " " " | 4th " " " |
| | (the 7th pair is | |
| | developed in this | |
| | segment later). | |
+------------------------+---------------------+----------------------+
| 9th " " | Apodous. | 5th " " " |
+------------------------+---------------------+----------------------+
| 10th " " | " (last segment | 6th " " " |
| | in embryo). | |
+------------------------+---------------------+----------------------+
_The germinal layers and formation of organs._
The development of the organs of the Myriapoda, and the origin of the
germinal layers, are very imperfectly known: Myriapoda appear however
to be closely similar to Insects in this part of their development,
and the general question of the layers will be treated more fully in
connection with that group.
The greater part of the blastoderm gives rise to the epiblast, which
furnishes the skin, nervous system, tracheal system, and the stomodæum
and proctodæum.
The mesoblast arises in connection with the ventral thickening of the
blastoderm, but the details of its formation are not known.
Metschnikoff describes a longitudinal furrow which appears very early
in Strongylosoma, which is perhaps equivalent to the mesoblastic
furrows of Insects, and so connected with the formation of the
mesoblast.
The mesoblast is divided up into a series of protovertebra-like
bodies--the mesoblastic somites--the cavities of which become the body
cavity and the walls the muscles and probably the heart. They are
(Metschnikoff) prolonged into the legs, though the prolongations
become subsequently segmented off from the main masses. The splanchnic
mesoblast is, according to Metschnikoff, formed independently of the
somites, but this point requires further observation.
The origin of the hypoblast remains uncertain, but it appears probable
that it originates, in a large measure at least, from the yolk
segments. In the Chilognatha the mesenteron is formed in the interior
of the yolk segments, so that those yolk segments which are not
employed in the formation of the alimentary canal lie freely in the
body cavity. In the relation of the yolk segments to the alimentary
canal the Chilopoda present a strong contrast to the Chilognatha, in
that the greater part of the yolk lies within their mesenteron. The
mesenteron is at first a closed sack, but is eventually placed in
communication with the stomodæum and the proctodæum. The Malpighian
bodies arise as outgrowths from the blind extremity of the latter.
BIBLIOGRAPHY.
(397) G. Newport. "On the Organs of Reproduction and Development of
the Myriapoda." _Philosophical Transactions_, 1841.
(398) E. Metschnikoff. "Embryologie der doppeltfüssigen Myriapoden
(Chilognatha)." _Zeit. f. wiss. Zool._, Vol. XXIV. 1874.
(399) ---- "Embryologisches über Geophilus." _Zeit. f. wiss. Zool._,
Vol. XXV. 1875.
(400) Anton Stecker. "Die Anlage d. Keimblatter bei den Diplopoden."
_Archiv f. mik. Anatomie,_ Bd. XIV. 1877.
INSECTA[167].
[167] The following classification of the Insecta is employed in
this chapter:
I. Aptera. {(1) Collembola.
{(2) Thysanura.
II. Orthoptera. {(1) Orthoptera genuina
(_Blatta_, _Locusta_, etc.).
{(2) " pseudoneuroptera
(_Termes_, _Ephemera_, _Libellula_).
III. Hemiptera. {(1) Hemiptera heteroptera
(_Cimex_, _Notonecta_, etc.).
{(2) " homoptera (_Aphis_, _Cicada_, etc.).
{(3) " parasita (_Pediculus_, etc.).
IV. Diptera. {(1) Diptera genuina (_Musca_, _Tipula_, etc.).
{(2) " aphaniptera (_Pulex_, etc.).
{(3) " pupipara (_Braula_, etc.).
V. Neuroptera. {(1) Neuroptera planipennia (_Myrmeleon_, etc.).
{(2) " trichoptera (_Phryganea_, etc.).
VI. Coleoptera.
VII. Lepidoptera.
VIII. Hymenoptera. {(1) Hymenoptera aculeata
(_Apis_, _Formica_, etc.).
{(2) " entomophaga
(_Ichneumon_, _Platygaster_, etc.).
{(3) " phytophaga
(_Tenthredo_, _Sirex_, etc.).
The formation of the embryonic layers in Insects has not been followed
out in detail in a large number of types; but, as in so many other
instances, some of the most complete histories we have are due to
Kowalevsky (No. 416). The development of Hydrophilus has been worked
out by him more fully than that of any other form, and will serve as a
type for comparison with other forms.
[FIG. 176. FOUR EMBRYOS OF HYDROPHILUS PICEUS VIEWED FROM THE
VENTRAL SURFACE. (After Kowalevsky.)
The upper end is the anterior. _gg._ germinal groove; _am._ amnion.]
The segmentation has not been studied, but no doubt belongs to the
centrolecithal type (_vide_ pp. 110-120). At its close there is an
uniform layer of cells enclosing a central mass of yolk. These cells,
in the earliest observed stage, were flat on the dorsal, but columnar
on part of the ventral surface of the egg, where they form a
thickening which will be called the ventral plate. At the posterior
part of the ventral plate two folds, with a furrow between them, make
their appearance. They form a structure which may be spoken of as the
germinal groove (fig. 176 A, _gg_). The cells which form the floor of
the groove are far more columnar than those of other parts of the
blastoderm (fig. 177 A). The two folds on each side of it gradually
approach each other. They do so at first behind, and then in the
middle; from the latter point the approximation gradually extends
backwards and forwards (fig. 176 B and C). In the middle and hinder
parts of the ventral plate the groove becomes, by the coalescence of
the folds, converted into a canal (fig. 178 A, _gg_), the central
cavity of which soon disappears, while at the same time the cells of
the wall undergo division, become more rounded, and form a definite
layer (_me_)--the mesoblast--beneath the columnar cells of the
surface. Anteriorly the process is slightly different, though it leads
to the similar formation of mesoblast (fig. 177 B). The flat floor of
the groove becomes in front bodily converted into the mesoblast, but
the groove itself is never converted into a canal. The two folds
simply meet above, and form a continuous superficial layer.
[FIG. 177. TWO TRANSVERSE SECTIONS THROUGH EMBRYOS OF HYDROPHILUS
PICEUS. (After Kowalevsky.)
A. Section through an embryo of the stage represented in fig. 176 B,
at the point where the two germinal folds most approximate.
B. Section through an embryo somewhat later than the stage fig. 176
D, through the anterior region where the amnion has not completely
closed over the embryo.
_gg._ germinal groove; _me._ mesoblast; _am._ amnion; _yk._ yolk.]
[FIG. 178. SECTIONS THROUGH TWO EMBRYOS OF HYDROPHILUS PICEUS.
(After Kowalevsky.)
A. Section through the posterior part of the embryo fig. 176 D,
shewing the completely closed amnion and the germinal groove.
B. Section through an older embryo in which the mesoblast has grown
out into a continuous plate beneath the epiblast.
_gg._ germinal groove; _am._ amnion; _yk._ yolk; _ep._ epiblast.]
During the later stages of the process last described remarkable
structures, eminently characteristic of the Insecta, have made their
first appearance. These structures are certain embryonic membranes or
coverings, which present in their mode of formation and arrangement a
startling similarity to the true and false amnion of the Vertebrata.
They appear as a double fold of the blastoderm round the edge of the
germinal area, which spreads over the ventral plate, from behind
forwards, in a general way in the same manner as the amnion in, for
instance, the chick. The folds at their origin are shewn in surface
view in fig. 176 D, am, and in section in fig. 177 B, _am_. The folds
eventually meet, coalesce (fig. 178, am) and give rise to two
membranes covering the ventral plate, viz. an inner one, which is
continuous with the edge of the ventral plate; and an outer,
continuous with the remainder of the blastoderm. The vertebrate
nomenclature may be conveniently employed for these membranes. The
inner limb of the fold will therefore be spoken of as the amnion, and
the outer one, including the dorsal part of the blastoderm, as the
serous envelope[168]. A slight consideration of the mode of formation
of the membranes, or an inspection of the figures illustrating their
formation, makes it at once clear that the yolk can pass in freely
between the amnion and serous envelope (_vide_ fig. 181). At the hind
end of the embryo this actually takes place, so that the ventral plate
covered by the amnion appears to become completely imbedded in the
yolk: elsewhere the two membranes are in contact. At first (fig. 176)
the ventral plate occupies but a small portion of the ventral surface
of the egg, but during the changes above described it extends over the
whole ventral surface, and even slightly on the dorsal surface both in
front and behind. It becomes at the same time (fig. 179) divided by a
series of transverse lines into segments, which increase in number and
finally amount in all to seventeen, not including the most anterior
section, which gives off as lateral outgrowths the two procephalic
lobes (_pc.l_). The changes so far described are included within what
Kowalevsky calls his first embryonic period; at its close the parts
contained within the chorion have the arrangement shewn in fig. 178 B.
The whole of the body of the embryo is formed from the ventral plate,
and no part from the amnion or serous envelope.
[168] The reverse nomenclature to this is rather inconveniently
employed by Metschnikoff.
[FIG. 179. EMBRYO OF HYDROPHILUS PICEUS VIEWED FROM THE VENTRAL
SURFACE. (After Kowalevsky.)
_pc.l._ procephalic lobe.]
The general history of the succeeding stages may be briefly told.
[FIG. 180. TWO STAGES IN THE DEVELOPMENT OF HYDROPHILUS PICEUS.
(From Gegenbaur, after Kowalevsky.)
_ls._ labrum; _at._ antenna; _md._ mandible; _mx._ maxilla I.; _li._
maxilla II.; _p´ p´´ p´´´_. feet; _a._ anus.]
The appendages appear as very small rudiments at the close of the last
stage, but soon become much more prominent (fig. 180 A). They are
formed as outgrowths of both layers, and arise nearly simultaneously.
There are in all eight pairs of appendages. The anterior or antennæ
(_at_) spring from the procephalic lobes, and the succeeding
appendages from the segments following. The last pair of embryonic
appendages, which disappears very early, is formed behind the third
pair of the future thoracic limbs. Paired epiblastic involutions,
shewn as pits in the posterior segments in fig. 180 A, give rise to
the tracheæ; and the nervous system is formed as two lateral
epiblastic thickenings, one on each side of the mid-ventral line.
These eventually become split off from the skin; while between them
there passes in a median invagination of the skin (fig. 189 C). The
two nervous strands are continuous in front with the supra-oesophageal
ganglia, which are formed of the epiblast of the procephalic lobes.
These plates gradually grow round the dorsal side of the embryo, and
there is formed immediately behind them an oral invagination, in front
of which there appears an upper lip (fig. 180, _ls_). A proctodæum is
formed at the hind end of the body slightly later than the stomodæum.
The mesoblast cells become divided into two bands, one on each side of
the middle line (fig. 189 A), and split into splanchnic and somatic
layers. The central yolk mass at about the stage represented in fig.
179 begins to break up into yolk spheres. The hypoblast is formed
first on the ventral side at the junction of the mesoblast and the
yolk, and gradually extends and forms a complete sack-like mesenteron,
enveloping the yolk (fig. 185 _al_). The amnion and serous membrane
retain their primitive constitution for some time, but gradually
become thinner on the ventral surface, where a rupture appears
eventually to take place. The greater part of them disappears, but in
the closure of the dorsal parietes the serous envelope plays a
peculiar part, which is not yet understood. It is described on p. 404.
The heart is formed from the mesoblastic layers, where they meet in
the middle dorsal line (fig. 185 C, _ht_). The somatic mesoblast gives
rise to the muscles and connective tissue, and the splanchnic
mesoblast to the muscular part of the wall of the alimentary tract,
which accompanies the hypoblast in its growth round the yolk. The
proctodæum forms the rectum and Malpighian bodies[169], and the
stomodæum the oesophagus and proventriculus. The two epiblastic
sections of the alimentary tract are eventually placed in
communication with the mesenteron.
[169] This has not been shewn in the case of Hydrophilus.
The development of Hydrophilus is a fair type of that of Insects
generally, but it is necessary to follow with somewhat greater detail
the comparative history of the various parts which have been briefly
described for this type.
_The embryonic membranes and the formation of the layers._
All Insects have at the close of segmentation a blastoderm formed of a
single row of cells enclosing a central yolk mass, which usually
contains nuclei, and in the Poduridæ is divided up in the ordinary
segmentation into distinct yolk cells. The first definite structure
formed is a thickening of the blastoderm, which forms a ventral plate.
The ventral plate is very differently situated in relation to the yolk
in different types. In most Diptera, Hymenoptera and (?) Neuroptera
(Phryganea) it forms from the first a thickening extending over nearly
the whole ventral surface of the ovum, and in many cases extends in
its subsequent growth not only over the whole ventral surface, but
over a considerable part of the apparent dorsal surface as well
(Chironomus, Simulia, Gryllotalpa, etc.). In Coleoptera, so far as is
known, it commences as a less extended thickening either of the
central part (Donacia) or posterior part (Hydrophilus) of the ventral
surface, and gradually grows in both directions, passing over to the
dorsal surface behind.
Embryonic membranes. In the majority of Insects there are developed
enveloping membranes like those of Hydrophilus.
[FIG. 181. DIAGRAMMATIC LONGITUDINAL SECTIONS OF AN INSECT EMBRYO AT
TWO STAGES TO SHEW THE DEVELOPMENT OF THE EMBRYONIC ENVELOPES.
In A the amniotic folds have not quite met so as to cover the
ventral plate. The yolk is represented as divided into yolk cells.
In B the sides of the ventral plate have extended so as nearly to
complete the dorsal integument. The mesenteron is represented as a
closed sack filled with yolk cells. _am._ amnion; _se._ serous
envelope; _v.p._ ventral plate; _d.i._ dorsal integument; _me._
mesenteron; _st._ stomodæum; _an i._ proctodæum.]
The typical mode of formation of these membranes is represented
diagrammatically in fig. 181 A and B. A fold of the blastoderm arises
round the edge of the ventral plate. This fold, like the amniotic fold
of the higher Vertebrata, is formed of two limbs, an outer, the serous
membrane (_se_), and an inner, the true amnion (_am_). Both limbs
extend so as to cover over the ventral plate, and finally meet and
coalesce, so that a double membrane is present over the ventral plate.
At the same time (fig. 181 B) the point where the fold originates is
carried dorsalwards by the dorsal extension of the edges of the
ventral plate, which give rise to the dorsal integument (_d.i_). This
process continues till the whole dorsal surface is covered by the
integument. The amnion then separates from the dorsal integument, and
the embryo becomes enveloped in two membranes--an inner, the amnion,
and an outer, the serous membrane. In fig. 181 B the embryo is
represented at the stage immediately preceding the closure of the
dorsal surface.
By the time that these changes are effected, the serous membrane and
amnion are both very thin and not easily separable. The amnion appears
to be usually absorbed before hatching; but in hatching both
membranes, if present, are either absorbed, or else ruptured and
thrown off.
The above mode of development of the embryonic membranes has been
especially established by the researches of Kowalevsky (No. 416) and
Graber (No. 412) for various Hymenoptera (_Apis_), Diptera
(_Chironomus_), Lepidoptera and Coleoptera (_Melolontha_, _Lina_).
Considerable variations in the development of the enveloping membranes
are known.
When the fold which gives rise to the membranes is first formed, there
is, as is obvious in fig. 181 A, a perfectly free passage by which the
yolk can pass in between the amnion and serous membrane. Such a
passage of the yolk between the two membranes takes place posteriorly
in Hydrophilus and Donacia: in Lepidoptera the yolk passes in
everywhere, so that in this form the ventral plate becomes first of
all imbedded in the yolk, and finally, on the completion of the dorsal
integument, the embryo is enclosed in a complete envelope of yolk
contained between the amnion and the serous membrane. During the
formation of the dorsal integument the external yolk-sack communicates
by a dorsally situated umbilical canal with the yolk cavity within the
body. On the rupture of the amnion the embryo is nourished at the
expense of the yolk contained in the external yolk-sack.
In the Hemiptera and the Libellulidæ the ventral plate also becomes
imbedded in the yolk, but in a somewhat different fashion to the
Lepidoptera, which more resembles on an exaggerated scale what takes
place in Hydrophilus.
[FIG. 182. THREE STAGES IN THE DEVELOPMENT OF THE EMBRYO OF
CALOPTERYX. (After Brandt.)
The embryo is represented in the egg-shell.
A. Embryo with ventral plate.
B. Commencing involution of ventral plate.
C. Involution of ventral plate completed.
_ps._ ventral plate; _g._ edge of ventral plate; _am._ amnion; _se._
serous envelope.]
In the Libellulidæ (_Calopteryx_) there is first of all formed
(Brandt, No. 403) a small ventral and posterior thickening of the
blastoderm (fig. 182 A). The hinder part of this becomes infolded into
the yolk as a projection (fig. 182 B), which consists of two laminæ,
an anterior and a posterior, continuous at the apex of the
invagination. The whole structure, which is completely imbedded within
the yolk, rapidly grows in length, and turns towards the front end of
the egg (fig. 182 C). Its anterior lamina remains thick and gives rise
to the ventral plate (_ps_), the posterior (_am_) on the other hand
becomes very thin, and forms a covering corresponding with the amnion
of the more ordinary types. The remainder of the blastoderm covering
the yolk (_se_) forms the homologue of the serous membrane of other
types. The ventral surface of the ventral plate is turned towards the
dorsal side (retaining the same nomenclature as in ordinary cases) of
the egg, and the cephalic extremity is situated at the point of origin
of the infolding.
The further history is however somewhat peculiar. The amnion is at
first (fig. 182 C) continuous with the serous envelope on the
posterior side only, so that the serous envelope does not form a
continuous sack, but has an opening close to the head of the embryo.
In the Hemiptera parasita this opening (Melnikow, No. 422) remains
permanent, and the embryo, after it has reached a certain stage of
development, becomes everted through it, while the yolk, enclosed in
the continuous membrane formed by the amnion and serous envelope,
forms a yolk-sack on the dorsal surface. In the Libellulidæ however
and most Hemiptera, a fusion of the two limbs of the serous membrane
takes place in the usual way, so as to convert it into a completely
closed sack (fig. 183 A). After the formation of the appendages a
fusion takes place between the amnion and serous envelope over a small
area close to the head of the embryo. In the middle of this area a
rupture is then effected, and the head of the embryo followed by the
body is gradually pushed through the opening (fig. 183 B and C). The
embryo becomes in the process completely rotated, and carried into a
position in the egg-shell identical with that of the embryos of other
orders of Insects (fig. 183 C).
[FIG. 183. THREE STAGES IN THE DEVELOPMENT OF CALOPTERYX. (After
Brandt.)
The embryo is represented in the egg-shell; B. and C. shew the
inversion of the embryo.
_se._ serous envelope; _am._ amnion; _ab._ abdomen; _v._ anterior
end of head; _at._ antennæ; _md._ mandible; _mx1._ maxilla 1;
_mx2._ maxilla 2; _p1-p3._ three pairs of legs;
_oe._ oesophagus.]
Owing to the rupture of the embryonic envelopes taking place at the
point where they are fused into one, the yolk does not escape in the
above process, but is carried into a kind of yolk-sack, on the dorsal
surface of the embryo, formed of the remains of the amnion and serous
envelope. The walls of the yolk-sack either assist in forming the
dorsal parietes of the body, or are more probably enclosed within the
body by the growth of the dorsal parietes from the edge of the ventral
plate.
[FIG. 184. THREE LARVAL STAGES OF HYDROPHILUS FROM THE DORSAL SIDE,
SHEWING THE GRADUAL CLOSING IN OF THE DORSAL REGION WITH THE
FORMATION OF THE PECULIAR DORSAL ORGAN _do._ (After Kowalevsky.)
_do._ dorsal organ; _at._ antennæ.]
In Hydrophilus and apparently in the Phryganidæ also, there are
certain remarkable peculiarities in the closure of the dorsal surface.
The fullest observations on the subject have been made by Kowalevsky
(No. 416), but Dohrn (No. 408) has with some probability thrown doubts
on Kowalevsky's interpretations. According to Dohrn the part of the
serous envelope which covers the dorsal surface becomes thickened, and
gives rise to a peculiar dorsal plate which is shewn in surface view
in fig. 184 A, _do_, and in section in fig. 185 A, _do._ The ventral
parts of the amnion and serous membrane have either been ruptured or
have disappeared. While the dorsal plate is being formed, the
mesoblast, and somewhat later the lateral parts of the epiblast of the
ventral plate gradually grow towards the dorsal side and enclose the
dorsal plate, the wall of which in the process appears to be folded
over so as first of all to form a groove and finally a canal. The
stages in this growth are shewn from the surface in fig. 184 B and C
and in section in fig. 185 B, _do._ The canal is buried on the dorsal
part of the yolk, but for some time remains open by a round aperture
in front (fig. 184 C). The whole structure is known as the dorsal
canal. It appears to atrophy without leaving a trace. The heart when
formed lies immediately dorsal to it[170].
[170] According to Kowalevsky the history of the dorsal plate is
somewhat different. He believes that on the absorption of the
amnion the ventral plate unites with the serous membrane, and
that the latter directly gives rise to the dorsal integument,
while the thickened part of it becomes involuted to form the
dorsal tube already described.
[FIG. 185. THREE TRANSVERSE SECTIONS THROUGH ADVANCED EMBRYOS OF
HYDROPHILUS.
A. Section through the posterior part of the body of the same age as
fig. 184 A.
B. Section through the embryo of the same age as fig. 184 C.
C. Section through a still older embryo.
_do._ dorsal plate; _vn._ ventral nerve cord; _al._ mesenteron;
_ht._ heart.
The large spaces at the sides are parts of the body cavity.]
In the Poduridæ the embryonic membranes appear to be at any rate
imperfect. Metschnikoff states in his paper on Geophilus that in some
ants no true embryonic membranes are found, but merely scattered cells
which take their place. In the Ichneumonidæ the existence of two
embryonic membranes is very doubtful.
Formation of the embryonic layers. The formation of the layers has
been studied in sections by Kowalevsky (No. 416), Hatschek (No. 414),
and Graber (No. 412), etc. From their researches it would appear that
the formation of the mesoblast always takes place in a manner closely
resembling that in Hydrophilus. The essential features of the process
(figs. 177 and 178) appear to be that a groove is formed along the
median line of the ventral plate, and that the sides of this groove
either (1) simply close over like the walls of the medullary groove in
Vertebrates, and so convert the groove into a tube, which soon becomes
solid and forms a mass or plate of cells internal to the epiblast; or
(2) that the cells on each side of the groove grow over it and meet in
the middle line, forming a layer external to the cells which lined the
groove. The former of these processes is the most usual; and in the
Muscidæ the dimensions of the groove are very considerable (Graber,
No. 411). In both cases the process is fundamentally the same, and
causes the ventral plate to become divided into two layers[171]. The
external layer or epiblast is an uniform sheet forming the main part
of the ventral plate (fig. 178 B, _ep_). It is continuous at its edge
with the amnion. The inner layer or mesoblast constitutes an
independent plate of cells internal to the epiblast (fig. 178 B,
_me_). The mesoblast soon becomes divided into two lateral bands.
[171] Tichomiroff (No. 420) denies the existence of a true
invagination to form the mesoblast, and also asserts that a
separation of mesoblast cells from the epiblast can take place at
other parts besides the median ventral line.
The origin of the hypoblast is still in dispute. It will be remembered
(_vide_ pp. 114 and 116) that after the segmentation a number of
nuclei remain in the yolk; and that eventually a secondary
segmentation of the yolk takes place around these nuclei, and gives
rise to a mass of yolk cells, which fill up the interior of the
embryo. These cells are diagrammatically shewn in figs. 181 and 189,
and it is probable that they constitute the true hypoblast. Their
further history is given below.
_Formation of the organs and their relation to the germinal
layers._
The segments and appendages. One of the earliest phenomena in the
development is the appearance of transverse lines indicating
segmentation (fig. 186). The transverse lines are apparently caused by
shallow superficial grooves, and also in many cases by the division of
the mesoblastic bands into separate somites. The most anterior line
marks off a præ-oral segment, which soon sends out two lateral
wings--the procephalic lobes. The remaining segments are at first
fairly uniform. Their number does not, however, appear to be very
constant. So far as is known they never exceed seventeen, and this
number is probably the typical one (figs. 186 and 187).
In Diptera the number appears to be usually fifteen though it may be
only fourteen. In Lepidoptera and in Apis there appear to be sixteen
segments. These and other variations affect only the number of the
segments which form the abdomen of the adult.
[FIG. 186. EMBRYO OF HYDROPHILUS PICEUS VIEWED FROM THE VENTRAL
SURFACE. (After Kowalevsky.)
_pc.l._ procephalic lobe.]
The appendages arise as paired pouch-like outgrowths of the epiblast
and mesoblast; and their number and the order of their appearance are
subject to considerable variation, the meaning of which is not yet
clear. As a rule they arise subsequently to the segmentation of the
parts of the body to which they belong. There is always formed one
pair of appendages which spring from the lateral lobes of the
procephalic region, or from the boundary line between these and the
median ventral part of this region. These appendages are the antennæ.
They have in the embryo a distinctly ventral position as compared to
that which they have in the adult.
In the median ventral part of the procephalic region there arises the
labrum (fig. 187, _ls_). It is formed by the coalescence of a pair of
prominences very similar to true appendages, though it is probable
that they have not this value[172].
[172] If these structures are equivalent to appendages, they may
correspond to one of the pairs of antennæ of Crustacea. From a
figure by Fritz Müller of the larva of Calotermes (_Jenaische
Zeit_. Vol. XI. pl. 11, fig. 12) it would appear that they lie in
front of the true antennæ, and would therefore on the above
hypothesis correspond to the first pair of antennæ of Crustacea.
Bütschli (No. 405) describes in the Bee a pair of prominences
immediately in front of the mandibles which eventually unite to
form a kind of underlip; they in some ways resemble true
appendages.
[FIG. 187. TWO STAGES IN THE DEVELOPMENT OF HYDROPHILUS PICEUS.
(From Gegenbaur, after Kowalevsky.)
_ls._ labrum; _at._ antenna; _md._ mandible; _mx._ maxilla I.; _li._
maxilla II.; _p´ p´´ p´´´._ feet; _a._ anus.]
The antennæ themselves can hardly be considered to have the same
morphological value as the succeeding appendages. They are rather
equivalent to paired processes of the præ-oral lobes of the Chætopoda.
From the first three post-oral segments there grow out the mandibles
and two pairs of maxillæ, and from the three following segments the
three pairs of thoracic appendages. In many Insects (cf. Hydrophilus)
a certain number of appendages of the same nature as the anterior ones
are visible in the embryo on the abdominal segments, a fact which
shews that Insects are descended from ancestors with more than three
pairs of ambulatory appendages.
In Apis according to Bütschli (No. 405) all the abdominal segments are
provided with appendages, which always remain in a very rudimentary
condition. All trace of them as well as of the thoracic appendages is
lost by the time the embryo is hatched. In the phytophagous
Hymenoptera the larva is provided with 9-11 pairs of legs.
In the embryo of Lepidoptera there would appear from Kowalevsky's
figures to be rudiments of ten pairs of post-thoracic appendages. In
the caterpillar of this group there are at the maximum five pairs of
such rudimentary feet, viz. a pair on the 3rd, 4th, 5th, and 6th, and
on the last abdominal segment. The embryos of Hydrophilus (fig. 187),
Mantis, etc. are also provided with additional appendages. In various
Thysanura small prominences are present on more or fewer of the
abdominal segments (fig. 192), which may probably be regarded as
rudimentary feet.
Whether all or any of the appendages of various kinds connected with
the hindermost segments belong to the same category as the legs is
very doubtful. Their usual absence in the embryo or in any case their
late appearance appears to me against so regarding them; but Bütschli
is of opinion that in the Bee the parts of the sting are related
genetically to the appendages of the penultimate and antepenultimate
abdominal segments, and this view is to some extent supported by more
recent observations (Kraepelin, etc.), and if it holds true for the
Bee must be regarded as correct for other cases also.
As to the order of the appearance of the appendages observations are
as yet too scanty to form any complete scheme. In many cases all the
appendages appear approximately at the same moment, _e.g._
Hydrophilus, but whether this holds good for all Coleoptera is by no
means certain. In Apis the appendages are stated by Bütschli to arise
simultaneously, but according to Kowalevsky the two mouth appendages
first appear, then the antennæ, and still later the thoracic
appendages. In the Diptera the mouth appendages are first formed, and
either simultaneously with these, or slightly later, the antennæ. In
the Hemiptera and Libellulidæ the thoracic appendages are the first to
be formed, and the second pair of maxillæ makes its appearance before
the other cephalic appendages.
The history of the changes in the embryonic appendages during the
attainment of the adult condition is beyond the scope of this
treatise, but it may be noted that the second pair of maxillæ are
relatively very large in the embryo, and not infrequently (Libellula,
etc.) have more resemblance to the ambulatory than to the masticatory
appendages.
[FIG. 188. FIGURES ILLUSTRATING AQUATIC RESPIRATION IN INSECTS.
(After Gegenbaur.)
A. Hinder portion of the body of Ephemera vulgata. _a._ longitudinal
tracheal trunks; _b._ alimentary canal; _c._ tracheal gills.
B. Larva of Æschna grandis. _a._ superior longitudinal tracheal
trunks; _b._ their anterior end; _c._ portion branching on
proctodæum; _o._ eyes.
C. Alimentary canal of the same larva from the side. _a, b,_ and
_c._ as in B; _d._ inferior tracheal trunk; _e._ transverse branches
between upper and lower tracheal trunks.]
The exact nature of the wings and their relation to the other segments
is still very obscure. They appear as dorsal leaf-like appendages on
the 2nd and 3rd thoracic segments, and are in many respects similar to
the tracheal gills of the larvæ of Ephemeridæ and Phryganidæ (fig. 188
A), of which they are supposed by Gegenbaur and Lubbock to be
modifications. The undoubtedly secondary character of the _closed_
tracheal system of larvæ with tracheal gills tells against this view.
Fritz Müller finds in the larvæ of Calotermes rugosus (one of the
Termites) that peculiar and similar dorsal appendages are present on
the two anterior of the thoracic segments. They are without tracheæ.
The anterior atrophies, and the posterior acquires tracheæ and gives
rise to the first pair of wings. The second pair of wings is formed
from small processes on the third thoracic segment like those on the
other two. Fritz Müller concludes from these facts that the wings of
Insects are developed from dorsal processes of the body, not
equivalent to the ventral appendages. What the primitive function of
these appendages was is not clear. Fritz Müller suggests that they may
have been employed as respiratory organs in the passage from an
aqueous to a terrestrial existence, when the Termite ancestors lived
in moist habitations--a function for which processes supplied with
blood-channels would be well adapted. The undoubted affinity of
Insects to Myriapods, coupled with the discovery by Moseley of a
tracheal system in Peripatus, is however nearly fatal to the view that
Insects can have sprung directly from aquatic ancestors not provided
with tracheæ. But although this suggestion of Fritz Müller cannot be
accepted, it is still possible that the processes discovered by him
may have been the earliest rudiments of wings, which were employed
first as organs of propulsion by a water-inhabiting Insect ancestor
which had not yet acquired the power of flying.
The nervous system. The nervous system arises entirely from the
epiblast; but the development of the præ-oral and post-oral sections
may be best considered separately.
The post-oral section, or ventral cord of the adult, arises as two
longitudinal thickenings of the epiblast, one on each side of the
median line (fig. 189 B, _vn_), which are subsequently split off from
the superficial skin and give rise to the two lateral strands of the
ventral cord. At a later period they undergo a differentiation into
ganglia and connecting cords.
Between these two embryonic nerve cords there is at first a shallow
furrow, which soon becomes a deep groove (fig. 189 C). At this stage
the differentiation of the lateral elements into ganglia and
commissures takes place, and, according to Hatschek (No. 414), the
median groove becomes in the region of the ganglia converted into a
canal, the walls of which soon fuse with those of the ganglionic
enlargements of the lateral cords, and connect them across the middle
line. Between the ganglia on the other hand the median groove
undergoes atrophy, becoming first a solid cord interposed between the
lateral strands of the nervous system, and finally disappearing
without giving rise to any part of the nervous system. It is probable
that Hatschek is entirely mistaken about the entrance of a median
element into the ventral cord, and that the appearances he has
described are due to shrinkage. In Spiders the absence of a median
element can be shewn with great certainty, and, as already stated,
this element is not present in Peripatus. Hatschek states that in the
mandibular segment the median element is absorbed, and that the two
lateral cords of that part give rise to the oesophageal commissures,
while the sub-oesophageal ganglion is formed from the fusion of the
ganglia of the two maxillary segments.
[FIG. 189. THREE TRANSVERSE SECTIONS THROUGH THE EMBRYO OF
HYDROPHILUS. (After Kowalevsky.)
A. Transverse section through the larva represented in fig. 187 A.
B. Transverse section through a somewhat older embryo in the region
of one of the stigmata.
C. Transverse section through the larva represented in fig. 187 B.
_vn._ ventral nerve cord; _am._ amnion and serous membrane; _me._
mesoblast; _me.s._ somatic mesoblast; _hy._ hypoblast(?);
_yk._ yolk cells (true hypoblast); _st._ stigma of trachea.]
The præ-oral portion of the nervous system consists entirely of the
supra-oesophageal ganglion. It is formed, according to Hatschek, of
three parts. Firstly and mainly, of a layer separated from the
thickened inner part of the cephalic lobe on each side; secondly, of
an anterior continuation of the lateral cords; and thirdly, of a pit
of skin invaginated on each side close to the dorsal border of the
antennæ. This pit is at first provided with a lumen, which is
subsequently obliterated; while the walls of the pit become converted
into true ganglion cells. The two supra-oesophageal ganglia remain
disconnected on the dorsal side till quite the close of embryonic
life.
The tracheæ and salivary glands. The tracheæ, as was first shewn by
Bütschli (No. 405), arise as independent segmentally arranged paired
invaginations of the epiblast (fig. 189 B and C, _st_). Their openings
are always placed on the outer sides of the appendages of their
segments, where such are present.
Although in the adult stigmata are never found in the space between
the prothorax and head[173], in the embryo and the larva tracheal
invaginations may be developed in all the thoracic (and possibly in
the three jaw-bearing segments) and in all the abdominal segments
except the two posterior.
[173] In Smynthurus, one of the Collembola, there are, according
to Lubbock, only two stigmata, which are placed on the head.
In the embryo of the Lepidoptera, according to Hatschek (No. 414),
there are 14 pairs of stigmata, belonging to the 14 segments of the
body behind the mouth; but Tichomiroff states that Hatschek is in
error in making this statement for the foremost post-oral segments.
The last two segments are without stigmata. In the larvæ of
Lepidoptera as well as those of many Hymenoptera, Coleoptera and
Diptera, stigmata are present on all the postcephalic segments except
the 2nd and 3rd thoracic and the two last abdominal. In Apis there are
eleven pairs of tracheal invaginations according to Kowalevsky (No.
416), but according to Bütschli (No. 405) only ten, the prothorax
being without one. In the Bee they appear simultaneously, and before
the appendages.
The blind ends of the tracheal invaginations frequently (_e.g._ Apis)
unite together into a common longitudinal canal, which forms a
longitudinal tracheal stem. In other cases (_e.g._ Gryllotalpa,
_Dohrn_, No. 408) they remain distinct, and each tracheal stem has a
system of branches of its own.
The development of the tracheæ strongly supports the view, arrived at
by Moseley from his investigations on Peripatus, that they are
modifications of cutaneous glands.
The salivary and spinning glands are epiblastic structures, which in
their mode of development are very similar to the tracheæ, and perhaps
have a similar origin. The salivary glands arise as paired epiblastic
invaginations, not, as might be expected, of the Stomodæum, but of the
ventral plate behind the mouth on the inner side of the mandibles. At
first independent, they eventually unite in a common duct, which falls
into the mouth. The spinning glands arise on the inner side of the
second pair of maxillæ in Apis and Lepidoptera, and form elongated
glands extending through nearly the whole length of the body. They are
very similar in their structure and development to salivary glands,
and are only employed during larval life. They no doubt resemble the
mucous glands of the oral papillæ of Peripatus, with which they have
been compared by Moseley. The mucous glands of Peripatus may perhaps
be the homologous organs of the first pair of maxillæ, for the
existence of which there appears to be some evidence amongst Insects.
Mesoblast. It has been stated that the mesoblast becomes divided in
the region of the body into two lateral bands (fig. 189 A). These
bands in many, if not all forms, become divided into a series of
somites corresponding with the segments of the body. In each of them a
cavity appears--the commencing perivisceral cavity--which divides them
into a somatic plate in contact with the epiblast, and a splanchnic
plate in contact with the hypoblast (fig. 189). In the interspaces
between the segments the mesoblast is continuous across the median
ventral line. The mesoblast is prolonged into each of the appendages
as these are formed, and in the appendages there is present a central
cavity. By Metschnikoff these cavities are stated to be continuous, as
in Myriapods and Arachnida, with those of the somites; but by Hatschek
(No. 414) they are stated to be independent of those in the somites
and to be open to the yolk.
The further details of the history of the mesoblast are very
imperfectly known, and the fullest account we have is that by Dohrn
(No. 408) for Gryllotalpa. It would appear that the mesoblast grows
round and encloses the dorsal side of the yolk earlier than the
epiblast. In Gryllotalpa it forms a pulsating membrane. As the
epiblast extends dorsalwards the median dorsal part of the membrane is
constricted off as a tube which forms the heart. At the same time the
free space between the pulsating membrane and the yolk is obliterated,
but transverse passages are left at the lines between the somites,
through which the blood passes from the ventral part of the body to
corresponding openings in the wall of the heart. The greater part of
the membrane gives rise to the muscles of the trunk.
Ventrally the mesoblastic bands soon meet across the median line. The
cavities in the appendages become obliterated and their mesoblastic
walls form the muscles, etc. The cavities in the separate mesoblastic
somites also cease to be distinctly circumscribed.
The splanchnic mesoblast follows the hypoblast in its growth, and
gives rise to the connective tissue and muscular parts of the walls of
the alimentary tract. The mesoblastic wall of the proctodæum is
probably formed independently of the mesoblastic somites. In the head
the mesoblast is stated to form at first a median ventral mass, which
does not pass into the procephalic lobe; though it assists in forming
both the antennæ and upper lip.
The alimentary canal. The alimentary tract of Insects is formed of
three distinct sections (fig. 181)--a mesenteron or middle section
(_me_), a stomodæum (_st_) and a proctodæum (_an_). The stomodæum and
proctodæum are invaginations of the epiblast, while the mesenteron is
lined by the hypoblast. The distinction between the three is usually
well marked in the adult by the epiblastic derivatives being lined by
chitin. The stomodæum consists of mouth, oesophagus, crop, and
proventriculus or gizzard, when such are present. The mesenteron
includes the stomach, and is sometimes (Orthoptera, etc.) provided at
its front end with pyloric diverticula--posteriorly it terminates just
in front of the Malpighian bodies. These latter fall into the
proctodæum, which includes the whole of the region from their
insertion to the anus.
The oral invagination appears nearly coincidently with the first
formation of segments at the front end of the groove between the
lateral nerve cords, and the anal invagination appears slightly later
at the hindermost end of the ventral plate.
The Malpighian bodies arise as _two pairs of outgrowths of the
epiblast of the proctodæum_, whether solid at first is not certain.
The subsequent increase which usually takes place in their number is
due to sproutings (at first solid) of the two original vessels.
The glandular walls of the mesenteron are formed from the hypoblast;
but the exact origin of the layer has not been thoroughly worked out
in all cases. In Hydrophilus it is stated by Kowalevsky (No. 416) to
appear as two sheets split off from the lateral masses of mesoblast,
which gradually grow round the yolk, and a similar mode of formation
would seem to hold good for Apis. Tichomiroff (No. 420) confirms
Kowalevsky on this point, and further states that these two masses
meet first ventrally and much later on the dorsal side. In
Lepidoptera, on the other hand, Hatschek finds that the hypoblast
arises as a median mass of polygonal cells in the anterior part of the
ventral plate. These cells increase by absorbing material from the
yolk, and then gradually extend themselves and grow round the yolk.
Dohrn (No. 408) believes that the yolk cells, the origin of which has
already been spoken of, give rise to the hypoblastic walls of the
mesenteron, and this view appears to be shared by Graber (No. 412),
though the latter author holds that some of the yolk cells are derived
by budding from the blastoderm[174].
[174] Graber's view on this point may probably be explained by
supposing that he has mistaken a passage of yolk cells into the
blastoderm for a passage of blastoderm cells into the yolk. The
former occurrence takes place, as I have found, largely in
Spiders, and probably therefore also occurs in Insects.
From the analogy of Spiders I am inclined to accept Dohrn's and
Graber's view. It appears to me probable that Kowalevsky's
observations are to be explained by supposing that the hypoblast
plates which he believes to be split off from the mesoblast are really
separated from the yolk.
It will be convenient to add here a few details to what has already
been stated as to the origin of the yolk cells. As mentioned above,
the central yolk breaks up at a period, which is not constant in the
different forms, into polygonal or rounded masses, in each of which a
nucleus has in many instances been clearly demonstrated although in
others such nuclei have not been made out. It is probable however that
nuclei are in all cases really present, and that these masses must be
therefore regarded as cells. They constitute in fact the yolk cells.
The periphery of the yolk breaks up into cells while the centre is
still quite homogeneous.
The hypoblastic walls of the mesenteron appear to be formed in the
first instance laterally (fig. 189 B and C, _hy_). They then meet
ventrally (fig. 185 A and B), and finally close in the mesenteron on
the dorsal side.
The mesenteron is at first a closed sack, independent of both
stomodæum and proctodæum; and in the case of the Bee it so remains
even after the close of embryonic life. The only glandular organs of
the mesenteron are the not unfrequent pyloric tubes, which are simple
outgrowths of its anterior end. It is possible that in some instances
they may be formed _in situ_ around the lateral parts of the yolk.
In many instances the whole of the yolk is enclosed in the walls of
the mesenteron, but in other cases, as in Chironomus and Simulia
(Weismann, No. 430; Metschnikoff, No. 423), part of the yolk may be
left between the ventral wall of the mesenteron and the ventral plate.
In Chironomus the mass of yolk external to the mesenteron takes the
form of a median and two lateral streaks. Some of the yolk cells
either prior to the establishment of the mesenteron, or derived from
the unenclosed portions of the yolk, pass into the developing organs
(Dohrn, 408) and serve as a kind of nutritive cell. They also form
blood corpuscles and connective-tissue elements. Such yolk cells may
be compared to the peculiar bodies described by Reichenbach in
Astacus, which form the secondary mesoblast. Similar cells play a very
important part in the development of Spiders.
Generative organs. The observations on the development of the
generative organs are somewhat scanty. In Diptera certain cells--known
as the pole cells--are stated by both Metschnikoff (No. 423) and
Leuckart to give rise to the generative organs. The cells in question
(in Chironomus and Musca vomitoria, Weismann, No. 430) appear at the
hinder end of the ovum before any other cells of the blastoderm. They
soon separate from the blastoderm and increase by division. In the
embryo, produced by the viviparous larva of Cecidomyia, there is at
first a single pole cell, which eventually divides into four, and the
resulting cells become enclosed within the blastoderm. They next
divide into two masses, which are stated by Metschnikoff (No. 423) to
become surrounded by indifferent embryonic cells[175]. Their
protoplasm then fuses, and their nuclei divide, and they give rise to
the larval ovaries, for which the enclosing cells form the tunics.
[175] This point requires further observation.
In _Aphis_ Metschnikoff (No. 423) detected at a very early stage a
mass of cells which give rise to the generative organs. These cells
are situated at the hind end of the ventral plate; and, except in the
case of one of the cells which gives rise by division to a green mass
adjoining the fat body, the protoplasm of the separate cells fuses
into a syncytium. Towards the close of embryonic life the syncytium
assumes a horse-shoe form. The mass is next divided into two, and the
peripheral layer of each part gives rise to the tunic, while from the
hinder extremity of each part an at first solid duct--the
egg-tube--grows out. The masses themselves form the germogens. The
oviduct is formed by a coalescence of the ducts from each germogen.
Ganin derives the generative organs in Platygaster (_vide_ p. 347)
from the hind end of the ventral plate close to the proctodæum; while
Suckow states that the generative organs are outgrowths of the
proctodæum. According to these two sets of observations the generative
organs would appear to have an epiblastic origin--an origin which is
not incompatible with that from the pole cells.
In Lepidoptera the genital organs are present in the later periods of
embryonic life as distinct paired organs, one on each side of the
heart, in the eighth postcephalic segment. They are elliptical bodies
with a duct passing off from the posterior end in the female or from
the middle in the male. The egg-tubes or seminal tubes are outgrowths
of the elliptical bodies.
In other Insects the later stages in the development of the generative
organs closely resemble those in the Lepidoptera, and the organs are
usually distinctly visible in the later stages of embryonic life.
It may probably be laid down, in spite of some of Metschnikoff's
observations above quoted, that the original generative mass gives
rise to both the true genital glands and their ducts. It appears also
to be fairly clear that _the genital glands of both sexes have an
identical origin_.
_Special types of larvæ._
Certain of the Hymenopterous forms, which deposit their eggs in the
eggs or larvæ of other Insects, present very peculiar modifications in
their development. Platygaster, which lays its egg in the larvæ of
Cecidomyia, undergoes perhaps the most remarkable development amongst
these forms. It has been studied especially by Ganin (No. 410), from
whom the following account is taken.
[FIG. 190. A SERIES OF STAGES IN THE DEVELOPMENT OF PLATYGASTER.
(From Lubbock; after Ganin.)]
The very first stages are unfortunately but imperfectly known, and the
interpretations offered by Ganin do not in all cases appear quite
satisfactory. In the earliest stage after being laid the egg is
enclosed in a capsule produced into a stalk (fig. 190 A). In the
interior of the egg there soon appears a single spherical body,
regarded by Ganin as a cell (fig. 190 B). In the next stage three
similar bodies appear in the vitellus, no doubt derived from the first
one (fig. 190 C). The central one presents somewhat different
characters to the two others, and, according to Ganin, gives rise to
_the whole embryo_. The two peripheral bodies increase by division,
and soon appear as nuclei imbedded in a layer of protoplasm (fig. 190
D, E, F). The layer so formed serves as a covering for the embryo,
regarded by Ganin as equivalent to the amnion (? serous membrane) of
other Insect embryos. In the embryo cell new cells are stated to be
formed by a process of endogenous cell formation (fig. 190 D, E). It
appears probable that Ganin has mistaken nuclei for cells in the
earlier stages, and that a blastoderm is formed as in other Insects,
and that this becomes divided in a way not explained into a
superficial layer which gives rise to the serous envelope, and a
deeper layer which forms the embryo. However this may be, a
differentiation into an epiblastic layer of columnar cells and a
hypoblastic layer of more rounded cells soon becomes apparent in the
body of the embryo. Subsequently to this the embryo grows rapidly,
till by a deep transverse constriction on the ventral surface it
becomes divided into an anterior cephalothoracic portion and a
posterior caudal portion (fig. 190 F). The cephalothorax grows in
breadth, and near its anterior end an invagination appears, which
gives rise to the mouth and oesophagus. On the ventral side of the
cephalothorax there is first formed a pair of claw-like appendages on
each side of the mouth, then a posterior pair of appendages near the
junction of the cephalothorax and abdomen, and lastly a pair of short
conical antennæ in front.
At the same time the hind end of the abdomen becomes bifid, and gives
rise to a fork-like caudal appendage; and at a slightly later period
four grooves make their appearance in the caudal region, and divide
this part of the embryo into successive segments. While these changes
have been taking place in the general form of the embryo, the epiblast
has given rise to a cuticle, and the hypoblastic cells have become
differentiated into a central hypoblastic axis--the mesenteron--and a
surrounding layer of mesoblast, some of the cells of which form
longitudinal muscles.
With this stage closes what may be regarded as the embryonic
development of Platygaster. The embryo becomes free from the amnion,
and presents itself as a larva, which from its very remarkable
characters has been spoken of as the Cyclops larva by Ganin.
The larvæ of three species have been described by Ganin, which are
represented in fig. 191 A, B, C. These larvæ are strangely dissimilar
to the ordinary Hexapod type, whether larval or adult. They are formed
of a cephalothoracic shield with the three pairs of appendages (_a_,
_kf_, _lfg_), the development of which has already been described, and
of an abdomen formed of five segments, the last of which bears the
somewhat varying caudal appendages. The nervous system is as yet
undeveloped.
The larvæ move about in the tissues of their hosts by means of their
claws.
The first larval condition is succeeded by a second with very
different characters, and the passage from the first to the second is
accompanied by an ecdysis.
The ecdysis commences at the caudal extremity, and the whole of the
last segment is completely thrown off. As the ecdysis extends forwards
the tail loses its segmentation and becomes strongly compressed, the
appendages of the cephalothorax are thrown off, and the whole embryo
assumes an oval form without any sharp distinction into different
regions and without the _slightest indication of segmentation_ (fig.
191 D). Of the internal changes which take place during the shedding
of the cuticle, the first is the formation of a proctodæum (_gh_) by
an invagination, which ends blindly in contact with the mesenteron.
Shortly after this a thickening of the epiblast (_bsm_) appears along
the ventral surface, which gives rise mainly to the ventral nerve
cord; this thickening is continuous behind with the epiblast which is
invaginated to form the proctodæum, and in front is prolonged on each
side into two procephalic lobes, in which there are also thickenings
of the epiblast (_gsae_), which become converted into supra-oesophageal
ganglia, and possibly other parts.
[FIG. 191. A SERIES OF STAGES IN THE DEVELOPMENT OF PLATYGASTER.
(From Lubbock; after Ganin.)
A. B. C. Cyclops larvæ of three species of Platygaster. D. Second
larval stage. E. Third larval stage.
_mo._ mouth; _a._ antenna; _hf._ hooked feet; _lfg._ lateral feet;
_f._ branches of tail; _ul._ lower lip; _slkf._ oesophagus;
_gsae._ supra-oesophageal ganglion; _bsm._ ventral epiblastic
plate; _lm._ lateral muscles (the letters also point in D to the
salivary glands); _gh._ proctodæum; _ga._ generative organs;
_md._ mandibles; _ag._ ducts of salivary glands; _sp._ (in E)
salivary glands; _mls._ stomach; _ed._ intestine; _ew._ rectum;
_ao._ anus; _tr._ tracheæ; _fk._ fat body.]
Towards the close of the second larval period the muscles (_lm_)
become segmentally arranged, and give indications of the segmentation
which becomes apparent in the third larval period. The third and last
larval stage (fig. 191 E) of Platygaster, during which it still
remains in the tissues of its host, presents no very peculiar
features. The passage from the second to the third form is accompanied
by an ecdysis.
Remarkable as are the larvæ just described, there can I think be no
reason, considering their parasitic habits, for regarding them as
ancestral.
_Metamorphosis and heterogamy._
Metamorphosis. The majority of Insects are born in a condition in
which they obviously differ from their parents. The extent of this
difference is subject to very great variations, but as a rule the
larvæ pass through a very marked metamorphosis before reaching the
adult state. The complete history of this metamorphosis in the
different orders of Insects involves a far too considerable amount of
zoological detail to be dealt with in this work; and I shall confine
myself to a few observations on the general characters and origin of
the metamorphosis, and of the histological processes which take place
during its occurrence[176].
[176] For a systematic account of this subject the reader is
referred to Lubbock (No. 420) and to Graber (No. 411). He will
find in Weismann (Nos. 430 and 431) a detailed account of the
internal changes which take place.
In the Aptera the larva differs from the adult only in the number of
facets in the cornea and joints in the antennæ.
In most Orthoptera and Hemiptera the larvæ differ from the adult in
the absence of wings and in other points. The wings, etc., are
gradually acquired in the course of a series of successive moultings.
In the Ephemeridæ and Libellulidæ, however, the metamorphosis is more
complicated, in that the larvæ have provisional tracheal gills which
are exuviated before the final moult. In the Ephemeridæ there are
usually a great number of moultings; the tracheal gills appear after
the second moult, and the rudiments of the wings when the larva is
about half grown. Larval life may last for a very long period.
In all the other groups of Insects, viz. the Diptera, Neuroptera,
Coleoptera, Lepidoptera, and Hymenoptera, the larva passes--with a few
exceptions--through a quiescent stage, in which it is known as a pupa,
before it attains the adult stage. These forms are known as the
Holometabola.
In the Diptera the larvæ are apodous. In the true flies (Muscidæ) they
are without a distinct head and have the jaws replaced by hooks. In
the Tipulidæ there is on the other hand a well-developed head with the
normal appendages. The pupæ of the Muscidæ are quiescent, and are
enclosed in the skin of the larva which shrinks and forms a firm oval
case. In the Tipulidæ the larval skin is thrown off at the pupa stage,
and in some cases the pupæ continue to move about.
The larvæ of the Neuroptera are hexapodous voracious forms. When the
larva becomes a pupa all the external organs of the imago are already
established. The pupa is often invested in a cocoon. It is usually
quiescent, though sometimes it begins to move about shortly before the
imago emerges.
In the Coleoptera there is considerable variety in the larval forms.
As a rule the larvæ are hexapodous and resemble wingless Insects. But
some herbivorous larvæ (_e.g._ the larva of Melolontha) closely
resemble true caterpillars, and there are also grub-like larvæ without
feet (Curculio) which resemble the larvæ of Hymenoptera. The pupa is
quiescent, but has all the parts of the future beetle plainly visible.
The most interesting larvæ among the Coleoptera are those of Sitaris,
one of the Meloidæ (Fabre, No. 409). They leave the egg as active
hexapodous larvæ which attach themselves to the bodies of Hymenoptera,
and are thence transported to a cell filled with honey. Here they eat
the ovum of the Hymenopterous form. They then undergo an ecdysis, in
which they functionally lose their appendages, retaining however small
rudiments of them, and become grubs. They feed on the honey and after
a further ecdysis become pupæ.
In the Lepidoptera the larva has the well-known form of a caterpillar.
The caterpillars have strong jaws, adapted for biting vegetable
tissues, which are quite unlike the oral appendages of the adult. They
have three pairs of jointed thoracic legs, and a variable number
(usually five) of pairs of rudimentary abdominal legs--the so-called
pro-legs. The larva undergoes numerous ecdyses, and the external parts
of the adult such as the wings, etc., are formed underneath the
chitinous exoskeleton before the pupa stage. The pupa is known as a
chrysalis and in some Lepidoptera is enveloped in a cocoon.
The Hymenoptera present considerable variations in the character of
the larvæ. In the Aculeata, many Entomophaga, the Cynipidæ, etc., the
larvæ are apodous grubs, incapable of going in search of their food;
but in the Siricidæ they are hexapodous forms like caterpillars, which
are sometimes even provided with pro-legs. In some of the Entomophaga
the larvæ have very remarkable characters which have already been
described in a special section, _vide_ pp. 418, 419.
Before proceeding to the consideration of the value of the various
larval forms thus shortly enumerated, it is necessary to say a few
words as to the internal changes which take place during the
occurrence of the above metamorphosis. In the simplest cases, such as
those of the Orthoptera and Hemiptera, where the metamorphosis is
confined to the gradual formation of the wings, etc. in a series of
moults, the wings first appear as two folds of the epidermis beneath
the cuticle on the two posterior thoracic segments. At the next moult
these processes become covered by the freshly formed cuticle, and
appear as small projections. At every successive moult these
projections become more prominent owing to a growth in the epidermis
which has taken place in the preceding interval. Accompanying the
formation of such organs as the wings, internal changes necessarily
take place in the arrangement of the muscles, etc. of the thorax,
which proceed _pari passu_ with the formation of the organs to which
they belong. The characters of the metamorphosis in such forms as the
Ephemeridæ only differ from the above in the fact that provisional
organs are thrown off at the same time that the new ones are formed.
In the case of the Holometabola the internal phenomena of the
metamorphosis are of a very much more remarkable character. The
details of our knowledge are largely due to Weismann (Nos. 430 and
431). The larvæ of the Holometabola have for the most part a very
different mode of life to the adults. A simple series of transitions
between the two is impossible, because intermediate forms would be for
the most part incapable of existing. The transition from the larval to
the adult state is therefore necessarily a more or less sudden one,
and takes place during the quiescent pupa condition. Many of the
external adult organs are however formed prior to the pupa stage, but
do not become visible on the surface. The simplest mode of
Holometabolic metamorphosis may be illustrated by the development of
Corethra plumicornis, one of the Tipulidæ. This larva, like that of
other Tipulidæ, is without thoracic appendages, but before the last
larval moult, and therefore shortly before the pupa stage, certain
structures are formed, which Weismann has called imaginal discs. These
imaginal discs are in Corethra simply invaginations of the epidermis.
There are in the thorax six pairs of such structures, three dorsal and
three ventral. The three ventral are attached to the terminations of
the sensory nerves, and the limbs of the imago are formed as simple
outgrowths of them, which as they grow in length take a spiral form.
In the interior of these outgrowths are formed the muscles, tracheæ,
etc., of the limbs; which are believed by Weismann (it appears to me
without sufficient ground) to be derived from a proliferation of the
cells of the neurilemma. The wings are formed from the two posterior
dorsal imaginal discs. The hypodermis of the larva passes directly
into that of the imago.
The pupa stage of Corethra is relatively very short, and the changes
in the internal parts which take place during it are not considerable.
The larval abdominal muscles pass for the most part unchanged into
those of the imago, while the special thoracic muscles connected with
the wings, etc., develop directly during the latest larval period from
cords of cells already formed in the embryo.
In the Lepidoptera the changes in the passage from the larval to the
adult state are not very much more considerable than those in
Corethra. Similar imaginal discs give rise during the later larval
periods to the wings, etc. The internal changes during the longer pupa
period are somewhat more considerable. Important modifications and new
formations arise in connection with the alimentary tract, the nervous
and muscular systems.
The changes which take place in the true flies (Muscidæ) are far more
complicated than either those in Corethra or in the Lepidoptera. The
abdomen of the larva of Musca becomes bodily converted into the
abdomen of the imago as in the above types, but the whole epidermis
and appendages of the head and thorax are derived from imaginal discs
which are formed within and (so far as is known) independently of the
epidermis of the larva or embryo. These imaginal discs are simple
masses of apparently indifferent cells, which for the most part appear
at the close of embryonic life, and are attached to nerves or tracheæ.
They grow in size during larval life, but during the relatively long
pupa stage they unite together to give rise to a continuous epidermis,
from which the appendages grow out as processes. The epidermis of the
anterior part of the larva is simply thrown off, and has no share in
forming the epidermis of the adult.
There are a pair of cephalic imaginal discs and six pairs of thoracic
discs. Two pairs, a dorsal and a ventral, give rise to each thoracic
ring, and the appendages attached to it.
Though, as mentioned above, no evidence has yet been produced to shew
that the imaginal discs of Musca are derived from the embryonic
epiblast, yet their mode of growth and eventual fate proves beyond the
shadow of a doubt that they are homologous with the imaginal discs of
Corethra. Their earliest origin is well worth further investigation.
The metamorphosis of the internal organs is still more striking than
that of the external. There is a disruption, total or partial, of all
the internal organs except the generative organs. In the case of the
alimentary tract, the Malpighian vessels, the heart and the central
nervous system, the disruption is of a partial kind, which has been
called by Weismann histolysis. The cells of these organs undergo a
fatty degeneration, the nuclei alone in some cases remaining. The kind
of plasma resulting from this degeneration retains the shape of the
organs, and finally becomes built up again into the corresponding
organs of the imago. The tracheæ, muscles and peripheral nerves, and
an anterior part of the alimentary tract, are entirely disrupted. They
seem to be formed again from granular cells derived from the enormous
fat body.
The phenomena of the development of the Muscidæ are undoubtedly of
rather a surprising character. Leaving for the moment the question of
the origin of the pupa stage to which I return below, it will be
admitted on all hands that during the pupa stage the larva undergoes a
series of changes which, had they taken place by slow degrees, would
have involved, in such a case as Musca, a complete though gradual
renewal of the tissues. Such being the case, the cells of the organs
common to the larva and the imago would, in the natural course of
things, not be the same cells as those of the larva but descendants of
them. We might therefore expect to find in the rapid conversion of the
larval organs into those of the adult some condensation, so to speak,
of the process of ordinary cell division. Such condensations are
probably represented in the histolysis in the case of the internal
organs, and in the formation of imaginal discs in the case of the
external ones, and I think it probable that further investigation will
shew that the imaginal discs of the Muscidæ are derivatives of the
embryonic epiblast. The above considerations by no means explain the
whole of Weismann's interesting observations, but an explanation is I
believe to be found by following up these lines.
More or less parallel phenomena to those in Insects are found in the
development of the Platyelminthes and Echinoderms. The four disc-like
invaginations of the skin in many larval Nemertines (_vide_ p. 198),
which give rise to the permanent body wall of the Nemertine, may be
compared to the imaginal discs. The subsequent throwing off of the
skin of Pilidium or larva of Desor is a phenomenon like the absorption
of part of the larval skin of Musca. The formation of an independent
skin within the first larval form in the Distomeæ and in the Cestoda
may be compared to the apparently independent formation of the
imaginal discs in Musca.
The fact that in a majority of instances it is possible to trace an
intimate connection between the surroundings of a larva and its
organization proves in the clearest way _that the characters of the
majority of existing larval forms of Insects have owed their origin to
secondary adaptations_. A few instances will illustrate this point.
[FIG. 192. ANTERIOR HALF OF CAMPODEA FRAGILIS. (From Gegenbaur;
after Palmen.)
_a._ antennæ; _p._ feet; _p´._ post-thoracic rudimentary feet; _s._
stigma.]
In the simplest types of metamorphosis, _e.g._ those of the Orthoptera
genuina, the larva has precisely the same habits as the adult. We find
that a caterpillar form is assumed by phytophagous larvæ amongst the
Lepidoptera, Hymenoptera and Coleoptera. Where the larva has not to go
in search of its nutriment the grub-like apodous form is assumed. The
existence of such an apodous larva is especially striking in the
Hymenoptera, in that rudiments of thoracic and abdominal appendages
are present in the embryo and disappear again in the larva. The case
of the larva of Sitaris, already described (p. 421), affords another
very striking proof that the organization of the larva is adapted to
its habits.
It follows from the above that the development of such forms as the
Orthoptera genuina is more primitive than that of the holometabolous
forms; a conclusion which tallies with the fact that both
palæontological and anatomical evidence shew the Orthoptera to be a
very primitive group of Insects.
The above argument probably applies with still greater force to the
case of the Thysanura; and it seems to be probable that this group is
more nearly related than any other to the primitive wingless ancestors
of Insects[177]. The characters of the oral appendages in this group,
the simplicity of their metamorphosis, and the presence of abdominal
appendages (fig. 192), all tell in favour of this view, while the
resemblance of the adult to the larvæ of the Pseudoneuroptera, etc.,
points in the same direction. The Thysanura and Collembola are not
however to be regarded as belonging to the true stock of the ancestors
of Insects, but as degenerated relations of this stock; much as
Amphioxus and the Ascidians are degenerate relations of the ancestral
stock of Vertebrates, and Peripatus of that of the Tracheata. It is
probable that all these forms have succeeded in retaining their
primitive characters from their degenerate habits, which prevented
them from entering into competition in the struggle for existence with
their more highly endowed relatives. While in a general way it is
clear that the larval forms of Insects cannot be expected to throw
much light on the nature of Insect ancestors, it does nevertheless
appear to me probable that such forms as the caterpillars of the
Lepidoptera are not without a meaning in this respect. It is easy to
conceive that even a secondary larval form may have been produced by
the prolongation of one of the embryonic stages; and the general
similarity of a caterpillar to Peripatus, and the retention by it of
post-thoracic appendages, are facts which appear to favour this view
of the origin of the caterpillar form.
[177] Brauer and Lubbock (No. 421) have pointed out the primitive
characters of these forms, especially of Campodea.
The two most obscure points which still remain to be dealt with in the
metamorphosis of Insects are (1) the origin of the quiescent pupa
stage; (2) the frequent dissimilarity between the masticatory
apparatus of the larva and adult.
These two points may be conveniently dealt with together, and some
valuable remarks about them will be found in Lubbock (No. 420).
On grounds already indicated it may be considered certain that the
groups of Insects without a pupa stage, and with a larva very
similarly organised to the adult, preceded the existing holometabolic
groups. The starting point in the metamorphosis of the latter groups
was therefore something like that of the Orthoptera. Suppose it became
an advantage to a species that the larva and adult should feed in a
somewhat different way, a difference in the character of their mouth
parts would soon make itself manifest; and, since an intermediate type
of mouth parts would probably be disadvantageous, there would be a
tendency to concentrate into a single moult the transition from the
larval to the adult form of mouth parts. At each ordinary moult there
is a short period of quiescence, and this period of quiescence would
naturally become longer in the important moult at which the change in
the mouth parts was effected. In this way a rudimentary pupa stage
might be started. The pupa stage, once started, might easily become a
more important factor in the metamorphosis. If the larva and imago
diverged still more from each other, a continually increasing amount
of change would have to be effected at the pupa stage. It would
probably be advantageous to the species that the larva should not have
rudimentary functionless wings; and the establishment of the wings as
external organs would therefore become deferred to the pupa stage. The
same would probably apply to other organs.
Insects usually pass through the pupa stage in winter in cold climates
and during the dry season in the tropics, this stage serving therefore
apparently for the protection of the species during the inclement
season of the year. These facts are easily explained on the
supposition that the pupa stage has become secondarily adapted to play
a part in the economy of the species quite different from that to
which it owes its origin.
Heterogamy. The cases of alternations of generations amongst Insects
all fall under the heading already defined in the introduction as
Heterogamy. Heterogamy amongst Insects has been rendered possible by
the existence of parthenogenesis, which, as stated in the
introduction, has been taken hold of by natural selection, and has led
to the production of generations of parthenogenetic forms, by which a
clear economy in reproduction is effected. Parthenogenesis without
heterogamy occurs in a large number of forms. In Bees, Wasps, and a
Sawfly (Nematus ventricosus) the unfertilized ova give rise to males.
In two Lepidopterous genera (Psyche and Solenobia) the unfertilized
ova give rise mainly, if not entirely, to females. Heterogamy occurs
in none of the above types, but in Psyche and Solenobia males are only
occasionally found, so that a series of generations producing female
young from unfertilized ova are followed by a generation producing
young of both sexes from fertilized ova. It would be interesting to
know if the unimpregnated female would not after a certain number of
generations give rise to both males and females; such an occurrence
might be anticipated on grounds of analogy. In the cases of true
heterogamy parthenogenesis has become confined to special generations,
which differ in their character from the generations which reproduce
themselves sexually. The parthenogenetic generations generally
flourish during the season when food is abundant; while the sexual
generations occur at intervals which are often secondarily regulated
by the season, supply of food, etc.
A very simple case of this kind occurs, if we may trust the recent
researches of Lichtenstein[178], in certain Gall Insects (Cynipidæ).
He finds that the female of a form known as Spathegaster baccarum, of
which both males and females are plentiful, pricks a characteristic
gall in certain leaves, in which she deposits the fertilized eggs. The
eggs from these galls give rise to a winged and apparently adult form,
which is not, however, Spathegaster, but is a species considered to
belong to a distinct genus known as Neuroterus ventricularis. Only
females of Neuroterus are found, and they lay unfertilized ova in
peculiar galls which develop into Spathegaster baccarum. Here we have
a true case of heterogamy, the females which produce parthenogenetically
having become differentiated from those which produce sexually.
Another interesting type of heterogamy is that which has been long
known in the Aphides. In the autumn impregnated eggs are deposited by
females, which give rise in the course of the spring to females which
produce parthenogenetically and viviparously. The viviparous females
always differ from the females which lay the fertilized eggs. The
generative organs are of course differently constituted, and the ova
of the viviparous females are much smaller than those of the oviparous
females, as is generally the case in closely allied viviparous and
oviparous forms; but in addition the former are usually without wings,
while the latter are winged. The reverse is however occasionally the
case. An indefinite number of generations of viviparous females may be
produced if they are artificially kept warm and supplied with food;
but in the course of nature the viviparous females produce in the
autumn males and females which lay eggs with firm shells, and so
preserve the species through the winter. The heterogamy of the allied
Coccidæ is practically the same as that of the Aphidæ. In the case of
Chermes and Phylloxera the parthenogenetic generations lay their eggs
in the normal way.
[178] _Petites Nouvelles Entomologiques_, May, 1878.
The complete history of Phylloxera quercus has been worked out by
Balbiani (No. 401). The apterous females during the summer lay eggs
developing parthenogenetically into apterous females, which continue
the same mode of reproduction. In the autumn, however, the eggs which
are laid give rise in part to winged forms and in part to apterous
forms. Both of these forms lay smaller and larger eggs, which develop
respectively into very minute males and females without digestive
organs. The fertilized eggs laid by these forms probably give rise to
the parthenogenetic females.
A remarkable case of heterogamy accompanied by pædogenesis was
discovered by Wagner to take place in certain species of Cecydomyia
(Miastor), a genus of the Diptera. The female lays a few eggs in the
bark of trees, etc. These eggs develop in the winter into larvæ, in
which ovaries are early formed. The ova become detached into the body
cavity, surrounded by their follicles, and grow at the cost of the
follicles. They soon commence to undergo a true development, and on
becoming hatched they remain for some time in the body cavity of the
parent, and are nourished at the expense of its viscera. They finally
leave the empty skin of their parent, and subsequently reproduce a
fresh batch of larvæ in the same way. After several generations the
larvæ undergo in the following summer a metamorphosis, and develop
into the sexual form.
Another case of pædogenesis is that of the larvæ of Chironomus, which
have been shewn by Grimm (No. 413) to lay eggs which develop exactly
in the same way as fertilized eggs into larvæ.
BIBLIOGRAPHY.
(401) M. Balbiani. "Observations s. la reproduction d. Phylloxera du
Chêne." _An. Sc. Nat._ Ser. V. Vol. XIX. 1874.
(402) E. Bessels. "Studien ü. d. Entwicklung d. Sexualdrüsen bei den
Lepidoptera." _Zeit. f. wiss. Zool._ Bd. XVII. 1867.
(403) Alex. Brandt. "Beiträge zur Entwicklungsgeschichte d.
Libellulida u. Hemiptera, mit besonderer Berücksichtigung d.
Embryonalhüllen derselben." _Mém. Ac. Pétersbourg_, Ser. VII. Vol.
XIII. 1869.
(404) Alex. Brandt. _Ueber das Ei u. seine Bildungsstätte_. Leipzig,
1878.
(405) O. Bütschli. "Zur Entwicklungsgeschichte d. Biene." _Zeit. f.
wiss. Zool._ Bd. XX. 1870.
(406) H. Dewitz. "Bau u. Entwicklung d. Stachels, etc." _Zeit. f.
wiss. Zool._ Vols. XXV. and XXVIII. 1875 and 1877.
(407) H. Dewitz. "Beiträge zur Kenntniss d. Postembryonalentwicklung
d. Gliedmassen bei den Insecten." _Zeit. f. wiss. Zool._ XXX.
Supplement. 1878.
(408) A. Dohrn. "Notizen zur Kenntniss d. Insectenentwicklung."
_Zeitschrift f. wiss. Zool._ Bd. XXVI. 1876.
(409) M. Fabre. "L'hypermétamorphose et les moeurs des Méloïdes." _An.
Sci. Nat._ Series IV. Vol. VII. 1857.
(410) Ganin. "Beiträge zur Erkenntniss d. Entwicklungsgeschichte d.
Insecten." _Zeit. f. wiss. Zool._ Bd. XIX. 1869.
(411) V. Graber. _Die Insecten._ München, 1877.
(412) V. Graber. "Vorlauf. Ergeb. üb. vergl. Embryologie d. Insecten."
_Archiv f. mikr. Anat._ Vol. XV. 1878.
(413) O. v. Grimm. "Ungeschlechtliche Fortpflanzung einer Chironomus-Art.
u. deren Entwicklung aus dem unbefruchteten Ei." _Mém. Acad.
Pétersbourg._ 1870.
(414) B. Hatschek. "Beiträge zur Entwicklung d. Lepidopteren."
_Jenaische Zeitschrift_, Bd. XI.
(415) A. Kölliker. "Observationes de primâ insectorum genese, etc."
_Ann. Sc. Nat._ Vol. XX. 1843.
(416) A. Kowalevsky. "Embryologische Studien an Würmern u.
Arthropoden." _Mém. Ac. imp. Pétersbourg_, Ser. VII. Vol. XVI. 1871.
(417) C. Kraepelin. "Untersuchungen üb. d. Bau, Mechanismus u. d.
Entwick, des Stachels d. bienartigen Thiere." _Zeit. f. wiss. Zool._
Vol. XXIII. 1873.
(418) C. Kupffer. "Faltenblatt an d. Embryonen d. Gattung Chironomus."
_Arch. f. mikr. Anat._ Vol. II. 1866.
(419) R. Leuckart. _Zur Kenntniss d. Generationswechsels u. d.
Parthenogenese b. d. Insecten._ Frankfurt, 1858.
(420) Lubbock. _Origin and Metamorphosis of Insects._ 1874.
(421) Lubbock. _Monograph on Collembola and Thysanura._ Ray Society,
1873.
(422) Melnikow. "Beiträge z. Embryonalentwicklung d. Insecten."
_Archiv f. Naturgeschichte_, Bd. XXXV. 1869.
(423) E. Metschnikoff. "Embryologische Studien an Insecten." _Zeit. f.
wiss. Zool._ Bd. XVI. 1866.
(424) P. Meyer. "Ontogenie und Phylogenie d. Insecten." _Jenaische
Zeitschrift_, Vol. X. 1876.
(425) Fritz Müller. "Beiträge z. Kenntniss d. Termiten." _Jenaische
Zeitschrift_, Vol. IX. 1875.
(426) A. S. Packard. "Embryological Studies on Diplex, Perithemis, and
the Thysanurous genus Isotoma." _Mem. Peabody Acad. Science_, 1. 2.
1871.
(427) Suckow. "Geschlechtsorgane d. Insecten." Heusinger's
_Zeitschrift f. organ. Physik_, Bd. II. 1828.
(428) Tichomiroff. "Ueber die Entwicklungsgeschichte des Seidenwurms."
_Zoologischer Anzeiger_, II. Jahr. No. 20 (Preliminary Notice).
(429) Aug. Weismann. "Zur Embryologie d. Insecten." _Archiv f. Anat.
und Phys._ 1864.
(430) Aug. Weismann. "Entwicklung d. Dipteren." _Zeit. f. wiss. Zool._
Vols. XIII. and XIV. Leipzig, 1863-4.
(431) Aug. Weismann. "Die Metamorphose d. Corethra plumicornis."
_Zeit. f. wiss. Zool._ Vol. XVI. 1866.
(432) N. Wagner. "Beitrag z. Lehre d. Fortpflanzung d. Insectenlarven."
_Zeit. f. wiss. Zool._ Vol. XIII. 1860.
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Gliederthiere._ Berlin, 1854.
ARACHNIDA[179].
[179] The classification of the Arachnida adopted in the present
work is shewn below:
I. Arthrogastra. {Scorpionidæ.
{Pedipalpi.
{Pseudoscorpionidæ.
{Solifugæ.
{Phalangidæ.
II. Araneina. {Tetrapneumones.
{Dipneumones.
III. Acarina.
The development of several divisions of this interesting group has
been worked out; and it will be convenient to deal in the first
instance with the special history of each of these divisions, and then
to treat in a separate section the development of the organs for the
whole group.
[FIG. 193. OVUM OF SCORPION WITH THE ALREADY FORMED BLASTODERM
SHEWING THE PARTIAL SEGMENTATION. (After Metschnikoff.)
_bl_. blastoderm.]
Scorpionidæ. The embryonic development always takes place within the
female Scorpion. In Buthus it takes place within follicle-like
protuberances of the wall of the ovary. In Scorpio also development
commences while the egg is still in the follicle, but when the trunk
becomes segmented the embryo passes into the ovarian tube. The chief
authority for the development of the Scorpionidæ is Metschnikoff (No.
434).
At the pole of the ovum facing the ovarian tube there is formed a
germinal disc which undergoes a partial segmentation (fig. 193 _bl_).
A somewhat saucer-shaped one-layered blastoderm is then formed, which
soon becomes thickened in the centre and then divided into two layers.
The outer of these is the epiblast, the inner the mesoblast. Beneath
the mesoblast there subsequently appear granular cells, which form the
commencement of the hypoblast[180].
[180] The origin of the hypoblast cells, if such these cells are,
is obscure. Metschnikoff doubtfully derives them from the
blastoderm cells; from my investigations on Spiders it appears to
me more probable that they originate in the yolk.
During the formation of the blastoderm a cellular envelope is formed
round the embryo. Its origin is doubtful, though it is regarded by
Metschnikoff as probably derived from the blastoderm and homologous
with the amnion of Insects. It becomes double in the later stages
(fig. 195).
[FIG. 194. THREE SURFACE VIEWS OF THE VENTRAL PLATE OF A DEVELOPING
SCORPION. (After Metschnikoff.)
A. Before segmentation.
B. After five segments have become formed.
C. After the appendages have begun to be formed.]
During the differentiation of the three embryonic layers the germinal
disc becomes somewhat pyriform, the pointed end being the posterior.
At this extremity there is a special thickening which is perhaps
equivalent to the primitive cumulus of Spiders. The germinal disc
continues gradually to spread over the yolk, but the original pyriform
area is thicker than the remainder, and is marked off anteriorly and
posteriorly by a shallow furrow. It constitutes a structure
corresponding with the ventral plate of other Tracheata. It soon
becomes grooved by a shallow longitudinal furrow (fig. 194 A) which
subsequently becomes less distinct. It is then divided by two
transverse lines into three parts.[181]
[181] The exact fate of the three original segments is left
somewhat obscure by Metschnikoff. He believes however that the
anterior segment forms the procephalic lobes, the posterior
probably the telson and five adjoining caudal segments, and the
middle one the remainder of the body. This view does not appear
to me quite satisfactory, since on the analogy of Spiders and
other Arthropoda the fresh somites ought to be added by a
continuous segmentation of the posterior lobe.
In succeeding stages the anterior of the three parts is clearly marked
out as the procephalic lobe, and soon becomes somewhat broader. Fresh
segments are added from before backwards, and the whole ventral plate
increases rapidly in length (fig. 194 B).
When ten segments have become formed, appendages appear as paired
outgrowths of the nine posterior segments (fig. 194 C). The second
segment bears the pedipalpi, the four succeeding segments the four
ambulatory appendages, and the four hindermost segments smaller
provisional appendages which subsequently disappear, with the possible
exception of the second. The foremost segment, immediately behind the
procephalic lobes, is very small, and still without a rudiment of the
cheliceræ, which are subsequently formed on it. It would appear from
Metschnikoff's figures to be developed later than the other post-oral
segments present at this stage. The still unsegmented tail has become
very prominent and makes an angle of 180° with the remainder of the
body, over the ventral surface of which it is flexed.
[FIG. 195. A FAIRLY ADVANCED EMBRYO OF THE SCORPION ENVELOPED IN ITS
MEMBRANES. (After Metschnikoff.)
_ch._ cheliceræ; _pd._ pedipalpi; _p1-p4._ ambulatory appendages;
_ab._ post-abdomen (tail).]
By the time that twelve segments are definitely formed, the
procephalic region is distinctly bilobed, and in the median groove
extending along it the stomodæum has become formed (fig. 196 A). The
cheliceræ (ch) appear as small rudiments on the first post-oral
segment, and the nerve cords are distinctly differentiated and
ganglionated. In the embryonic state there is one ganglion for each
segment. The ganglion in the first segment (that bearing the
cheliceræ) is very small, but is undoubtedly post-oral.
At this stage, by a growth in which all the three germinal layers have
a share, the yolk is completely closed in by the blastoderm. It is a
remarkable fact with only few parallels, and those amongst the
Arthropoda, that the blastopore, or point where the embryonic
membranes meet in closing in the yolk, is situated on the dorsal
surface of the embryo.
The general relations of the embryo at about this stage are shewn in
fig. 195, where the embryo enclosed in its double cellular membrane is
seen in a side view. This embryo is about the same age as that seen
from the ventral surface in fig. 196 A.
The general nature of the further changes may easily be gathered from
an inspection of fig. 196 B and C, but a few points may be noted.
An upper lip or labrum is formed as an unpaired organ in the line
between the procephalic lobes. The pedipalpi become chelate before
becoming jointed, and the cheliceræ also early acquire their
characteristic form. Rudimentary appendages appear on the six segments
behind the ambulatory legs, five of which are distinctly shewn in fig.
195; they persist only on the second segment, where they appear to
form the comb-like organs or pectines. The last abdominal segment,
_i.e._ that next the tail, is without provisional appendages. The
embryonic tail is divided into six segments including the telson (fig.
196 C, _ab_). The lungs (_st_) are formed by paired invaginations, the
walls of which subsequently become plicated, on the four last segments
which bear rudimentary limbs, and simultaneously with the
disappearance of the rudimentary limbs.
[FIG. 196. THREE STAGES IN THE DEVELOPMENT OF THE SCORPION. THE
EMBRYOS ARE REPRESENTED AS IF SEEN EXTENDED ON A PLANE. (After
Metschnikoff.)
_ch._ cheliceræ; _pd._ pedipalpi; _p1-p4._ ambulatory appendages;
_pe._ pecten; _st._ stigmata; _ab._ post abdomen (tail).]
Pseudoscorpionidæ. The development of Chelifer has been investigated
by Metschnikoff (436), and although (except that it is provided with
tracheæ instead of pulmonary sacks) it might be supposed to be closely
related to Scorpio, yet in its development is strikingly different.
The eggs after being laid are carried by the female attached to the
first segment of the abdomen. The segmentation (_vide_ p. 93) is
intermediate between the types of complete and superficial
segmentation. The ovum, mainly formed of food-yolk, divides into two,
four, and eight equal segments (fig. 197 A). There then appear one or
more clear segments on the surface of these, and finally a complete
layer of cells is formed round the central yolk spheres (fig. 197 B),
which latter subsequently agglomerate into a central mass. The
superficial cells form what may be called a blastoderm, which soon
becomes divided into two layers (fig. 197 C). There now appears a
single pair of appendages (the pedipalpi) (fig. 198 A, _pd_), while at
the same time the front end of the embryo grows out into a remarkable
proboscis-like prominence--a temporary upper lip (concealed in the
figure behind the pedipalpus), and the abdomen (_ab_) becomes bent
forwards towards the ventral surface. In this very rudimentary
condition, after undergoing an ecdysis, the larva is hatched, although
it still remains attached to its parent. After hatching it grows
rapidly, and becomes filled with a peculiar transparent material. The
first pair of ambulatory appendages is formed behind the pedipalpi and
then the three succeeding pairs, while at the same time the cheliceræ
appear as small rudiments in front. External signs of segmentation
have not yet appeared, but about this period the nervous system is
formed. The supra-oesophageal ganglia are especially distinct, and
provided with a central cavity, probably formed by an invagination, as
in other Arachnida. In the succeeding stages (fig. 198 B) four
provisional pairs of appendages (shewn as small knobs at _ab_) appear
behind the ambulatory feet. The abdomen is bent forwards so as to
reach almost to the pedipalpi. In the later stages (fig. 198 C) the
adult form is gradually attained. The enormous upper lip persists for
some time, but subsequently atrophies and is replaced by a normal
labrum. The appendages behind the ambulatory feet atrophy, and the
tail is gradually bent back into its final position. The segmentation
and the gradual growth of the limbs do not call for special
description, and the formation of the organs, so far as is known,
agrees with other types.
[FIG. 197. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER.
(After Metschnikoff.)
In A the ovum is divided into a number of separate segments. In B a
number of small cells have appeared (_bl_) which form a blastoderm
enveloping the large yolk spheres. In C the blastoderm has become
divided into two layers.]
The segmentation of Chthonius is apparently similar to that of
Chelifer (Stecker, No. 437).
Phalangidæ. Our knowledge of the development of Phalangium is
unfortunately confined to the later stages (Balbiani, No. 438). These
stages do not appear however to differ very greatly from those of true
Spiders.
Araneina. The eggs of true Spiders are either deposited in nests made
specially for them, or are carried about by the females. Species
belonging to a considerable number of genera, viz. Pholcus, Epeira,
Lycosa, Clubione, Tegenaria and Agelena have been studied by Claparède
(No. 442), Balbiani (No. 439), Barrois (No. 441) and myself (No. 440),
and the close similarity between their embryos leaves but little doubt
that there are no great variations in development within the group.
[FIG. 198. THREE STAGES IN THE DEVELOPMENT OF CHELIFER. (After
Metschnikoff.)
_pd._ pedipalpi; _ab._ abdomen; _an.i._ anal invagination; _ch._
cheliceræ.]
The ovum is enclosed in a delicate vitelline membrane, enveloped in
its turn by a chorion secreted by the walls of the oviduct. The
chorion is covered by numerous rounded prominences, and occasionally
exhibits a pattern corresponding with the areas of the cells which
formed it. The segmentation has already been fully described, pp. 118
and 119. At its close there is present an enveloping blastoderm formed
of a single layer of large flattened cells. The yolk within is formed
of a number of large polygonal segments; each of which is composed of
large yolk-spherules, and contains a nucleus surrounded by a layer of
protoplasm, which is prolonged into stellate processes holding
together the yolk-spherules. The nucleus, surrounded by the major part
of the protoplasm of each yolk cell, appears, as a rule, to be
situated not at the centre, but on one side of its yolk segment.
The further description of the development of Spiders applies more
especially to Agelena labyrinthica, the species which formed the
subject of my own investigations.
The first differentiation of the blastoderm consists in the cells of
nearly the whole of one hemisphere becoming somewhat more columnar
than those of the other hemisphere, and in the cells of a small area
near one end of the thickened hemisphere becoming distinctly more
columnar than elsewhere, and two layers thick. This area forms a
protuberance on the surface of the ovum, originally discovered by
Claparède, and called by him the primitive cumulus. In the next stage
the cells of the thickened hemisphere of the blastoderm become still
more columnar; and a second area, at first connected by a whitish
streak with the cumulus, makes its appearance. In the second area the
blastoderm is also more than one cell deep (fig. 199). It will be
noticed that the blastoderm, though more than one cell thick over a
large part of the ventral surface, is not divided into distinct
layers. The second area appears as a white patch and soon becomes more
distinct, while the streak continued to it from the cumulus is no
longer visible. It is shewn in surface view in fig. 200 A. Though my
observations on this stage are not quite satisfactory, yet it appears
to me probable that there is a longitudinal thickened ridge of the
blastoderm extending from the primitive cumulus to the large white
area. The section represented in fig. 199, which I believe to be
oblique, passes through this ridge at its most projecting part.
[FIG. 199. SECTION THROUGH THE EMBRYO OF AGELENA LABYRINTHICA.
The section is from an embryo of the same age as fig. 200 A, and is
represented with the ventral plate upwards. In the ventral plate
is seen a keel-like thickening, which gives rise to the main mass
of the mesoblast.
_yk._ yolk divided into large polygonal cells, in several of which
nuclei are shewn.]
The nuclei of the yolk cells during the above stages multiply rapidly,
and cells are formed in the yolk which join the blastoderm; there can
however be no doubt that the main increase in the cells of the
blastoderm has been due to the division of the original blastoderm
cells.
In the next stage I have been able to observe there is, in the place
of the previous thickened half of the blastoderm, a well-developed
ventral plate with a procephalic lobe in front, a caudal lobe behind,
and an intermediate region marked by about three transverse grooves,
indicating a division into segments. This plate is throughout two or
more rows of cells thick, and the cells which form it are divided
_into two distinct layers_--a columnar superficial layer of epiblast
cells, and a deeper layer of mesoblast cells (fig. 203 A). In the
latter layer there are several very large cells which are in the act
of passing from the yolk into the blastoderm. The identification of
the structures visible in the previous stage with those visible in the
present stage is to a great extent a matter of guess-work, but it
appears to me probable that the primitive cumulus is still present as
a slight prominence visible in surface views on the caudal lobe, and
that the other thickened patch persists as the procephalic lobe.
However this may be, the significance of the primitive cumulus appears
to be that it is the part of the blastoderm where two rows of cells
become first established[182].
[182] Various views have been put forward by Claparède and
Balbiani about the position and significance of the primitive
cumulus. For a discussion of which _vide_ self, No. 440.
The whole region of the blastoderm other than the ventral plate is
formed of a single row of flattened epiblast cells. The yolk retains
its original constitution.
By this stage the epiblast and mesoblast are distinctly
differentiated, and the homologue of the hypoblast is to be sought for
in the yolk cells. The yolk cells are not however entirely
hypoblastic, since they continue for the greater part of the
development to give rise to fresh cells which join the mesoblast.
The Spider's blastoderm now resembles that of an Insect (except for
the amnion) after the establishment of the mesoblast, and the mode of
origin of the mesoblast in both groups is very similar, in that the
longitudinal ridge-like thickening of the mesoblast shewn in fig. 199
is probably the homologue of the mesoblastic groove of the Insects'
blastoderm.
The ventral plate continues to grow rapidly, and at a somewhat later
stage (fig. 200 B) there are six segments interposed between the
procephalic and caudal lobes. The two anterior of these (_ch_ and
_pd_), especially the foremost, are less distinct than the remainder;
and it is probable that both of them, and in any case the anterior
one, are formed later than the three segments following. These two
segments are the segments of the cheliceræ and pedipalpi. The four
segments following belong to the four pairs of ambulatory legs. The
segments form raised transverse bands separated by transverse grooves.
There is at this stage a faintly marked groove extending along the
median line of the ventral plate. This groove is mainly caused by the
originally single mesoblastic plate having become divided throughout
the whole region of the ventral plate, except possibly the procephalic
lobes, into two bands, one on each side of the middle line (fig. 203
B).
[FIG. 200. FOUR STAGES IN THE DEVELOPMENT OF AGELENA LABYRINTHICA.
A. Stage when the ventral plate is very imperfectly differentiated.
_pr.c._ primitive cumulus.
B. Ovum viewed from the side when the ventral plate has become
divided into six segments. _ch._ segment of cheliceræ imperfectly
separated from procephalic lobe; _pd._ segment of pedipalpi.
C. Ventral plate ideally unrolled after the full number of segments
and appendages are established. _st._ stomodæum between the two
præ-oral lobes. Behind the six pairs of permanent appendages are
seen four pairs of provisional appendages.
D and E. Two views of an embryo at the same stage. D ideally
unrolled, E seen from the side. _st._ stomodæum; _ch._ cheliceræ;
on their inner side is seen the ganglion belonging to them. _pd._
pedipalpi; _pr.p._ provisional appendages.]
The segments continue to increase in number by the continuous addition
of fresh segments between the one last formed and the caudal lobe. By
the stage with nine segments the first rudiments of the limbs make
their appearance. The first rudiments to appear are those of the
pedipalpi and four ambulatory limbs: the cheliceræ, like the segment
to which they belong, lag behind in development. The limbs appear as
small protuberances at the borders of their segments. By the stage
when they are formed the procephalic region has become bilobed, and
the two lobes of which it is composed are separated by a shallow
groove.