Difference between revisions of "The Works of Francis Balfour 2-12"

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(358) R. v. Willemoes-Suhm. " Biologische Beobachtungen U. niedrige  
(358) R. v. Willemoes-Suhm. " Biologische Beobachtungen U. niedrige  
Meeresthiere." Zeit. f. wiss. Zool. Bd. xxi. 1871.  
Meeresthiere." Zeit. f. wiss. Zool. Bd. xxi. 1871.
==Chapter XIII. Discophora==
THE eggs of the Discophora, each enclosed in a delicate
membrane, are enveloped in a kind of mucous case formed by a
secretion of the integument, which hardens into a capsule or
cocoon. In each cocoon there are a limited number of eggs
surrounded by albumen. The cocoons are attached to waterplants, etc. In Clepsine the embryos leave the cocoon very
soon after they get rid of the egg membrane, but in Nephelis
they remain within the cocoon for a very much longer period
(27 28 days after hatching). The young of Clepsine, after
their liberation, attach themselves to the ventral surface of their
Our knowledge of the development of the Discophora is in a
very unsatisfactory state ; but sufficient is known to shew that it
has very many points in common with that of the Oligochaeta,
and that the Discophora are therefore closely related to the
Chaetopoda. In Clepsine there is an epibolic gastrula, and
mesoblastic bands like those in Euaxes are also formed. In
Nephelis however the segmentation is very abnormal, and the
formation of the germinal layers cannot easily be reduced to an
invaginate gastrula type, though probably it is modified from
such a type. Mesoblastic bands similar to those in the Oligochaeta occur in this form also.
The embryology of Clepsine, which will serve as type for the
Leeches without jaws (Rhyncobdellidae), has recently been
studied by Whitman (No. 365), and that of Nephelis, which will
1 The Discophora are divided into the following groups.
I. Rhyncobdellidse.
II. Gnathobdellidae.
III. Branchiobdellidffi.
serve as type for the Leeches with jaws (Gnathobdellidae), has
been studied by Butschli (No. 359). The early history of both
types is imperfectly known 1 .
Formation of the layers.
Clepsine. It is necessary to give a full account of the segmentation
of Clepsine, as the formation of the germinal layers would be otherwise
Segmentation commences with the division of the ovum into two unequal
spheres by a vertical cleavage passing from the animal to the vegetative
pole. By a second vertical cleavage the large segment is divided into two
unequal parts, and the small one into two equal parts. Of the four segments
so produced three are relatively small, and one, placed at the posterior end,
is large. Each of the four segments next gives rise to a small cell at the
animal pole. These small cells form the commencement of the epiblast,
and, according to Whitman, the mouth is eventually placed in their centre.
Such a position for the mouth, at the animal pole, is extremely unusual, and
the statements on this head require further confirmation.
The posterior large segment now divides into two, one of which is dorsal,
and the other and larger ventral. The former I shall call
with Whitman the neuroblast,
and the latter the mesoblast.
The mesoblast very shortly
divides again. During the formation of the neuroblast and
mesoblast additional epiblastic
small cells are added from the
three spheres which give rise
to three of the primitive epiblast cells, which may now be
called the vitelline spheres.
The neuroblast next divides
into ten cells, of which the two
smaller are soon broken up
into epiblastic cells, while the
remaining eight arrange themselves in two groups of four
each, one group on each side
at the posterior border of the epiblastic cap. The two mesoblasts also take up
a position on the right and left sides immediately ventral to the four neuroblasts of each side. The neuroblasts and mesoblasts now commence to
CLEPSINE. (After Whitman.)
o. oral extremity ; m mouth ; pr. germinal
A. This figure shews the blastoderm (shaded)
with a thickened edge formed by the primitive
(i.e. mesoblastic) streaks with the four so-called
neuroblasts posteriorly. The vitelline spheres
are left without shading.
B. represents an embryo in which the blastoderm has enclosed the yolk, and in which the
division into segments has taken place. At the
hind end are shewn the so-called neuroblasts
forming the termination of the germinal streak.
1 Hoffmann's account (No. 36) is so different from that of other observers that
I have been unable to make any use of it.
proliferate at their anterior border, and produce on each side a thickened
band of cells underneath the edge of the cap of epiblast cells. Each of these
bands is formed of a superficial quadruple 1 row of neuroblasts budded off
from the four primary neuroblasts, and a deeper row of mesoblasts. The
compound streaks so formed may be called the germinal streaks.
The general appearance of the embryo as seen from the dorsal surface,
after the appearance of the two germinal streaks, may be gathered from
fig. 158 A. The epiblastic cap in this figure is shaded. The epiblastic cap,
accompanied by the germinal streaks, now rapidly extends and encloses the
three vitelline spheres by a process equivalent to that of an ordinary epibolic
gastrula; but the front and hind ends of the streaks remain practically
stationary. Owing to this mode of growth the edges of the epiblastic cap
and the germinal streaks meet in a linear fashion along the ventral surface
of the embryo (fig. 159, A and B). The germinal streaks first meet anteriorly
(B) and their junction is then gradually continued backwards. The process
is completed at about the time of hatching.
During the above changes the nuclei of the vitelline spheres pass to the
surface and rapidly divide. Eventually, together with part of the protoplasm
of the vitelline spheres, they appear to give rise to a layer of hypoblastic
cells. This layer encloses the remains of the vitelline spheres, which
become the yolk.
At the front end of the germinal streaks, in a position corresponding with that
of the four original epiblast cells,
two depressions appear which
coalesce to form the single oral
invagination ; in the centre of
which are formed the mouth and
pharynx by a second epiblastic
The most important point in FIG. 159. Two EMBRYOS OF CLEPSINE IN
connection with the above history WHICH THE GERMINAL STREAKS HAVE PARTIis the fate of what have been ALLY^MET ALONG THE VENTRAL LINE. (After
called the germinal streaks. Ac- ^ germinal, i.e. mesoblastic streaks,
cording to Whitman they are The area covered by epiblast is shaded .
composed of two kinds of cells, The so-called neuroblasts at the end of the
viz. four rows of smaller super- germinal streaks are shewn in B.
ficial cells, which he calls neuroblasts, and, in the later stages at any rate, a
row of deeper large cells, which he calls mesoblasts. As to the eventual fate
of these cells he states that the neuroblasts uniting together in the median
line form the rudiment of the ventral ganglionic chain, while the mesoblasts
equally coalesce and give rise to the mesoblast. Such a mode of origin for a
ventral ganglionic chain is, so far as I know, without a parallel in the whole
animal kingdom ; and whatever evidence Whitman may have that the cells
1 According to Robin it is more usual for there to be only a triple row of primary
in question really do give rise to the nervous system he has not thought fit to
produce it in his paper. He figures a section with the eight neuroblastic cells
in the middle ventral line, and in the next stage described the nervous
system is divided up into ganglia ! The first stage, in which the so-called
nervous system has the form of a single row of eight cells, is quite unlike
any rudiment of the nervous system such as is usually met with in the
Chaetopoda, and not a single stage between this and a ganglionated cord is
described or figured. Whitman, whose views seem to have been influenced
by a peculiar, and in my opinion erroneous, theory of Rauber's about the
relation of the neural groove of Vertebrata to the blastopore, does not seem
to be aware that his determination of the fate of his neuroblasts requires any
special support.
He quotes the formation of these parts in Euaxes (vide preceding
Chapter, p. 324) as similar to that in Clepsine. In this comparison it
appears to me probable that he may be quite correct, but the result of the
comparison would be to shew that the neuroblasts and mesoblasts composed
together a mesoblastic band similar to that of the Oligochaeta. Till more
evidence is brought forward by Whitman or some other observer in support
of the view that the so-called neuroblasts have any share in forming the
nervous system, they must in my opinion be regarded as probably forming,
in conjunction with the mesoblasts, two simple mesoblastic bands. Kowalevsky has moreover briefly stated that he has satisfied himself that the
nervous system in Clepsine originates from the epiblast a statement which
certainly could not be brought into harmony with Whitman's account.
Nephelis. Nephelis will form my type of the Gnathobdellidae. The
segmentation of this form has not yet been thoroughly investigated, but
Biitschli's (No. 359) observations are probably the most trustworthy.
The ovum first divides into two, and then into four segments of which
two are slightly smaller than the others. Four small cells which form the
commencement of the epiblast are now formed. Three of them are derived
by budding from the two larger and one of the smaller of the four cells,
and the fourth from a subsequent division of one of the larger cells 1 .
The three cells which assisted in the formation of the epiblast cells again
give rise each to a small cell ; and the small cells so formed constitute a
layer underneath the epiblast which is the commencement of the hypoblast,
while the cells from which they originated form the vitelline spheres.
Shortly after the formation of the hypoblast, the large sphere which has
hitherto been quiescent divides into two, one of which then gives rise
in succession to two small epiblastic elements.
The two large spheres, resulting from the division of the originally
quiescent sphere, next divide again on the opposite side of the embryo,
and form a layer of epiblast there ; so that there is now on one side of
the embryo (the ventral according to Robin) a layer of epiblast formed
1 Doubts have been cast by Whitman on the above account of the origin of the
four epiblast cells.
of six cells, and on the opposite side a layer formed of four cells. The
two layers meet at the front border of the embryo and between them are
placed the three large vitelline spheres. The two patches of epiblast cells
now rapidly increase, and gradually spread over the three large vitelline
spheres. Except where they meet -each other at the front edge they leave
uncovered a large part of the margin of the vitelline spheres.
While these changes have been taking place on the exterior, the
hypoblast cells have increased in number (additional cells being probably
derived from the three large vitelline spheres) and fill up in a column-like
fashion a space which is bounded behind by the three vitelline spheres, and
in front by the epiblast of the anterior end of the embryo. At the sides of
the hypoblast the mesoblast has become established, probably as two lateral
bands. The origin of the cells forming it has not yet been determined.
The hypoblast cells in the succeeding stage arrange themselves round a
central archenteric cavity, and at the same time rapidly increase in size
and become filled with a secondary deposit of food-yolk. Shortly afterwards a mouth and thick-walled oesophagus are formed, probably from an
epiblastic invagination. The mesoblast now forms two curved lateral
bands at the two sides of the body, equivalent to the mesoblastic bands
of the Chaetopoda. The three vitelline spheres, still largely uncovered by
the epiblast, lie at the posterior end of the body. The embryo grows
rapidly, especially anteriorly, and the three vitelline spheres become
covered by a layer of flattened epiblast cells. Around the oesophagus a
cavity traversed by muscular fibres is established. Elsewhere there is no
trace of such a cavity. The cephalic region becomes ciliated, and the
dorsal part of it, which represents a rudimentary prae-oral lobe, is especially
prominent. The cilia of the oral region are continued into the lumen of
the oesophagus, and at a later period are prolonged, as in Lumbricus, along
the median line of the ventral surface.
The mesoblastic bands would seem from Biitschli's observations, which
receive confirmation from Kleinenberg's researches on Lumbricus, to be prolonged dorsally to the oesophagus into the cephalic region. Posteriorly they
abut on the large vitelline spheres, which were supposed by Kowalevsky
to give origin to them, and to play the same part as the large mesoblasts in Lumbricus. It has already been shewn that the function of the
large cells in Lumbricus has been exaggerated, and Biitschli denies to
them in Nephelis any share in the production of the mesoblast. It seems
in fact probable that they are homologous with the three vitelline spheres
of Clepsine ; and that their primitive function is to give origin to the
hypoblast. They are visible for a long time at the hind end of the embryo,
but eventually break up into smaller cells, the fate of which is unknown.
The embryo of Hirudo would appear from the researches of Robin
to develop in nearly the same way as that of Nephelis. The anterior
part is not however ciliated. The three large posterior cells disappear
relatively early.
General history of the larva.
The larva of Clepsine, at the time when the mesoblastic
bands have met along the ventral line, is represented in fig.
158 B. It is seen to be already segmented, the process having
proceeded pari passu with the ventral coalescence of the mesoblastic bands. The segments are formed from before backwards
as in Chaetopoda. The dorsal surface is flat and short, and the
ventral very convex. The embryo about this time leaves its
capsule, and attaches itself to its parent. It rapidly elongates,
and the dorsal surface, growing more rapidly than the ventral,
becomes at last the more convex. Eventually thirty-three postoral segments become formed ; of which the eight last coalesce
to form the posterior sucker.
The general development of the body of Nephelis and
Hirudo is nearly the same as that of Clepsine. The embryo
passes from a spherical to an oval, and then to a vermiform
shape. For full details the reader is referred to Robin's
The presence of a well-marked protuberance above the
oesophagus, which forms the rudiment of a prae-oral lobe, has
already been mentioned as characteristic of the embryo of
Nephelis ; no such structure is found in Clepsine.
History of the germinal layers and development of organs.
The epiblast. The epiblast is formed of a single layer of
cells and early develops a delicate cuticle which is clearly formed
quite independently of the egg membrane. It becomes raised
into a series of transverse rings which bear no relation to the
true somites of the mesoblast.
The nervous system. The nervous system is probably
derived from the epiblast, but its origin still requires further
investigation. The ventral cord breaks up into a series of
ganglia, which at first correspond exactly with the somites of
the mesoblast. Of these, four or perhaps three eventually coalesce to form the sub-cesophageal ganglion, and seven or eight
become united in the posterior sucker.
It would appear from Biitschli's statements that the supra
cesophageal ganglion arises, as in Oligochaeta, independently of
the ventral cord.
Mesoblast. It has already been indicated that the mesoblast probably takes its origin both in Nephelis and Clepsine
from the two mesoblastic bands which unite in the median
ventral line. The further history of these bands is only imperfectly known. They become segmented from before backwards. The somites formed by the segmentation gradually
grow upwards and meet in the dorsal line. Septa are formed
between the somites probably in the same way as in the
In Clepsine the mesoblastic bands are stated by Kowalevsky to become split into somatic and splanchnic layers, between which are placed
the so-called lateral sinuses. These sinuses form, according to Whitman,
a single continuous tube investing the alimentary tract ; a tube which
differs therefore to a very small extent from the normal body cavity of
the Chaetopoda. The somatic layer of mesoblast no doubt gives rise to
the circular and longitudinal muscular layers of the embryo. The former
is stated to appear the earliest, while the latter, as in the Oligochaeta,
first takes its origin on the ventral side.
A delicate musculature, formed mainly of transverse but also of longitudinal fibres, would appear to be developed independently of the mesoblastic bands in Nephelis and Hirudo (Rathke, Leuckart, Robin, and
Biitschli). It develops apparently from certain stellate cells which are
found between the walls of the alimentary tract and the skin, and which
probably correspond to the system of contractile fibres which pass from
the body wall to the alimentary tract through the segmentation cavity in
the larva of Chaetopoda, various Vermes and Mollusca 1 .
The mesoblast, so far as is known, gives rise, in addition to
the parts already mentioned, to the excretory organs, generative
organs, vascular system, etc.
Excretory organs. There are found in the embryo of Nephelis and Hirudo certain remarkable provisional excretory organs
the origin and history of which is not yet fully made out. In
Nephelis they appear as one (according to Robin, No. 364), or
(according to Biitschli, No. 359) as two successive pairs of
1 According to Robin this system of muscles becomes gradually strengthened and
converted into the permanent system. Rathke on the other hand states that it is
provisional, and that it is replaced by the muscles developed from the mesoblastic
somites. It is possible to suppose that it may really become incorporated in the latter
convoluted tubes on the dorsal side of the embryo, which are
stated by the latter author to develop from the scattered mesoblast cells underneath the skin. At their fullest development
they extend, according to Robin, from close to the head to near
the ventral sucker. Each of them is U-shaped, with the open
end forwards, each limb of the U being formed by two tubes
united in front. No external opening has been clearly made
out. Semper believed that the tubes were continuous with
the three posterior vitelline cells, but this has been shewn not
to be the case. Fiirbringer 1 is inclined from his own researches to believe that they open laterally. They contain a
clear fluid.
In Hirudo, Leuckart (No. 362) has described three similar
pairs of organs the structure of which he has fully elucidated.
They are situated in the posterior part of the body, and each of
them commences with an enlargement from which a convoluted
tube is continued for some distance backwards ; it then turns
forwards again and afterwards bends upon itself to open to the
exterior. The anterior part is broken up into a kind of labyrinthic network.
The true segmental organs are found in a certain number
of the segments and are stated (Whitman) to develop from
groups of mesoblast cells. Their origin requires however further
A double row of colossal cells on each side of the body has been
described in Clepsine by Whitman as derived from the mesoblastic plates.
These cells (fig. 58 B), which he calls segment-cells, lie opposite the walls of
the septa. The inner row is stated to be connected with the segmental
organs. Their eventual history is unknown, but they are conjectured
by Whitman to be the mother cells of the testes.
The alimentary tract. This is formed primitively of two
parts the epiblastic stomodaeum forming mouth, pharynx,
and oesophagus, and the hypoblastic mesenteron. The anus is
formed very late as a simple perforation immediately dorsal to
the posterior sucker.
In Clepsine, where there is an epibolic gastrula, the rudiment
1 Morphologisches Jahrbuch, Vol. iv. p. 676. He further speaks of the tube as
" feinverzweigt u. netzformig verastelt," but whether from his own observations is
not clear.
B. II. 23
of the mesenteron is at first formed of the three vitelline
spheres, from the surface of which a true hypoblastic layer
enclosing a central yolk mass becomes differentiated, as already
described. The mesenteric sack so formed is constricted by the
growth of the mesoblastic septa into a series of lobes, while the
posterior part forms a narrow and at first very short tube opening by the anus.
The lobed region forms the sacculated stomach of the adult.
The sacculations of the stomach by their mode of origin necessarily correspond with the segments. In the adult however the
anterior lobe is really double and has two divisions for the two
segments it fills, while the posterior lobe, which, as is well
known, extends backwards parallel with the rectum, is composed
of five segmental sacculations. In connection with the stomodaeum a protrusible pharynx is developed.
In Hirudo and Nephelis the mesenteron has from the first a
sack-like form. The cells which compose the sack give rise to a
secondary deposit of food-yolk. The further changes are practically the same as in Clepsine. In Hirudo the posterior sacculation of the stomach is primitively unpaired. The jaws are
formed at about the same time as the eyes as protuberances on
the wall of the oral cavity.
(359) O. Biitschli. " Entwicklungsgeschichtliche Beitrage (Nephelis)." Zeit.
f. wiss. Zool. Vol. xxix. 1877.
(360) E. Grube. Untersuchnngen iib. d. Entwicklung d. Aniiclidcu. Konigsl)crg, 1844.
(361) C.K.Hoffmann. " Zur Entwicklungsgeschichte d. Clepsineen." Niederldnd. Archiv f. Zool. Vol. iv. 1877.
(362) R. Leuckart. Die mcnschlichen Parasiten (Hirudo), Vol. i. |>. 686,
et seq.
(363) II. Rathke. Beit. z. Entwicklungsgesch. d. Hirudineen. Leipzig, 1862.
(364) Ch. Robin. Mfm. sur le Dhjeloppcment embryogenique des Hirudwccs.
1'aris, 1875.
(365) C. O. Whitman. " Embryology of Clepsine." Quart. J. of Micro.
Science, Vol. xvm. 1878.
[Vide also C. Semper (No. 355) and Kowalevsky (No. 342) for isolated observations.]
IT is convenient for the purposes of embryology to divide
the Gephyrea into two groups, viz. (i) Gephyrea nuda or true
Gephyrea; and (2) Gephyrea tubicola formed by the genus
Segmentation and formation of the layers.
An embolic or epibolic gastrula is characteristic of the
Gephyrea, and the blastopore appears, in some cases at any rate
(Phascolosoma, Thalassema), to become the mouth.
Bonellia. In Bonellia (Spengel, No. 370) the segmentation
is unequal but complete, and, as in many Molluscs etc., the
ovum exhibits before its commencement a distinction into a
protoplasmic and a yoke pole. The ovum first divides into four
equal segments, each of them formed of the same constituents as
the original ovum. At the animal pole four small cells, entirely
formed of protoplasm, are next formed by an equatorial furrow.
They soon place themselves in the intervals between the large
spheres. Four small cells are again budded off from the large
spheres and the eight small cells then divide. By a further
continuation of the division of the existing small cells, and the
formation of fresh ones from the large spheres, a layer of small
1 The following scheme shews the classification of the Gephyrea adopted in the
present chapter :
i. Gephyrea nuda. {
ii. Gephyrea tubicola (Phoronis).
cells is eventually formed, which completely envelops the four
large spheres except for a small blastopore at the vegetative pole
of the ovum (fig. 160 A). The large spheres continue to give
rise to smaller cells which however no longer take a superficial
position but lie within the layer of small cells, and give rise to
the hypoblast (fig. r6o B). The small cells become the epiblast,
and at the blastopore they curl inwards (fig. 160 B) and give
A. Stage when the four hypoblast cells are nearly enclosed.
B. Stage after the formation of the mesoblast has commenced by an infolding of
the lips of the blastopore.
ep. epiblast ; me. mesoblast ; bl. blastopore.
rise to a layer of cells, which appears to extend as an unbroken
sheet between the epiblast and hypoblast, and to form the
mesoblast. The blastopore now closes up, but its position in
relation to the parts of the embryo has not been made out.
In Phascolosoma (Selenka, No. 369) the ovum, enclosed in a
porous zona radiata, divides into two unequal spheres, of which
the smaller next divides into two and then into four. An
invagination takes place which is intermediate between the
embolic and the epibolic types. The small cells, the number of
which is increased by additions from the large sphere, divide, and
grow round the large sphere. The latter in the meantime also
divides, and the cells produced from it form on the one hand a
small sack which opens by the blastopore, and on the other they
fill up the segmentation cavity, and become the mesoblast and
blood corpuscles. Tin- Mastoporc becomes the permanent
Larval forms and development of organs.
Amongst the Gephyrea armata the larva has as a rule
(Thalassema, Echiurus) the characters of a trochosphere, and
closely approaches the typical form characteristic of the larva of
Polygordius, often known as Loven's larva. In Bonellia this
larval form is less perfectly preserved.
Echiurus. In Echiurus (Salensky, No. 368) the youngest
known larva has all the typical trochosphere characters (fig. 161).
It is covered with cilia and divided into a prae-oral lobe and
post-oral region of nearly equal dimensions. There is a double
ciliated ring which separates the two sections of the body as in
the larva of Polygordius : the mouth (m) opens between its two
elements. The alimentary canal is divided into a stomodaeum
with a ventral opening, a large stomach, and a short intestine
opening by a terminal anus (an). Connecting the oesophagus
with the apex of the prae-oral lobe is the usual contractile band,
and at the insertion of this band -is a thickening of the epiblast
which probably represents the rudiment of the supra-oesophageal
ganglion. A ventral nerve cord is stated by Salensky to be
present, but his observations on this point are not quite satisfactory.
The metamorphosis is accompanied by the loss of swimming
power, and consists in the
enlargement of the post-oral
portion of the trunk, and in
the simultaneous reduction
of the prae-oral lobe, which
remains however permanently as the cylindrical
proboscis. A groove which
terminates posteriorly at the
mouth is very early formed
on its ventral side. The
ciliated rings gradually disappear during the metamorphosis.
(After Salensky.)
_ m. mouth ; an. anus ; sg. supra-ceso
Of the further external phageal ganglion (?).
changes the most important are (i) the early appearance
round the anal end of the body of a ring of bristles ; and (2) the
appearance of a pair of ventral setae in the anterior part of the
body. The anterior ring of bristles characteristic of the adult
Echiurus does not appear till a late period.
Of the internal changes the earliest is the formation of the
anal respiratory sacks. With the growth of the posterior part
of the trunk the intestine elongates, and becomes coiled.
Bonellia. The embryo of Bonellia, while still within the
egg, retains a spherical form and acquires an equatorial band of
cilia, behind which a second narrower band is soon established,
while in front of the first one a pair of eye-spots becomes
A. Larva with two ciliated bands and two eye-spots.
B. Ripe larva from the dorsal surface.
C. Young female Bonellia from the side.
a/, alimentary tract ; m. mouth ; sc. provisional excretory tube ; s. ventral hook ;
an.-', anal vesicle.
formed (fig. 162 A). The embryo on becoming hatched rapidly
elongates, while at the same time it becomes dorso-vcntrally
flattened and acquires a complete coating of cilia (fig. 162 B).
According to Spengel it resembles at this time in its form and
habits a rhabdoccelous Turbcllarian. The anterior part is
however somewhat swollen and presents an indication of a
pre-oral lobe.
During the above changes important advances are made in the formation of the organs from the embryonic layers.
The epiblast acquires a superficial cuticula, which is perhaps directly
derived from the vitelline membrane. The nervous system is also formed,
probably from the epiblast. The band-like supra-cesophageal ganglion is
the first part of the nervous system formed, and appears to be undoubtedly
derived from the epiblast. The ventral cord arises somewhat later, but the
first stages in its development have not been satisfactorily traced. It is
continuous with the supra-cesophageal band which completely girths the
oesophagus without exhibiting any special dorsal enlargement. After the
ventral cord has become completely separated from the epiblast a central
fibrous mass becomes differentiated in it, while the lateral parts are composed
of ganglion cells. In the arrangement of its cells it presents indications of
being composed of two lateral halves. It is, however, without ganglionic
The mesoblast, though at first very thin, soon exhibits a differentiation
into a splanchnic and somatic layer though the two do not become
distinctly separated by a body cavity. The somatic layer rapidly becomes
thicker, and enlarges laterally to form two bands united dorsally and
ventrally by narrow, thinner bands. The outermost parts of each of these
bands become differentiated into an external circular and an internal
longitudinal layer of muscles. In the pras-oral lobe the mesoblast assumes
a somewhat vacuolated character.
The hypoblast cells form a complete layer round the four yolk cells from
which they arise (fig. 162 B, al\ but at first no alimentary lumen is developed.
The oesophagus appears during this period as an, at first solid, but subsequently hollow, outgrowth of the hypoblast towards the epiblast.
The metamorphosis of the larva into the adult female
Bonellia commences with the conversion of many of the indifferent mesoblast cells into blood corpuscles, and the introduction into the body cavity of a large amount of fluid, which
separates the splanchnic and somatic layers of mesoblast. The
fluid is believed by Spengel to be sea-water, introduced by two
anal pouches, the development of which is described below.
The body cavity is lined by a peritoneum, and very soon
distinct vessels, formed by folds of the peritoneum, become
established. Of these there are three trunks, two lateral and a
median in the prae-oral lobe (proboscis), and in the body a
ventral trunk above the nerve cord, and an intestinal trunk
opening anteriorly into the ventral one. The vessels appear
to communicate with the body cavity.
In the course of the above changes the two ciliated bands
disappear, the hinder one first. The cilia covering the general
surface become atrophied, with the exception of those on the
ventral side of the prae-oral lobe. The latter structure becomes
more prominent ; the stellate mesoblast cells, which fill up its
interior, become contractile, and it gives rise to the proboscis
(fig. 162 C).
At the point where the cesophageal protuberance joined the epiblast at
a previous stage the mouth becomes established (fig. 162 C, ;//), and though
it is formed subsequently to the atrophy of the anterior ciliated band, yet
there is evidence that it is potentially situated behind this band. The lumen
of the alimentary canal becomes established by the absorption of the
remains of the four central cells. The anus is formed on the ventral side
of the posterior end of the body, and close to it the pouches already noticed
grow out from the hindermost part of the alimentary tract (fig. 162 C, an.v\
They are at first simple blind pouches, but subsequently open into the
body cavity 1 . They become the anal pouches of the adult. There is present
when the mouth is first formed a peculiar process of the alimentary tract
projecting into the prae-oral lobe, which appears to atrophy shortly afterwards.
After the formation of the mouth, there are formed on the ventral side of
and slightly behind it (i) anteriorly a pair of tubes, which appear to be
provisional excretory organs and soon disappear (fig. 162 C, sc}\ and (2)
behind them a pair of bristles (s) which remain in the adult. The formation
of the permanent excretory (?) organ (oviduct and uterus) has not been
followed out. The ovary appears very early as a differentiation of the
epithelium lining the ventral vessel.
The larvae, which become the minute parasitic males, undergo
a very different and far less complete metamorphosis than those
which become females. They attach themselves to the proboscis of an adult female, and lose their ciliated bands. Germinal cells make their appearance in the mesoblast, which form
spherical masses, and, like the germinal balls in the female
ovary, consist of a central cell, and an epithelium around it.
The central cell becomes very large, while the peripheral cells
give rise to the spermatozoa. A body cavity becomes developed
in the larvae, into which the spermatic balls are dehisced.
Neither mouth nor anus is formed. The further changes have
not been followed out.
1 The fact that these pouches are outgrowths of the alimentary tract appears to
preclude the possibility of their being homologous with excretory tubes of the Plalyelminthes and Rotifera.
The larval males make their way into the oesophagus of the
female, where they no doubt live for some time, and probably
become mature, though the seminal pouch of the adult is not
found in many of the males living in the oesophagus. When
mature the males leave the oesophagus, and pass into the
Phascolosoma. Cilia appear in Phascolosoma (Selenka,
No. 369) while the ovum is still segmenting. After segmentation
they form a definite band immediately behind the mouth, which
divides the Jarva into two hemispheres a prae-oral and a postoral. A prae-oral band of cilia is soon formed close to the postoral band, and at the apex of the prae-oral lobe a tuft of cilia
also appears.
The larva has now the characters of a trochosphere, but
differs from the typical trochosphere in the post-oral part of the
ciliated equatorial ring being more important than the prae-oral,
and in the absence of an anus.
The metamorphosis commences very early. The trunk
rapidly elongates, and the prae-oral lobe becomes relatively less
and less conspicuous. The zona radiata becomes the larval
Three pairs of bristles are formed on the trunk, of which the
posterior pair appears first, then the anterior, and finally the
middle pair : an order of succession which clearly proves they
can have no connection with a true segmentation.
The tentacles become developed between the two parts of the
ciliated ring, and finally the prse-oral lobe, unlike what takes
place in the Gephyrea armata, nearly completely vanishes.
The anus appears fairly late on the dorsal surface, and the
ventral nerve cord is established as an unganglionated thickening
of the ventral epiblast.
The larva of Phoronis was known as Actinotrocha long
before its connection with Phoronis was established by Kowalevsky (No. 372). There is a complete segmentation leading to
the formation of a blastosphere, which is followed by an invagination, the opening of which is said by Kowalevsky to remain as
the mouth 1 . It is at first terminal, but on the development of a
large prse-oral lobe it assumes a ventral position. The anus
is formed at a later period at the posterior end of the body.
ACTINOTROCHA. (After Metschnikoff.)
A. Young larva.
H. Larva after the formation of post-oral ring of tentacles.
C. Larva with commencing invagination to form the body of Phoronis.
D. Invagination partially everted.
E. Invagination completely everted.
m. mouth ; an. anus; iv. invagination to form the body of Phoronis.
The youngest free larva observed by Metschnikoff (No. 373)
was less developed than the oldest larva found by Kowalevsky.
1 Kowalevsky states that whnt I have called the mouth is the anus, but his subsequent descriptions shew that he has transposed the mouth and anus in the embryo,
and that the opening, which he asserts to be the anus, is in reality the mouth.
It probably belongs to a different species. The body is uniformly ciliated (fig. 163 A). There is a large contractile prse-oral
lobe, and the body ends behind in two processes. The mouth
(m) is ventral, and the anus (an) dorsal, and not terminal as in
Kowalevsky's larva.
The alimentary tract is divided into stomodseum, stomach
and intestine. The two processes at the hind end of the body
are the rudiments of the first-formed pair of the arms which are
so characteristic of the fully developed Actinotrocha. A second
pair of arms next become established on the dorsal side of the
previously existing pair, and the region where the anus is placed
grows out as a special process. New pairs of arms continue to
be formed in succession dorsalwards and forwards, and soon
constitute a complete oblique post-oral ring (fig. 163 B). They are
covered by long cilia. Round the anal process a very conspicuous
ciliated ring also becomes established.
At the period when five pairs of arms are present a delicate membrane
becomes visible on the ventral side of the intestine which joins the somatic
mesoblast anteriorly. This membrane is the rudiment of the future ventral
vessel. The somatic mesoblast is present even before this period as a
delicate layer of circular muscular fibres.
When six pairs of arms have become formed an involution
(fig. 163 C, iv) appears on the ventral side, immediately behind
the ring of arms. This involution consists both of the epiblast
and somatic mesoblast. It grows inwards towards the intestine,
and, increasing greatly in length, becomes at the same time
much folded.
When it has reached its full development the critical period
of the metamorphosis of Actinotrocha into Phoronis is reached,
and is completed in about a quarter of an hour. The ventral
involution becomes evoluted (fig. 163 D), just as one might turn
out the finger of a glove which had been pulled inwards. When
the involution has been to a certain extent everted, the alimentary canal passes into it, and at the same time the body of the
larva becomes violently contracted. By the time the evagination is completed it forms (fig. 162 E) a long conical body,
containing the greater part of the alimentary tract, and constituting the body of the young- Phoronis. The original anal process
remains on the dorsal side as a small papilla (fig. 162 E, an).
364 \< T1NOTROCHA.
While these changes have been taking place the prae-oral
lobe has become much contracted, and partly withdrawn into
the stomodajum. At the same time the arms have become bent
forward, so as to form a ring round the mouth. Their bases
become much thickened. The metamorphosis is completed by
the entire withdrawal of the prae-oral lobe within the oesophagus,
and by the casting off of the ends of the arms, their bases
remaining as the circumoral ring of tentacles, which form
however a lophophore rather than a complete ring. The
perianal ring of cilia is also thrown off, and the anal process
withdrawn into the body of the young Phoronis. There are now
three longitudinal vascular trunks, united anteriorly by a circular
vessel which is prolonged into the tentacles.
General Considerations.
The development of Phoronis is so different from that of the
other Gephyrea that further investigations are required to shew
whether Phoronis is a true Gephyrean. Apart from its peculiar
metamorphosis Actinotrocha is a very interesting larval form, in
that it is without a prae-oral ciliated ring, and that the tentacles
of the adult are derived from a true post-oral ring, prolonged
into arm-like processes.
The other Gephyrea present in their development an obvious
similarity to the normal Chaetopoda, but their development stops
short of that of the Chaetopoda, in that they are clearly without
any indications of a true segmentation. In the face of what is
known of their development it is hardly credible that they can
represent a degenerate Chaetopod phylum in which segmentation
has become lost. Further than this the Gephyrea armata seem
in one respect to be a very primitive type in that they retain
through life a well-developed pra-oral lobe, which constitutes
their proboscis. In almost all other forms, except Balanoglossus,
the larval prae-oral lobe becomes reduced to a relatively insignificant anterior part of the head.
Gephyrea nuda.
(366) A. Kowalevsky. Sitz. d. zool. Abth. d. III. Vcrsam. russ. Naturj.
(Thalasscma). Zeit.f. wiss. Zool. Vol. xxn. 1872, p. 284.
(367) A. Krohn. "Ueb. d. Larve d. Sipunculus nudus nebst Bemerkungen,"
etc. Miiller's Archiv, 1857.
(368) M. Salensky. "Ueber die Metamorphose d. Echiurus." Morphologisches
Jahrbuch, Bd. 11.
(369) E. Selenka. "Eifurchung u. Larvenbildung von Phascolosoma elongatum." Zeit.f. wiss. ZooL 1875, Bd. xxv. p. i.
(370) J. W. Spengel. "Beitrage z. Kenntniss d. Gephyreen (Bonellia)." Mittheil. a. d. zool. Station z. Neapel, Vol. I. 1879.
Gephyrea tubicola (Actinotrocha).
(371) A. Krohn. " Ueb. Pilidium u. Actinotrocha." Miiller's Archiv, 1858.
(372) A. Kowalevsky. "On anatomy and development of Phoronis," Petersbourg, 1867. 2 PI. Russian. Vide Leuckart's Bericht, 1866-7.
(373) E. Metschnikoff. " Ueber d. Metamorphose einiger Seethiere (Actinotrocha)." Zeit.f. wiss. Zool. Bd. xxi. 1871.
(374) J. Miiller. " Bericht lib. ein. Thierformen d. Nordsee." Miiller's Archiv,
(375) An. Schneider. "Ueb. d. Metamorphose d. Actinotrocha branchiata."
Miiller's Arch. 1862.
THE present chapter deals with three small isolated groups,
which only resemble each other in that the systematic position of
all of them is equally obscure.
The discoveries of Kowalevsky (No. 378) confirmed by
Btitschli (No. 376) with reference to the development of Sagitta,
though they have not brought us nearer to a knowledge of the
systematic position of this remarkable form, are nevertheless of
Hiitschli and B after Kowalevsky.) The three embryos are represented in the same
A. The gastrula stage.
li. A succeeding stage in which the primitive archenteron is commencing to be
divided into three parts, the two lateral of which are destined to form the body
C. A later stage in which the mouth involution (/;/) has become continuous with
the alimentary tract, and the blastopore has become closed.
m. mouth; al. alimentary canal ; ae. archenteron ; bl.p. blastopore; pv. perivisceral
cavity; sf>. splanchnopleuric mesoblast; so. somatopleuric mesoblast ; ge. generative
great value for the more general problems of embryology. The
development commences after the eggs are laid. The segmentation is uniform, and a blastosphere, formed of a single layer of
columnar cells, is the product of it. An invagination takes
place, the opening of which narrows to a blastopore situated
at the pole of the embryo opposite that at which the mouth
subsequently appears (fig. 164 A). The simple archenteron soon
becomes anteriorly divided into three lobes, which communicate
freely with the still single cavity behind (fig. 164 B). The two
lateral lobes are destined to form the body cavity, and the
median lobe the alimentary tract of the adult. An invagination
soon arises at the opposite pole of the embryo to the blastopore
and forms the mouth and oesophagus (fig. 164 B and C, m).
At the gastrula stage there is formed a paired mass destined
to give rise to the generative organs. It arises as a prominence
of six cells, projecting from the hypoblast at the anterior pole of
the archenteron, and soon separates itself as a mass, or probably
a pair of masses, lying freely in the cavity of the archenteron
(fig. 164 A. y ge). When the folding of the primitive cavity takes
place the generative rudiment is situated at the hind end of the
median lobe of the archenteron in the position represented
in fig. 164 C, ge.
An elongation of the posterior end of the embryo now takes
place, and the embryo becomes coiled up in the egg, and when
eventually hatched sufficiently resembles the adult to be recognisable as a young Sagitta.
Before hatching takes place various important changes
become manifest. The blastopore disappears after being carried
to the ventral surface. The middle section of the trilobed region
of the archenteron becomes separated from the unpaired
posterior part, and forms a tube, blind behind, but opening
in front by the mouth (fig. 165 A, al). It constitutes the permanent alimentary tract, and is formed of a pharyngeal epiblastic
invagination, and a posterior hypoblastic section derived from
the primitive archenteron. The anus is apparently not formed
till comparatively late. After the isolation of the alimentary
tract the remainder of the archenteron is formed of two cavities
in front, which open freely into a single cavity behind (fig.
165 A). The whole of it constitutes the body cavity and its walls
3 68
f/ic mesoblast. The anterior paired part becomes partitioned off
into a head section and a trunk section (fig. 165 A and B). The
former constitutes a pair of distinct cavities (c.pv) in the head,
and the latter two cavities opening freely into the unpaired
portion behind. At the junction of the paired cavities with the
unpaired cavity are situated the generative organs (ge). The
inner wall of each of the paired cavities forms the splanchnopleuric mesoblast, and the outer wall of the whole the somatic
mesoblast. The inner walls of the posterior cavities unite above
and below the alimentary tract, and form the dorsal and ventral
mesenteries, which divide the body cavity into two compartments
in the adult. Before the hatching of the embryo takes place
this mesentery is continued backwards so as to divide the
primitively unpaired caudal part of the body cavity in the
same way.
From the somatic mesoblast of the trunk is derived the
single layer of longitudinal muscles of Sagitta, and part of the
epithelioid lining of the body cavity. The anterior termination
of the trunk division of the body cavity is marked in the adult
by the mesentery dividing into two laminae, which bend outwards
to join the body wall.
The cephalic section of the body cavity
seems to atrophy, and
its walls to become converted into the complicated system of muscles
present in the head of
the adult Sagitta.
In the presence of
a section of the body
cavity in the head the
embryo of Sagitta re
sembles Lumbricus,
Spiders, etc.
The generative rudiment of each side
divides into an anterior
and a posterior part
SV.ITTA. A. from the dorsal surface. I?, from the
tide. (After 15iitschli.)
m. mouth ; al. alimentary canal ; v.g. ventral
ganglion (thickening of epiblast) ; rp. epiblast ; c.pv,
cephalic section of body cavity; so. somatopleure ;
s/>. splanchnopleure ; ,;v. generative
(fig. 165, ge]. The former constitutes the ovary, and is situated
in front of the septum dividing the tail from the body ; and the
latter, in the caudal region of the trunk, forms the testis.
The nervous system originates from the epiblast. There is a
ventral thickening (fig. 165 B, v.g) in the anterior region of the
trunk, and a dorsal one in the head. The two are at first
continuous, and on becoming separated from the epiblast remain
united by thin cords.
The ventral ganglion is far more prominent during embryonic
life than in the adult. Its position and early prominence in the
embryo perhaps indicate that it is the homologue of the ventral
cord of Chaetopoda 1 .
(376) O. Biitschli. "Zur Entwicklungsgeschichte der Sagitta." Zeitschrift f.
wiss. Zoo!., Vol. xxni. 1873.
(377) C. Gegenbaur. " Uber die Entwicklung der Sagitta." Abhand. d. naturforschenden Gesellschaft in Halle, 1857.
(378) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthropoden." Mem. Acad. Petersbourg, VII. ser., Tom. XVI., No. 12. 1871.
The development of these peculiar parasites on Crinoids has been
investigated by Metschnikoff (No. 380), Semper (No. 381), and Graff
(No. 379).
The segmentation is unequal, and would appear to be followed by an
epibolic invagination. The outer layer of cells (epiblast) becomes covered
with cilia, and the inner is transformed into a non-cellular (?) central yolk
mass. At this stage the larva is hatched, and commences to lead a free
existence. In the next stage observed by Metschnikoff, the mouth, oesophagus, stomach, and anus had become developed ; and two pairs of feet
were present. In both of these feet Chaetopod-like setae were present, which
in the hinder pair were simple fine bristles without a terminal hook. The
papilliform portion of the foot is at first undeveloped. The feet become
successively added, like Chaetopod segments, and the stomach does not
become dendriform till the whole complement of feet (5 pairs) are present.
In the primitive covering of cilia, combined with a subsequent indication
1 Langerhans has recently made some important investigations on the nervous
system of Sagitta, and identifies the ventral ganglion with the parieto-splanchnic ganglia of Molluscs, while he has found a pair of new ganglia, the development of which
is unknown, which he calls the suboesophageal or pedal ganglia. The embryological facts do not appear to be in favour of these interpretations.
B. II. 24
of segments in the formation of the feet and setae, the larva of the Myzostomea shews an approximation to the Chaetopoda, and the group is
probably to be regarded as an early Chactopod type specially modified in
connection with its parasitic habits.
(379) L.Graff. Das Genus Myzostoma. Leipzig, 1877.
(380) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Myzostomum."
Zfit.f. wiss. Zool. y Vol. XVI. 1866.
(381) C. Semper. "Z. Anat. u. Entwick. d. Gat. Myzostomum." Ztit.f. wiss.
Zool., Vol. ix. 1858.
A few observations of Ludwig on the winter eggs of Ichthydium larus
shew that the segmentation is a total and apparently a regular one. It
leads to the formation of a solid morula. The embryo has a ventral
curvature, and the caudal forks are early formed as cuticular structures. By
the time the embryo leaves the egg, it has almost reached the adult state.
The ventral cilia arise some little time prior to the hatching.
(382) H. Ludwig. " Ueber die Ordnung Gastrotricha Mctschn" Zeit. f. wiss.
Zool., Vol. xxvi. 1876.
Nematoidea. Although the ova of various Nematodes have
formed some of the earliest, as well as the most frequent objects
of embryological observation, their development is still but very
imperfectly known. Both viviparous and oviparous forms are
common, and in the case of the oviparous forms the eggs are
usually enveloped in a hard shell. The segmentation is total
and nearly regular, though the two first segments are often
unequal. The relation of the segmentation spheres to the
germinal layers is however only satisfactorily established (through
the researches of Butschli (No. 383)) in the case of Cucullanus
elegans, a form parasitic in the Perch 2 .
The early development of this embryo takes place within
the body of the parent, and the egg is enveloped in a delicate
membrane. After the completion of the early stages of segmentation the embryo acquires the form of a thin flat plate
composed of two layers of cells (fig. 166 A and B). The two
layers of this plate give rise respectively to the epiblast and
hypoblast, and at a certain stage the hypoblastic layer ceases to
1 The following classification of the Nematoda is employed in this chapter :
r Ascaridae.
Trichinidse. II. Gordioidea.
I. Nematoidea. , Filarid8B . m . Chaetosomoidea.
[_ Anguillulidse.
2 The ova of Anguillula aceti are stated by Hallez to undergo a similar development to those of Cucullanus.
grow, while the growth of the epiblastic layer continues. As a
consequence of this the sides of the plate begin to fold over
towards the side of the hypoblast (fig. 166 D.) This folding
results in the formation of a remarkably constituted gastrula,
which has the form of a hollow two-layered cylinder with an
incompletely closed slit on one side (fig. 166 E, bl.p}. This slit
has the value of a blastopore. It becomes closed by the coalescence of the two edges, a process which commences posteriorly,
FIG. 166.
(From Biitschli.)
Surface view of flattened embryo at an early stage in the segmentation.
Side view of an embryo at a somewhat later stage, in optical section.
Flattened embryo at the completion of segmentation.
D. Embryo at the commencement of the gastrula stage.
E. Embryo when the blastopore is reduced to a mere slit.
F. Vermiform embryo after the division of the alimentary tract into oesophageal
and glandular divisions.
m. mouth; ep. epiblast; hy. hypoblast; me. mesoblast; a?, oesophagus; bl.p. blastopore.
and then gradually extends forwards. In front the blastopore
never becomes completely closed, but remains as the permanent
mouth. The embryo after these changes has a worm-like form,
which becomes the more obvious as it grows in length and
becomes curved (fig. 166 F).
The hypoblast of the embryo gives rise to the alimentary
canal, and soon becomes divided into an cesophageal section
(fig. 1 66 F, ce) formed of granular cells, and a posterior division
formed of clear cells. The mesoblast (fig. 166, me) takes its
origin from certain special hypoblast cells around the mouth,
and thence grows backwards towards the posterior end of the
The young Cucullanus becomes hatched while still in the
generative ducts of its parent, and is distinguished by the
presence of a remarkable thread-like tail. On the dorsal surface
is a provisional boring apparatus in the form of a conical papilla.
A firm cuticle enveloping the body is already present. In this
condition it leaves its parent and host, and leads for a time a
free existence in the water. Its metamorphosis is dealt with in
another section.
The ova of the Oxyuridae parasitic in Insects are stated by Galeb (No.
386) to take the form of a blastosphere at the close of segmentation. An
inner layer is then formed by delamination. What the inner layer gives rise
to is not clear, since the whole alimentary canal is stated to be derived from
two buds, which arise at opposite ends of the body, and grow inwards till
they meet.
The generative organs. The study of the development of
the generative organs of Nematodes has led to some interesting
results. In the case of both sexes the generative organs originate (Schneider, No. 390) from a single cell. This cell elongates
and its nuclei multiply. After assuming a somewhat columnar
form, it divides into (i) a superficial investing layer, and (2) an
axial portion.
In the female the superficial layer is only developed distinctly in the median part of the column. In the course of the
further development the two ends of the column become the
blind ends of the ovary, and the axial tissue they contain forms
the germinal tissue of nucleated protoplasm. The superficial
layer gives rise to the epithelium of the uterus and oviduct.
The germinal tissue, which is originally continuous, is interrupted
in the middle part (where the superficial layer gives rise to the
uterus and oviduct), and is confined to the two blind extremities
of the tube.
In the male the superficial layer, which gives rise to the epithelium of the vas deferens, is only formed at the hinder end of
the original column. In other respects the development takes
place as in the female.
Gordioidea. The ovum of Gordius undergoes a regular segmentation.
According to Villot (No. 391) it forms at the close of segmentation a morula,
which becomes two-layered by delamination. The embryo is at first
spherical, but soon becomes elongated.
By an invagination at the anterior extremity the head is formed. It
consists of a basal portion, armed with three rings of stylets, and a conical
proboscis, armed with three large stylets. When the larva becomes free
the head becomes everted, though it remains retractile. By the time the
embryo is hatched a complete alimentary tract is formed with an oral opening
at the end of the proboscis, and a subterminal ventral anal opening. It is
divided into an oesophagus and stomach, and a large gland opens into it at
the base of the proboscis.
The body has a number of transverse folds, which give it a ringed
Metamorphosis and life history.
Nematoidea. Although a large number of Nematodes have
a free existence and simple life history, yet the greater number
of known genera are parasitic, and undergo a more or less complicated metamorphosis 1 . According to this metamorphosis
they may be divided into two groups (which by no means
closely correspond with the natural divisions), viz. those which
have a single host, and those with two hosts. Each of these
main divisions may be subdivided again into two.
In the first group with one host the simplest cases are those
in which the adult sexual form of parasite lays its eggs in the
alimentary tract of its host, and the eggs are thence transported
to the exterior. The embryo still in the egg, if favoured by
sufficient warmth and moisture, completes its development up
to a certain point, and, if then swallowed by an individual of the
species in which it is parasitic in the adult condition, it is
denuded of its shell by the action of the gastric juice, and
develops directly into the sexual form.
Leuckart has experimentally established this metamorphosis in the case
of Trichocephalus affinis, Oxyurus ambigua, and Heterakis vermicularis.
The Oxyuridae of Blatta and Hydrophilus have a similar life history
1 The following facts are mainly derived from Leuckart's exhaustive treatise
(No. 388).
(Caleb, No. 386), and it is almost certain that the metamorphosis of the
human parasites, Ascaris lumbricoides and Oxyurus vermicularis, is of this
A slightly more complicated metamorphosis is common in
the genera Ascaris and Strongylus. In these cases the egg-shell
is thin, and the embryo becomes free externally, and enjoys for
a shorter or longer period a free existence in water or moist
earth. During this period it grows in size, and though not
sexual usually closely resembles the adult form of the permanently free genus Rhabditis. In some cases the free larva
becomes parasitic in a freshwater Mollusc, but without thereby
undergoing any change. It eventually enters the alimentary
tract of its proper host and there become sexual.
As examples of this form of development worked out by Leuckart may
be mentioned Uochmius trigonocephalus, parasitic in the dog, and Ascaris
acuminata, in the frog. The human parasite Dochmius duodenale undergoes the same metamorphosis as Dochmius trigonocephalus.
A remarkable modification of this type of metamorphosis is found in
Ascaris (Rhabdonema) nigrovenosa, which in its most developed condition
is parasitic in the lungs of the frog (Metschnikoff, Leuckart, No. 388). The
embryos pass through their first developmental phases in the body of the
parent. They have the typical Rhabditis form, and make their way after
birth into the frog's rectum. From this they pass to the exterior, and then
living either in moist earth, or the faeces of the frog, develop into a sexual
form, but are very much smaller than in the adult condition. The sexes are
distinct, and the males are distinguished from the females by their smaller
size, shorter and rounded tails, and thinner bodies. The females have
paired ovaries with a very small number of eggs, but the testis of the males
is unpaired. Impregnation takes place in the usual way, and in summer
time about four embryos are developed in each female, which soon burst
their egg-capsules, and then move freely in the uterus. Their active movements soon burst the uterine walls, and they then come to lie freely in the
body cavity. The remaining viscera of the mother are next reduced to a
finely granular material, which serves for the nutrition of the young forms
which continue to live in the maternal skin. The larvae eventually become
free, and though in many respects different from the parent form which gave
rise to them, have nevertheless the Rhabditis form. They live in water or
slime, and sometimes become parasitic in water-snails ; in neither case however do they undergo important changes unless eventually swallowed by a
frog. They then pass down the trachea into the lungs and there rapidly
develop into the adult form. No separate males have been found in the
lungs of the frog, but it has been shewn by Schneider (No. 390) that the
so-called females are really hermaphrodites ; the same gland giving origin
to both spermatozoa and ova, the former being developed before the latter 1 .
The remarkable feature of the above life history is the fact that in the stage
corresponding with the free larval stage of the previous forms the larvae of
this species become sexual, and give rise to a second free larval generation,
which develops into the adult form on again becoming parasitic in the
original host. It constitutes a somewhat exceptional case of heterogamy as
defined in the introduction.
Amongst the Nematodes with but a single host a remarkable parasite in
wheat has its place. This form, known as Anguillula scandens, inhabits in
the adult condition the ears of wheat, in which it lays its eggs. After
hatching, the larvae become encysted, but become free on the death of the
plant. They now inhabit moist earth, but eventually make their way into
the ears of the young wheat and become sexually mature.
The second group of parasitic Nematodes with two hosts
may be divided into two groups, according to whether the larva
has a free existence before passing into its first or intermediate
host, or is taken into it while still in the egg. In the majority
of cases the larval forms live in special connective tissue capsules, or sometimes free in the tissues of their intermediate
hosts ; but the adults, as in the cases of other parasitic Nematodes, inhabit the alimentary tract.
The life history of Spiroptera obtusa may be cited as an example of a
Nematode with two hosts in which the embryo is transported into its
intermediate host while still within the egg. The adult of this form is
parasitic in the mouse, and the ova pass out of the alimentary tract with the
excreta, and may commonly be found in barns, etc. If one of the ova is
now eaten by the meal-worm (larva of Tenebrio), it passes into the body
cavity of this worm and undergoes further development. After about five
weeks it becomes encapsuled between the ' fat bodies ' of the meal-worm.
It then undergoes an ecdysis, and, if the meal-worm with its parasites is
now eaten by the mouse, the parasites leave their capsule and develop into
the sexual form.
As examples of life histories in which a free state intervenes before the
intermediate host, Cucullanus elegans and Dracunculus may be selected.
The adult Cucullanus elegans is parasitic in the alimentary tract of the Perch
and other freshwater fishes. It is a viviparous form, and the young after
birth pass out into the water. They next become parasitic in Cyclops,
passing in through the mouth, so into the alimentary tract, and thence into
the body cavity. They soon undergo an ecdysis, in the course of which the
oesophagus becomes divided into a muscular pharynx and true glandular
1 Leuckart does not appear to be satisfied as to the hermaphroditism of these
forms ; and holds that it is quite possible that the ova may develop parthenogenetically.
oesophagus. They then grow rapidly in length, and at a second ecdysis
acquire a peculiar beaker-like mouth cavity approaching that of the adult.
They do not become encapsuled. No further development of the worm
takes place so long as it remains in the Cyclops, but, if the Cyclops is now
swallowed by a Perch, the worm undergoes a further ecdysis, and rapidly
attains to sexual maturity.
The observations of Fedschenko on Dracunculus medinensis 1 , which is
parasitic in the subcutaneous connective tissue in Man, would seem to shew
that it undergoes a metamorphosis very similar to that of Cucullanus. There
is moreover a striking resemblance between the larvae of the two forms.
The larvae of Dracunculus become transported into water, and then make
their way into the body cavity of a Cyclops by boring through the soft skin
between the segments on the ventral surface of the body. In the body cavity
the larvae undergo an ecdysis and further development. But on reaching
a certain stage of development, though they remain a long time in the
Cyclops, they grow no further. The remaining history is unknown, but
probably the next host is man, in which the larva comes to maturity. In the
adult condition only females of Dracunculus are known, and it has been
suggested by various writers that the apparent females are in reality hermaphrodites, like Ascaris nigrovenosa, in which the male organs come to
maturity before the female.
Another very remarkable human parasite belonging to the same group
as Dracunculus is the form known as Filaria sanguinis hominis, or Filaria
Bancrofti 2 .
The sexual form is parasitic in warm climates in the human tissues, and
produces multitudes of larvae which pass into the blood, and are sometimes
voided with the urine. The larvae in the blood do not undergo a further development, and unless transported to an intermediate host die before very long.
Some, though as yet hardly sufficient, evidence has been brought forward to
shew that if the blood of an infected patient is sucked by a mosquito the
larvae develop further in the alimentary tract of the mosquito, pass through a
more or less quiescent stage, and eventually grow considerably in size, and
on the death of the mosquito pass into the water. From the water they are
probably transported directly or indirectly into the human intestines, and
then bore their way into the tissues in which they are parasitic, and become
sexually mature.
The well-known Trichina spiralis has a life history unlike that of other
known Nematodes, though there can be little doubt that this form should
be classified in respect to its life history with the last- described forms.
The peculiarity of the life history of Trichina is that the embryos set free
in the alimentary canal pass through the walls into the muscular tissues and
there encyst ; but do not in a general way pass out from the alimentary
1 Vide Leuckart, D. men. Par., Vol. II. p. 704.
2 Vide D. P. Manson, " On the development of Filaria sanguinis hominis."
Journal of the Linnean Society, Vol. xiv. No. 75.
canal of one host and thence into a fresh host to encyst. It occasionally
however happens that this migration does take place, and the life history
of Trichina spiralis then becomes almost identical with that of some of the
forms of the third type. Trichina is parasitic in man, and in swine, and
also in the rat, mouse, cat, fox and other forms which feed upon them.
Artificially it can be introduced into various herbivorous forms (rabbit,
guinea-pig, horse) and even birds.
The sexual form inhabits the alimentary canal. The female is viviparous, and produces myriads of embryos, which pass into the alimentary
canal of their host, through the walls of which they make their way, and
travelling along lines of connective tissue pass into the muscles. Here the
embryos, which are born in a very imperfect condition, rapidly develop,
and eventually assume a quiescent condition in a space inclosed by sarcolemma. Within the sarcolemma a firm capsule is developed for each larva,
which after some months becomes calcified ; and after the atrophy of the
sarcolemma a connective tissue layer is formed around it. Within its
capsule the larva can live for many years, even ten or more, without
undergoing further development, but if at last the infected flesh is eaten by
a suitable form, e.g. the infected flesh of the pig by man, the quiescent
state of the larva is brought to a close, and sexual maturity is attained in
the alimentary tract of the new host.
Gordioidea. The free larva of Gordius already described usually penetrates into the larva of Chironomus where it becomes encysted. On the
Chironomus being eaten by some fish (Villot, No. 39) (Phoxinus laevis or
Cobitis barbatula), it penetrates into the wall of the intestine of its second
host, becomes again encysted and remains quiescent for some time. Eventually in the spring it leaves its capsule, and enters the intestine, and
passes to the exterior with the faeces. It then undergoes a gradual metamorphosis, in the course of which it loses its ringed structure and cephalic
armature, grows in length, acquires its ventral cord, and on the development of the generative organs loses the greater part of its alimentary tract.
Young examples of Gordius have often been found in various terrestrial
carnivorous Insecta, but the meaning of this fact is not yet clear.
(383) O. Biitschli. "Entwicklungsgeschichte d. Cucullanus elegans." Zdt.j.
wiss. Zool., B. xxvi. 1876.
(384) T. S. Cobbold. Entozoa. Groombridge and Son, 1864.
(385) T. S. Cobbold. Parasites; A Treatise on the Entozoa of Man mn/
Animals. Churchill, 1879.
(386) O. Galeb. "Organisation et developpement des Oxyurides," &c. Archives de Zool. expcr. et getter. , Vol. vn. 1878.
(387) R. Leu ck art. Untcrsufkutigcn itb. Trichina spiralis. 2nd ed. Leip/ig,
(388) R. Leuckart. Die tnenschlichcn Parasitcn, Bd. II. 1876.
(389) H. A. Pagenstecher. Die Trichinen nach Versitchen dargestellt. Leipzig, 1865.
(390) A.Schneider. Monographic d. Nemaioden. Berlin, 1866.
(391) A. Villot. "Monographic des Dragoneaux" (Gordioidea). Archives de
Zool. exper. et gener., Vol. ill. 1874.
The Acanthocephala appear to be always viviparous. At the time of
impregnation the ovum is a naked cell, and undergoes in this condition the
earlier phases of segmentation.
The segmentation is unequal (Leuckart, No. 393), but whether there is an
epibolic gastrula has not clearly been made out.
Before segmentation is completed there are formed round the ovum
thick protecting membranes, which are usually three in number, the middle
one being the strongest. After segmentation the central cells of the ovum
fuse together to give rise to a granular mass, while the peripheral cells at a
slightly later period form a more transparent syncytium. At the anterior
end of the embryo there appears a superficial cuticle bearing in front a ring
of hooks.
The embryo is now carried out with the excreta from the intestine of
the vertebrate host in which its parent lives. It is then swallowed by some
invertebrate host 1 .
In the intestine of the invertebrate host the larva is freed from its
membranes, and is found to have a somewhat elongated conical form, terminating anteriorly in an obliquely placed disc, turned slightly towards the
ventral surface and armed with hooks. Between this disc and the granular
mass, already described as formed from the central cells of the embryo, is a
rather conspicuous solid body. Leuckart supposes that this body may represent a rudimentary functionless pharynx, while the granular mass in
his opinion is an equally rudimentary and functionless intestine. The body
wall is formed of a semifluid internal layer surrounding the rudimentary
intestine, if such it be, and of a firmer outer wall immediately within the cuticle.
The adult Echinorhyncus is formed by a remarkable process of development within the body of the larva, and the skin is the only part of the
larva which is carried over to the adult.
In Echinorhyncus proteus the larva remains mobile during the formation of the adult, but in other forms the metamorphosis takes place during
a quiescent condition of the larva.
The organs of the adult are differentiated from a mass of cells which
appears to be a product of the central embryonic granular mass, and is
1 Echin. proteus, which is parasitic in the adult state in many freshwater fish,
passes through its larval condition in the body cavity of Gammarus pulex. Ech.
angustatus, parasitic in the Perch, is found in the larval condition in the body cavity
of Asellus aquaticus. Ech. gigas, parasitic in swine, is stated by Schneider (No. 394)
to pass through its larval stages in maggots.
called by Leuckart the embryonic nucleus. The embryonic nucleus becomes
divided into four linearly arranged groups of cells, of which the hindermost
but one is the largest, and very early differentiates itself into (i) a peripheral
layer, and (2) a central mass formed of two distinct bodies. The peripheral
layer of this segment grows forwards and backwards, and embraces the
other segments, with the exception of the front end of the first one which
is left uncovered. The envelope so formed gives rise to the splanchnic and
somatic mesoblast of the adult worm. Of the four groups of cells within it
the anterior gives rise to the proboscis, the next to the nerve ganglion, the
third, formed of two bodies, to the paired generatives, and the fourth to the
generative ducts. The whole of the above complex rapidly elongates, and
as it does so the enveloping membrane becomes split into two layers ; of
which the outer forms the muscular wall of the body (somatic mesoblast),
and the inner the muscular sheath of the proboscis and the so-called generative ligament enveloping the generative organs. The inner layer may be
called the splanchnic mesoblast in spite of the absence of an intestine.
The cavity between the two mesoblastic layers forms the body cavity.
The various parts of the adult continue to differentiate themselves as
the whole increases in size. The generative masses very early shew traces
of becoming differentiated into testes or ovaries. In the male the two
generative masses remain spherical, but in the female become elongated :
the rudiment of the generative ducts becomes divided into three sections
in both sexes. The most remarkable changes are, however, those undergone
by the rudiment of the proboscis.
In its interior there is formed a cavity, but the wall bounding the front
end of the cavity soon disappears. By the time that this has taken place
the body of the adult completely fills up the larval skin, to which it very
soon attaches itself. The hollow rudiment of the proboscis then becomes
everted, and forms a papilla at the end of the body, immediately adjoining the larval skin. This papilla, with the larval skin covering it,
constitutes the permanent proboscis. The original larval cuticle is either
now or at an earlier period thrown off and a fresh cuticle developed. The
hooks of the proboscis are formed from cells of the above papilla, which
grow through the larval skin as conical prominences, on the apex of which
a chitinous hook is modelled. The remainder of the larval skin forms the
skin of the adult, and at a later period develops in its deeper layer the
peculiar plexus of vessels so characteristic of the Acanthocephala. The
anterior oval appendages of the adult cutis, known as the lemnisci, are
outgrowths from the larval skin.
The Echinorhyncus has with the completion of these changes practically
acquired its adult structure ; but in the female the ovaries undergo at this
period remarkable changes, in that they break up into a number of spherical
masses, which lie in the lumen of the generative ligaments, and also make
their way into the body cavity.
The young Echinorhyncus requires to be transported to its permanent
host, which feeds on its larval host, before attaining to sexual maturity.
(392) R. Greeff. " Untersuchungen ii. d. Bau u. Entwicklung des Echin. miliarius." Archiv f. Naturgesch. 1864.
(393) R. Leuckart. Die menschlichen Parasiten. Vol. n. p. 80 1 et seq.
(394) An. Schneider. " Ueb. d. Bau d. Acanthocephalen." Archiv f. Anat.
u, Phys. 1868.
(395) G. R. Wagener. Beitrdge z. Entwicklungsgeschichte d. Eingeweidewiirmer. Haarlem, 1865.
THE remarkable researches of Moseley (No. 396) on Peripatus
(From Moseley.)
capensis have brought clearly to light the affinities of this form
with the tracheate Arthropoda ; and its numerous primitive
(After Moseley.)
A. Youngest stage hitherto observed before the appearance of the legs.
B. Later stage after the legs and antennae have become developed.
Both figures represent the larva as it appears within the egg.
i and i. First and second post-oral appendages.
characters, such as the generally distributed tracheal apertures,
the imperfectly segmented limbs, the diverging ventral nerve
cords with imperfectly marked ganglia, and the nephridia (segmental organs 1 ), would render its embryology of peculiar interest. Unfortunately Moseley was unable, from want of
material, to make so complete a study of its development as of
its anatomy. The youngest embryo observed was in part
distinctly segmented, and coiled up within the egg (fig. 168 A).
The procephalic lobes resemble those of the Arthropoda generally, and are unlike the prae-oral lobe of
Chaetopods or Discophora. They are not
marked off by a transverse constriction
from the succeeding segments. The three
embryonic layers are differentiated, and
the interior is filled with a brownish mass
the remnant of the yolk which is probably enclosed in a distinct intestinal wall,
and is lobed in correspondence with the
segmentation of the body. The mouth
invagination is not present, and but two
pairs of slight prominences mark the rudiments of the two anterior post-oral appendages.
The single pair of antennae is formed
in the next stage, and is followed by the
remaining post-oral appendages, which
arise in succession from before backwards
somewhat later than the segments to which
they appertain.
The posterior part of the embryo becomes uncoiled, and the whole embryo
bent double in the egg (fig. 168 B).
The mouth appears as a slit-like opening between and below the procephalic
lobes. On each side and somewhat behind it there grows out
an appendage the first post-oral pair (fig. 169, i) while in
front and behind it are formed the upper and lower lips. These
two appendages next turn inwards towards the mouth, and their
Slightly older than A in
fig. 168; unrolled. (After
a. antennae ; o. mouth ;
i. intestine ; c. procephalic
lobe, i, 2, 3, etc., postoral appendages.
1 F. M. Balfour, "On certain points in the Anatomy of Peripatus capensis."
Quart. Journ. of Micros. Science, Vol. xix. 1879.
bases become gradually closed over by two processes of the
procephalic region (fig. 170, m)
The whole of these structures
assist in forming a kind of
secondary mouth cavity, which
is at a later period further
completed by the processes of
the procephalic region meeting
above the mouth, covering over
the labrum, and growing backwards to near the origin of the
second pair of post-oral appendages.
The antennae early become
jointed, and fresh joints continue to be added throughout
embryonic life ; in the adult
there are present fully thirty
joints. It appears to me probable (though Mr Moseley takes
the contrary view) from the late development of the paired
processes of the procephalic lobes, which give rise to the circular
lip of the adult, that they
are not true appendages.
The next pair therefore
to the antennae is the first
post-oral pair. It is the
only pair connected with
the mouth. At their extremities there is formed a
pair of claws similar to
those of the ambulatory
legs (fig. 171). The next FIG. 171. HEAD OF AN EMBRYO PERIPA, . . r TUS. (From Moseley.)
and largest pair of appen- The figure shews the jaws (mamlil)lcs)> and
dagCS in the embryo are close to them epiblastic involutions, which
/. thickening of epiblast of procephalic lobe to form supra-oesophageal ganglion ; ///. process from procephalic lobe
growing over the first post-oral appendage ; o. mouth; e. eye; i and 2, first
and second pair of post-oral appendages.
the oral papillae. They
grow into the supra-oesophageal ganglia. The
antennae, oral cavity, and oral papilhe are also
are chiefly remarkable for shewn.
containing the ducts of the slime glands which open at their
bases. They are without claws. The succeeding appendages
become eventually imperfectly five-jointed ; two claws are
formed as cuticular investments of papillae in pockets of the
skin at the ends of their terminal joints.
I have been able to make a few observations on the internal structure of
the embryos from specimens supplied to me by Moseley. These are so far
confined to a few stages, one slightly earlier, the others slightly later, than
the embryo represented in fig. 168 B. The epiblast is formed of a layer of
columnar cells, two deep on the ventral surface, except along the median line
where there is a well-marked groove and the epiblast is much thinner (fig. 172).
The ventral cords of the trunk are formed as two independent epiblastic
thickenings. In my earlier stage these are barely separated from the
epiblast, but in the later ones are quite independent (fig. 172, v.n), and
partly surrounded by mesoblast.
The supra-cesophageal ganglia are formed as thickenings of the epiblast
of the ventral side of the procephalic lobes in front of the stomodaeum.
They are shewn at / in fig. 170. The thickenings of the two sides are at
first independent. At a somewhat later period an invagination of the
epiblast grows into each of these lobes. The openings of these invaginations
extend from the oral cavity forwards; and they are shewn in fig. 171 l .
Their openings become closed, and the walls of the invaginations constitute
a large part of the embryonic supra-cesophageal ganglia.
Similar epiblastic invaginations assist in forming the supra-cesophageal
ganglia of other Tracheata.
They are described in the sequel
for Insects, Spiders and Scorpions. The position of the supracesophageal ganglia on the ventral side of the procephalic lobes
is the same as that in other
The mesoblast is formed, in
the earliest of my embryos, of
scattered cells in the fairly wide
space between the mesenteron
and the epiblast. There are two
distinct bands of mesoblast on
the outer sides of the nervous
cords. In the later stage the
mesoblast is divided into distinct somatic and splanchnic layers, both very thin ; but the two
layers are connected by transverse strands (fig. 172). There
from which the section is taken was somewhat
younger than fig. 171.
sp.m. splanchnic mesoblast.
s.m. somatic mesoblast.
me. median section of body cavity.
k. lateral section of body cavity.
v.n. ventral nerve cord.
me. mesenteron.
1 This figure is taken from Moseley. The epiblastic invaginations are represented
in it very accurately, and though not mentioned in the text of the paper, Moseley
informs me that he has long been aware of the homology of these folds with those in
various other Tracheata.
B. II.
2 5
are two special longitudinal septa dividing the body cavity into three
compartments, a median (me), containing the mesenteron, and two lateral
(Ic) containing the nerve cords. This division of the body cavity persists,
as I have elsewhere shewn, in the adult. A similar division is found in
some Chaetopoda, e.g. Polygordius.
I failed to make out that the mesoblast was divided into somites, and
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 supracesophageal ganglia.
The alimentary tract is formed of a mesenteron (fig. 172), a stomodaeum, and proctodaeum. 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 stomodaeum 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 proctodaeum is a moderately deep pit near the hinder end of the body.
The existence of a tracheal system 1 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 Metschnikoffs figure of Geophilus (No. 399) PI. xxi. fig. u,he
will be satisfied on this point.
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 papillae to the first pair of maxillae, etc. Moseley is
somewhat doubtful about the homologies of the appendages,
and hesitates between considering the oral papillae as equivalent
to the second pair of maxillae (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
1 The specimens shewing tracheae 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.
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 Chelicerae (mandibles) of
Spiders, and there is some evidence in Insects for the existence of a gland
belonging to the first pair of maxillae, 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
(396) H. N. Moseley. "On the Structure and Development of Peripatus
capensis." Phil. Trans. Vol. 164, 1874.
Chilognatha. The first stages in the development of the
Chilognatha have been investigated by Metschnikoffand Stecker,
but their accounts are so contradictory as hardly to admit of
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 2 .
1 The classification of the Myriapoda employed in the present section is
I. Chilognatha. (Millipedes.)
II. Chilopoda. (Centipedes.)
2 Stecker's (No. 400) observations were made on the eggs of Julus fasciatus, Julus
fcetidus, 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
(After Metschnikoff.)
A. Embryo on eleventh day with commencing ventral flexure (*).
B. Embryo with three pairs of post-oral appendages.
C. Embryo with five pairs of post-oral appendages.
gs. ventral plate; at. antenme; 15 post-oral appendages; x. point of flexure of
the ventral plate.
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 Sleeker, 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.
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 antennae, 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 antennae two further pairs become
added, and at the same time oral and anal invaginations become
formed '(fig- 173 Q. In front of the oral opening an unpaired
upper lip is developed. The prse-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-cesophageal 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 antennae 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 whicJi tJie 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
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.
At the time when the embryo of Strongylosoma is hatched
(fig. 174 B) nine post-cephalic segments appear to be present.
(After Metschnikoff.)
A. A seventeen days' embryo, already segmented.
B. A just-hatched larva.
Of these segments the second is apparently (from MetschnikofT'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 1 .
Julus, at the time it leaves the chorion, is imperfectly segmented, but is
provided with antennas, mandibles, and maxillae, 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) 2 . 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.
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
1 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.
2 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.
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.
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.
a u . i
(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. antenna.-; an.t. proctodseum.
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 antennae 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
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 maxillae 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 maxillae
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 maxillae of Insects or to the first pair of limbs of Insects?
In favour of the former view is the fact (i) that in embryo Insects the
second pair of maxillae 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 maxillae 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 maxillae, though they
are limb-like and not connected with the head. The subjoined table shews
the probable homologies of the appendages.
so ma at time of birth).
CHILOPODA (Scolopendra
Pre-oral region.
ist Post-oral segment.
2nd ,, ,,
Maxillae i. (Four-lobed
plate in adult, but a simple pair of appendages
in embryo).
Maxillie i.
(Palp and bilobed median
(probably equivalent to
segment bearing 2nd pair
of maxillae in Insects).
ist pair of ambulatory
Limb-like appendages with
basal parts in contact.
4th ,, ,,
(?) Apodous.
Poison claws.
2nd pair of ambulatory
ist pair of ambulatory
3rd ,,
2nd ,,
4th and sth
(rudimentary. )
8th ,, ,,
(the 7th pair is developed
in this segment later).
9 th
loth ,,
,, (last segment in
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 stomodacum
and proctodaeum.
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 stomodaeum and the proctodasum. The Malpighian bodies arise as outgrowths from the blind extremity of the latter.
(397) G. N e wp or t. " On the Organs of Reproduction and Development of the
Myriapoda." Philosophical Transactions, 1841.
(398) E. Metschnikoff. ' ' Embryologie der doppeltflissigen Myriapoden (Chilognatha)." Zeit.f. wiss. Zool., Vol. xxiv. 1874.
(399) ' ' Embryologisches iiber Geophilus." Zeit. f. wiss. ZooL y Vol. xxv.
(400) Anton Stecker. "Die Anlage d. Keimblatter bei den Diplopoden."
Archivf. mik. Anatomie, Bd. xiv. 1877.
The formation of the embryonic layers in Insects has not
been followed out in detail in a large number of types ; but, as
1 The following classification of the Insecta is employed in this chapter,
((i) Collembola.
I. Aptera. | (a) Thysanura .
!(i) Orthoptera genuina (Blatta, Locusta, etc.).
(2) pseudoneuroptera (Termes, Ephemera,
!(i) Hemiptera heteroptera (Cimex, Notonecta, etc.).
(2) ,, homoptera (Aphis, Cicada, etc.).
(3) ,, parasita (Pediculus, etc.).
in so many other instances, some of the most complete histories
we have are due to Kowalevsky (No. 416). The development
VENTRAL SURFACE. (After Kowalevsky.)
The upper end is the anterior, gg. germinal groove; am. amnion.
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.
The segmentation has not been studied, but no doubt belongs
to the centrolecithal type (vide pp. no 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.
!(i) Diptera genuina (Musca, Tipula, etc.).
(2) aphaniptera (Pulex, etc.).
(3) ,, pupipara (Braula, etc.).
v .. ( (i) Neuroptera planipennia (Myrniclcon, etc.)
TOptera. j (a) ^ trichoptera (Phryganea, etc.).
VI. Coleoptera.
VII. Lepidoptera.
(i) Hymenoptera aculeata (Apis, Formica, etc.).
(a) ,, entomophaga (Ichneumon, Platy
gaster, etc).
(3) ,, phytophaga ( Tenthredo, Sirex, etc.).
VIII. Hymenoptera.
EMBRYOS OF HvDROPHiLUS 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
). 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.
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
gg. germinal groove ;
nion ; yk. yolk.
me. mesoblast ; am. am
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 1 . 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
OF HYDROPHILUS PICEUS. (After Kowalevsky.)
A. Section through the posterior part of the
embryo fig. 1 76 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 ;
cp. epiblast.
1 The reverse nomenclature to this is rather inconveniently employed by Metschnikoff.
pc.L procephalic lobe.
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
The general history of the succeeding
stages may be briefly told.
The appendages appear as very small
rudiments at the close of the last stage, but
soon become much more prominent (fig.
1 80 A). They are formed as outgrowths of both layers, and
arise nearly simultaneously. There
are in all eight pairs of appendages.
The anterior or antennae (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.
1 80 A, give rise to the tracheae;
and the nervous system is formed
as two lateral epiblastic thickenings, one on each side of the midventral line. These eventually become split off from the skin ; while
between them there passes in a
median invagination of the skin
PICEUS. (From Gegenbaur, after
Is. labrum ; at. antenna ; md.
(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, Is). A proctodaeum is formed
at the hind end of the body slightly later than the stomodaeum.
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,
hi]. 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 proctodaeum
forms the rectum and Malpighian bodies 1 , and the stomodseum
the oesophagus and proventriculus. The two epiblastic sections
of the alimentary tract are eventually placed in communication
with the mesenteron.
The development of Hydrophilus is a fair type of that of
Insects generally, but it is necessary to follow with somewhat
greater detail the comparative history of the various parts which
have been briefly described for this type.
TJte 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,
1 This has not been shewn in the case of Hydrophilus,
which usually contains nuclei, and in the Poduridae 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.
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
thatadouble 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
B. II. 26
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.
stomodaeum ; an i. proctodaeum.
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, Lino).
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 Libellulidae 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
In the Libellulidas (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 laminae, 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
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 Libellulidae 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).
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
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. vefitral plate; g. edge of ventral plate; am.
amnion ; se- serous envelope.
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.
In Hydrophilus and
apparently in the Phryganidae 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
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
OF CALOPTERYX. (After Brandt.)
The embryo is represented in the egg-shell; B.
and C. shew the inversion of the embryo.
sf. serous envelope ; am. amnion ; ab. abdomen ;
v. anterior end of head ; at. antennae ; md. mandible ;
mx l . maxilla i ; mx*. maxilla 2 ; p 1 ^. three pairs
of legs; oe. oesophagus.
fig. 184 A, doi and in section in fig. 185 A, do. The
do. (After Kowalevsky.)
do. dorsal organ ; at. antennae.
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 1 .
Section through the posterior part of the body of the same age as fig. 184 A.
Section through the embryo of the same age as fig. 184 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 Poduridas 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 Ichneumonidas 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),
1 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
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
(i) 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 Muscidae 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 1 . 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.
The origin of the hypoblast is still in dispute. It will be
remembered (vide pp. 1 14 and 1 16) 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
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
1 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.
many cases by the division of the mesoblastic bands into
separate somites. The most anterior line marks off a prae-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.
The appendages arise as paired pouchlike 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 antennae.
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, Is}. 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 1 .
1 If these structures are equivalent to appendages, they may correspond to one of
the pairs of antennae of Crustacea. From a figure by Fritz Miiller of the larva of
Calotermes (Jenaische Zeit. Vol. XI. pi. n, fig. 12) it would appear that they lie in
front of the true antennae, and would therefore on the above hypothesis correspond to
the first pair of antennae of Crustacea. Biitschli (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.
(After Kowalevsky.)
pc. I. procephalic lobe.
The antennae themselves can hardly be considered to have
the same morphological value as the succeeding appendages.
They are rather equivalent to paired processes of the prae-oral
lobes of the Chaetopoda.
From the first three post-oral segments there grow out the
mandibles and two pairs of maxillae, 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 Biitschli (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 ii 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
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 Biitschli 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
PICEUS. (From Gegenbaur, after
Kowalevsky. )
Is. labrum; at. antenna; tnd.
mandible; nix. maxilla I.; li. maxilla II.; //>"/" feet; a. anus.
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 Biitschli to arise simultaneously, but according
to Kowalevsky the two mouth appendages first appear, then the antennae,
and still later the thoracic appendages. In the Diptera the mouth appendages are first formed, and either simultaneously with these, or slightly later,
the antennae. In the Hemiptera and Libellulidae the thoracic appendages
are the first to be formed, and the second pair of maxillae makes its appearance before the other cephalic appendages.
The history of the changes in the embryonic appendages during the
attainment of the 'adult con- .
dition is beyond the scope
of this treatise, but it may
be noted that the second
pair of maxillae are relatively very large in the
embryo, and not infrequently (Libellula, etc.)
have more resemblance to
the ambulatory than to the
masticatory appendages.
The exact nature of the
wings and their relation to
the other segments is still
very obscure. They appear as dorsal leaf-like appendages on the 2nd and
3rd thoracic segments, and
are in many respects similar to the tracheal gills
of the larvae of Ephemeridae and Phryganidae (fig.
1 88 A), of which they are
supposed by Gegenbaur
and Lubbock to be modifications. The undoubtedly
secondary character of the
closed tracheal system of
larvae with tracheal gills
tells against this view.
Fritz Miiller finds in the
larvae of Calotermes ru
A. Hinder portion of the body of Ephemera
vulgata. a. longitudinal tracheal trunks; b. alimentary canal ; c. tracheal gills.
B. Larva of ^Eschna grandis. a. superior longitudinal tracheal trunks ; b. their anterior end ; c. portion branching on proctodaeum ; 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.
gosus (one of the Termites) that peculiar and similar dorsal appendages are
present on the two anterior of the thoracic segments. They are without
tracheae. The anterior atrophies, and the posterior acquires tracheas 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 Miiller 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 Miiller 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 tracheae.
But although this suggestion of Fritz Miiller 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
The nervous system. The nervous system arises entirely
from the epiblast; but the development of the prae-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 ofif
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-cesophageal ganglion is formed
from the fusion of the ganglia of the two maxillary segments.
The prae-oral portion of the nervous system consists entirely
of the supra-cesophageal ganglion. It is formed, according to
Hatschek, of three parts. Firstly and mainly, of a layer sepa
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
rated 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
412 IN SECT A.
dorsal border of the antennae. 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-cesophageal ganglia remain disconnected on the dorsal
side till quite the close of embryonic life.
The tracheae and salivary glands. The tracheae, as was
first shewn by Butschli (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 1 , in the embryo and the larva
tracheal invaginations may be developed in all the thoracic (and
possibly in the three jaw-bearing segments) and in all the
abdominal segments except the two posterior.
In the embryo of the Lepidoptera, according to Hatschek (No. 414),
there are 14 pairs of stigmata, belonging to the 14 segments of the body
behind the mouth ; but Tichomiroff states that Hatschek is in error in
making this statement for the foremost post-oral segments. The last two
segments are without stigmata. In the larvae 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 Butschli (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 (eg. Gryllotalpa, Dohrn, No. 408) they remain distinct, and each tracheal
stem has a system of branches of its own.
The development of the tracheae 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
tracheae, and perhaps have a similar origin. The salivary glands
1 In Smynthurus, one of the Collembola, there are, according to Lubbock, only
two stigmata, which are placed on the head.
arise as paired epiblastic imaginations, not, as might be
expected, of the Stomodaeum, 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 maxillae 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
papillae 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 maxillae, 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 proctodaeum 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 antennae and upper
The alimentary canal. The alimentary tract of Insects is
formed of three distinct sections (fig. 181) a mesenteron or
middle section (me), a stomodaeum (st) and a proctodaeum (an).
The stomodaeum and proctodaeum 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 stomodaeum 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
proctodaeum, 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 t/te proctodceum, 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
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 1 .
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
stomodaeum and proctodaeum ; 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
1 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.
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 1 .
Their protoplasm then fuses, and their nuclei divide, and they give rise to
the larval ovaries, for which the enclosing cells form the tunics.
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
Ganin derives the generative organs in Platygaster (vide p. 347) from
the hind end of the ventral plate close to the proctodaeum ; while Suckow
states that the generative organs are outgrowths of the proctodicum.
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
1 This point requires further observation.
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 larva.
Certain of the Hymenopterous forms, which deposit their eggs in the
eggs or larvae of other Insects, present very peculiar modifications in their
development. Platygaster, which lays its egg in the larvae 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.
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
B. II. 27
OF PLATYGASTER. (From Lubbock ; after Ganin.)
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 cesophagus. 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 antennae 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 larvae of three species have been described by Ganin, which are represented in fig. 1 9 1 A, B, C. These larvae 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 larvae move about in the tissues of their hosts by means of their
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 proctodaeum (gfi) 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 proctodaeum, 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 supraoesophageal ganglia, and possibly other parts.
Towards the close of the second larval period the muscles (/;) become
segmentally arranged, and give indications of the segmentation which
(From Lubbock ; after Ganin.)
A. B. C. Cyclops larvae of three species of Platygaster.
D. Second larval stage. E. Third larval stage.
mo. mouth ; a. antenna ; kf. hooked feet ; Ifg. lateral feet ; /. branches of tail ;
ul. lower lip ; slkf. oesophagus ; gsae. supra- oesophageal ganglion ; bsm. ventral epiblastic plate ; Im. lateral muscles (the letters also point in D to the salivary glands) ;
gh. proctodseum ; ga. generative organs ; md. mandibles ; ag. ducts of salivary glands ;
sp. (in E) salivary glands ; mis. stomach ; ed. intestine ; ew. rectum ; ao. anus ;
tr. tracheae ; fk. fat body.
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 larvae 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 larvae 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 1 .
In the Aptera the larva differs from the adult only in the
number of facets in the cornea and joints in the antennae.
In most Orthoptera and Hemiptera the larvae 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 Ephemeridae and Libellulidae, however, the
metamorphosis is more complicated, in that the larvae have
provisional tracheal gills which are exuviated before the final
moult. In the Ephemeridae 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 larvae are apodous. In the true flies (Muscidae) they
are without a distinct head and have the jaws replaced by hooks. In the
Tipulidae there is on the other hand a well-developed head with the normal
appendages. The pupae of the Muscidae are quiescent, and are enclosed in
the skin of the larva which shrinks and forms a firm oval case. In the
1 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.
Tipulidae the larval skin is thrown off at the pupa stage, and in some cases
the pupae continue to move about.
The larvae 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 larvae are hexapodous and resemble wingless Insects. But some
herbivorous larvae (e.g. the larva of Melolontha) closely resemble true
caterpillars, and there are also grub-like larvae without feet (Curculio) which
resemble the larvae of Hymenoptera. The pupa is quiescent, but has all
the parts of the future beetle plainly visible. The most interesting larvae
among the Coleoptera are those of Sitaris, one of the Meloidae (Fabre, No.
409). They leave the egg as active hexapodous larvae 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 pupae.
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
larvae. In the Aculeata, many Entomophaga, the Cynipidae, etc., the larvae
are apodous grubs, incapable of going in search of their food ; but in the
Siricidse they are hexapodous forms like caterpillars, which are sometimes
even provided with pro-legs. In some of the Entomophaga the larvae 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 Ephemeridae 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 larvae 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 Tipulidae. This larva, like
that of other Tipulidae, 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, tracheae, 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
The changes which take place in the true flies (Muscidse) 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
tracheae. 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
424 1NSECTA.
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 tracheae, 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 Muscidse 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 Muscidae 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 Distomeaeand 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.
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 larvae
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
FIG. 102.
HALF OF CAMPODEA FRAGILIS. (From Gegenbaur; after Palmen.)
a. antennae ; p. feet ; j> ',
feet; s.
that both palaeontological 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 1 . The characters of the oral
1 Brauer and Lubbock (No. 421) have pointed out the primitive characters of these
forms, especially of Campodea.
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
larvae 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.
The two most obscure points which still remain to be dealt
with in the metamorphosis of Insects are (i) 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
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 1 , in certain Gall Insects
(Cynipidae). 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
1 Petites Nouvelles Entomolog iyues, May, 1878.
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
Coccidae is practically the same as that of the Aphidae. In the
case of Chermes and Phylloxera the parthenogenetic generations
lay their eggs in the normal way.
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 paedogenesis 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 larvae, 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 larvae in the same
way. After several generations the larvae undergo in the
following summer a metamorphosis, and develop into the sexual
Another case of paedogenesis is that of the larvae of Chironomus, which have been shewn by Grimm (No. 413) to lay eggs
which develop exactly in the same way as fertilized eggs into
(401) M. Balbiani. " Observations s. la reproduction d. Phylloxera du Chene."
An. Sc. Nat. Ser. v. Vol. xix. 1874.
(402) E. Bess els. " Studien u. d. Entwicklung d. Sexualdriisen bei den Lepidoptera." Ztit.f. wiss. Zool. Bd. xvii. 1867.
(403) Alex. Brandt. "Beitrage zur Entwicklungsgeschichte d. Libellulida u.
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(406) H. Dewitz. "Bau u. Entwicklung d. Stachels, etc." Zeit.f. wiss. Zool.
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(408) A. Dohrn. "Notizen zur Kenntniss d. Insectenentwicklung." Zeitschrift
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(409) M. Fabre. " L'hypermetamorphose et lesmoeursdes Meloides." An.Sci.
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(411) V. Graber. Die Insecten. MUnchen, 1877.
(412) V. Graber. "Vorlauf. Ergeb. lib. vergl. Embryologie d. Insecten."
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(413) O. v. Grimm. " Ungeschlechtliche Fortpflanzung einer Chironomus Art-u.
deren Entwicklung aus dem unbefruchteten Ei." Mem. Acad. Petersbourg. 1870.
(414) B. Hatschek. " Beitrage zur Entwicklung d. Lepidopteren." Jenaische
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(415) A. K 6 1 1 i k e r. " Observationes de prima insectorum genese, etc. " Ann. Sc.
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(416) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden."
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(417) C. Kraepelin. 4 ' Untersuchungen Ub. d. Bau, Mechanismus u. d. Entwick. des Stachels d. bienartigen Thiere." Zeit.f. wiss. Zool. Vol. xxni. 1873.
(418) C. Kupffer. "Faltenblatt an d. Embryonen d. Gattung Chironomus."
Arch.f. mikr. Anat. Vol. u. 1866.
(419) R. Leuckart. Zur Kenntniss d. Generationswechsels u. d. Parthenogenese
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(420) Lubbock. Origin and Metamorphosis of Insects. 1874.
(421) Lubbock. Monograph on Collembola and Thysanura. Ray Society, 1873.
(422) Melnikow. " Beitrage z. Embryonalentwicklung d. Insecten." Archiv
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(426) A. S. Packard. " Embryological Studies on Diplex, Perithemis, and
the Thysanurous genus Isotoma." Mem. Peabody Acad. Science, I. i. 1871.
(427) Suckow. " Geschlechtsorgane d. Insecten." Ileusinger's Zeitschrift f.
organ. Physik, Bd. n. 1828.
(428) Tichomiroff. " Ueber die Entwicklungsgeschichte des Seidenwiirms."
Zoologischer Anzeiger, n. 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. xin. 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. xin. 1860.
(433) Zaddach. Untersuchungen iib. d. Bau u. d. Entwicklungd. Gliederthiere.
Berlin, 1854.
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.
Scorpionidae. 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 Scorpionidae is Metschnikoff (No. 434).
At the pole of the ovum facing the ovarian tube there is
(After Metschnikoff.)
bl. blastoderm.
1 The classification of the Arachnida adopted in the present work is shewn below.
c Scorpionidse. . . ( Tetrapneumones.
Pedipalpi. IL Aranema - JDipneumones.
I. ArthrOgastra. \ Pseudoscorpionidae.
I Soiifugse. in. Acarina,
^ Phalangidse.
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 1 .
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).
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 FIG. 194. THREE SURFACE VIEWS OF THE
shallow longitudinal fur- (After Metschnikoff.)
A. Before segmentation.
B. After five segments have become formed.
C. After the appendages have begun to be
row (fig. 194 A) which
subsequently becomes
less distinct. It is then
divided by two transverse lines into three parts 2 .
1 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.
* The exact fate of the three original segments is left somewhat obscure by
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 chelicerae, 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
By the time that twelve
segments are definitely formed, the procephalic region is
distinctly bilobed, and in the
median groove extending
along it the stomodaeum has
become formed (fig. 196 A).
The chelicerae (ck) appear as
small rudiments on the first
post-oral segment, and the
ITS MEMBRANES. (After Metschnikoff. )
ch. chelicerae ; pd. pedipalpi ; p^p 4 .
ambulatory appendages ; al>. post-abdomen
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.
B. II. 28
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 chelicerse) 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 chelicerae 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, Le. 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.
PseudoscorpionidaB. The development of Qielifer has been investigated by Metschnikoff (436), and although (except that it is provided
with tracheae 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,/^/), 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
(After Metschnikoff.)
ch. chelicerae ; pd. pedipalpi ; p l />*. ambulatory appendages ; pe. pecten ; st.
stigmata ; ab. post abdomen (tail).
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 suceeding pairs, while at the
same time the chelicerae 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-cesophageal 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 ati] 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
(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.
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.
The segmentation of Chthonius is apparently similar to that of Chelifer
(Stecker, No. 437).
Phalangidae. 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 Agelcna
have been studied by Claparede (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.
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. 1 18 and 1 19. 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
' r .v-ii~-cr^ ^H 1
ooo^ o^*afe
(After Metschnikoff.)
pd. pedipalpi ; ab. abdomen ; an.i. anal invagination ; c/i. chelicerse.
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
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 Claparede, 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.
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
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
yk. yolk divided into large polygonal
cells, in several of which nuclei are shewn.
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 \
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
1 Various views have been put forward by Claparfede and Balbiani about the
position and significance of the primitive cumulus. For a discussion of which vide
self, No. 440.
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 chelicenc
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).
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 chelicerae, 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.
By a continuous elongation the ventral plate comes to form
a nearly complete equatorial ring round the ovum, the procephalic and caudal lobes being only separated by a very narrow
space, the undeveloped dorsal region of the embryo. This is
shewn in longitudinal section in fig. 204. In this condition the
embryo may be spoken of as having a dorsal flexure. By the
time that this stage is reached (fig. 200 C) the full number of
segments and appendages has become established. There are
in all sixteen segments (including the caudal lobe). The first
six of these bear the permanent appendages of the adult ; the
next four are provided with provisional appendages ; while the
last six are without appendages. The further features of this
stage which deserve notice are (i) the appearance of a shallow
depression (st) the rudiment of the stomodaeum between the
hinder part of the two procephalic lobes ; (2) the appearance of
A. Stage when the ventral plate is very imperfectly differentiated, pr.c. primitive
B. Ovum viewed from the side when the ventral plate has become divided into
six segments, ch. segment of chelicerae 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. stomodoeum between the two proe-oral lobes.
Behind the six pairs of permanent appendages are seen four pairs of provisional
D and E. Two views of an embryo at the same stage. D ideally unrolled,
E seen from the side. st. stomodseum ; ch. chelicerse ; on their inner side is seen
the ganglion belonging to them. pd. pedipalpi ; pr.p. provisional appendages.
raised areas on the inner side of the six anterior appendagebearing segments. These are the rudiments of the ventral
ganglia. It deserves to be especially noted that the segment of
the chelicera, like the succeeding segments, is provided with
ganglia ; and that the ganglia of the chelicerae are quite distinct
from the supra-cesophageal ganglia derived from the procephalic
lobes. (3) The pointed form of the caudal lobe. In Pholcus
(Claparede, No. 442) the caudal lobe forms a projecting structure
which, like the caudal lobe of the Scorpion, bends forward so as
to face the ventral surface of the part of the body immediately
in front. In most Spiders such a projecting caudal lobe is not
found. While the embryo still retains its dorsal flexure considerable changes are effected in its general constitution. The
appendages (fig. 200 D and E) become imperfectly jointed, and
grow inwards so as to approach each other in the middle line.
Even in the stage before this, the ventral integument between
the rudiments of the ganglia had become very much thinner,
and had in this way divided the ventral plate into two halves.
At the present stage the two halves of the ventral plate are still
further separated, and there is a wide space on the ventral side
only covered by a delicate layer of epiblast. This is shewn in
surface view (fig. 200 D) and in section in fig. 203 C.
The stomodaeum (j/) is much more conspicuous, and is
bounded in front by a prominent upper lip, and by a less
marked lip behind. The upper lip becomes less conspicuous in
later stages, and is perhaps to be compared with the provisional
upper lip of Chelifer. Each procephalic lobe is now marked by
a deep semicircular groove.
The next period in the development is characterised by the
gradual change in the flexure of the embryo from a dorsal to a
ventral one ; accompanied by the division of the body into an
abdomen and cephalo-thorax, and the gradual assumption of the
adult characters.
The change in the flexure of the embryo is caused by the
elongation of the dorsal region, which has hitherto been hardly
developed. Such an elongation increases the space on the
dorsal surface between the procephalic and caudal regions, and
therefore necessarily separates the caudal and procephalic lobes ;
but, since the ventral plate does not become shortened in the
process, and the embryo cannot straighten itself in the egg-shell,
it necessarily becomes ventrally flexed.
If there were but little food yolk this flexure would naturally
cause the whole embryo to be bent in so as to have the ventral
surface concave. But instead of this the flexure is at first confined to the two bands which form the ventral plate. These
bands, as shewn in fig. 201 A, acquire a true ventral flexure, but
the yolk forms a projection a kind of yolk sack as Barrois
(No. 441) calls it distending the thin integument between the
two ventral bands. This yolk sack is shewn in surface view in
A. Embryo from the side at the stage when there is a large ventral protuberance
of yolk. The angle between the line of insertion of the permanent and provisional
appendages shews the extent of the ventral flexure.
B. Embryo nearly ready to be hatched. The abdomen which has not quite
acquired its permanent form is seen to be pressed against the ventral side of the
prJ. procephalic lobe; pd. pedipalpi ; ch. chelicerae ; c,L caudal lobe; pr.p. provisional appendages.
fig. 20 1 A and in section in fig. 206. At a later period, when
the yolk has become largely absorbed, the true nature of the
ventral flexure becomes quite obvious, since the abdomen of the
young Spider, while still in the egg, is found to be bent over so
as to press against the ventral surface of the thorax (fig. 201 B).
The general character of the changes which take place
during this period in the development is shewn in fig. 201 A and
B representing two stages in it. In the first of these stages
there is no constriction between the future thorax and abdomen.
The four pairs of provisional appendages exhibit no signs of
atrophy ; and the extent of the ventral flexure is shewn by the
angle formed between the line of their insertion and that of the
appendages in front. The yolk has enormously distended the
integument between the two halves of the ventral plate, as is
illustrated by the fact that, at a somewhat earlier stage than
that figured, the limbs cross each other in the median ventral
line, while at this stage they do not nearly meet The limbs
have acquired their full complement of joints, and the pedipalpi
bear a cutting blade on their basal joint.
The dorsal surface between the prominent caudal lobe and
the procephalic lobes forms more than a semicircle. The terga
are fully established, and the boundaries between them, especially
in the abdomen, are indicated by transverse markings. A large
lower lip now bounds the stomodaeum, and the upper lip has
somewhat atrophied. In the later stage (fig. 201 B) the greater
part of the yolk has passed into the abdomen, which is now to
some extent constricted off from the cephalo-thorax. The
appendages of the four anterior abdominal somites have disappeared, and the caudal lobe has become very small. In front
of it are placed two pairs of spinning mammillae. A delicate
cuticle has become established, which is very soon moulted.
Acarina. The development of the Acarina, which has been mainly
investigated by Claparede (No. 446), is chiefly remarkable from the frequent
occurrence of several larval forms following each other after successive
ecdyses. The segmentation (vide p. 116) ends in the formation of a blastoderm of a single layer of cells enclosing a central yolk mass.
A ventral plate is soon formed as a thickening of the blastoderm, in which
an indistinct segmentation becomes early observable. In Myobia, which is
parasitic on the common mouse, the ventral plate becomes divided by five
constrictions into six segments (fig. 202 A), from the five anterior of which
paired appendages very soon grow out (fig. 202 B) The appendages are the
chelicerae (ch} and pedipalpi (pd] and the first three pairs of limbs (p^fi 1 }.
On the dorsal side of the chelicerae a thickened prominence of the ventral
plate appears to correspond to the procephalic lobes of other Arachnida.
The part of the body behind the five primitive appendage-bearing segments
appears to become divided into at least two segments. In other mites the
same appendages are formed as in Myobia, but the preceding segmentation
of the ventral plate is not always very obvious.
In Myobia two moultings take place while the embryo is still within the
primitive egg-shell. The first of these is accompanied by the apparently
total disappearance of the three pediform appendages, and the complete
coalescence of the two gnathiform appendages into a proboscis (fig. 202 C).
The feet next grow out again, and a second ecdysis then takes place. The
embryo becomes thus inclosed within three successive membranes, viz. the
original egg-shell and two cuticular membranes (fig. 202 D). After the
second ecdysis the appendages assume their final form, and the embryo
leaves the egg as an hexapodous larva. The fourth pair of appendages is
(After Claparede.)
J 1 j 4 . post-oral segments ; ch. chelicerae ; pd. pedipalpi ; pr. proboscis formed by
the coalescence of the chelicerse and pedipalpi ; p l , /*, etc. ambulatory appendages.
acquired by a post-embryonic metamorphosis. From the proboscis are
formed the rudimentary palpi of the second pair of appendages, and two
elongated needles representing the chelicerae.
In the cheese mite (Tyroglyphus) the embryo has two ecdyses which are
not accompanied by the peculiar changes observable in Myobia : the
cheliceras and pedipalpi fuse however to form the proboscis. The first
larval form is hexapodous, and the last pair of appendages is formed at a
subsequent ecdysis.
In Atax Bonzi, a form parasitic on Unio, the development and metamorphosis are even more complicated than in Myobia. The first ecdysis
occurs before the formation of the limbs, and shortly after the ventral plate
has become divided into segments. Within the cuticular membrane resulting
from the first ecdysis the anterior five pairs of limbs spring out in the usual
fashion. They undergo considerable differentiation ; the chelicerae and
pedipalpi approaching each other at the anterior extremity of the body, and
the three ambulatory legs becoming segmented and clawed. An oesophagus,
a stomach, and an oesophageal nerve-ring are also formed. When the larva
has attained this stage the original egg-shell is split into two valves and
eventually cast off, but the embryo remains enclosed within the cuticular
membrane shed at the first ecdysis. This cuticular membrane is spoken of
by Claparede as the deutovum. In the deutovum the embryo undergoes
further changes ; the chelicerae and pedipalpi coalesce and form the
proboscis ; a spacious body cavity with blood corpuscles appears ; and the
alimentary canal enclosing the yolk is formed.
The larva now begins to move, the cuticular membrane enclosing it is
ruptured, and the larva becomes free. It does not long remain active, but
soon bores its way into the gills of its host, undergoes a fresh moult, and
becomes quiescent. The cuticular membrane of the moult just effected
swells up by the absorption of water and becomes spherical. Peculiar
changes take place in the tissues, and the limbs become, as in Myobia,
nearly absorbed, remaining however as small knobs. The larva swims
about as a spherical body within its shell. The feet next grow out afresh,
and the posterior pair is added. From the proboscis the palpi (of the
pedipalpi) grow out below. The larva again becomes free, and amongst
other changes the chelicerae grow out from the proboscis. A further ecdysis,
with a period of quiescence, intervenes between this second larval form and
the adult state.
The changes in the appendages which appear common to the Mites
generally are (i) the late development of the fourth pair of appendages, which
results in the constant occurrence of an hexapodous larva ; and (2) the early
fusion of the chelicerae and pedipalpi to form a proboscis in which no trace
of the original appendages can be discerned. In most instances palpi and
stilets of variable form are subsequently developed in connexion with the
proboscis, and, as indicated in the above descriptions, are assumed to correspond with the two original embryonic appendages.
TJie history of tJie germinal layers.
It is a somewhat remarkable fact that each of the groups of
the Arachnida so far studied has a different form of segmentation. The types of Chelifer and the Spiders are simple modifications of the centrolecithal type, while that of Scorpio, though
apparently meroblastic, is probably to be regarded in the same
light (vide p. 120 and p. 434). The early development begins in
the Scorpion and Spiders with the formation of a ventral plate,
and there can be but little doubt that Chelifer is provided
with an homologous structure, though very probably modified,
owing to the small amount of food-yolk and early period of
The history of the layers and their conversion into the organs
has been studied in the case of the Scorpion (Metschnikoff, No.
434), and of the Spiders ; and a close agreement has been found
to obtain between them.
It will be convenient to take the latter group as type, and
simply to call attention to any points in which the two groups
The epiblast. The epiblast, besides giving rise to the skin
(hypodermis and cuticle), also supplies the elements for the
nervous system and organs of sense, and for the respiratory
sacks, the stomodaeum and proctodaeum.
At the period when the mesoblast is definitely established,
the epiblast is formed of a single layer of columnar cells in the
region of the ventral plate, and of a layer of flat cells over other
parts of the yolk.
When about six segments are present the first changes take
place. The epiblast of the ventral plate then becomes somewhat
thinner in the median line than at the two sides (fig. 203 B). In
succeeding stages the contrast between the median and the
lateral parts becomes still more marked, so that the epiblast
becomes finally constituted of two lateral thickened bands, which
meet in front in the procephalic lobes, and behind in the caudal
lobe, and are elsewhere connected by a very thin layer (fig.
203 C). Shortly after the appendages begin to be formed, the
first rudiments of the ventral nerve-cord become established as
epiblastic thickenings on the inner side of each of the lateral
bands. The thickenings of the epiblast of the two sides are
quite independent, as may be seen in fig. 203 C, vn, taken from a
stage somewhat subsequent to their first appearance. They are
developed from before backwards, but either from the first, or in
any case very soon afterwards, cease to form uniform thickenings,
but constitute a linear series of swellings the future ganglia
connected by very short less prominent thickenings of the epiblast (fig. 200 C). The rudiments of the ventral nerve-cord are
for a long time continuous with the epiblast, but shortly after the
establishment of the dorsal surface of the embryo they become
separated from the epiblast and constitute two independent
cords, the histological structure of which is the same as in other
Tracheata (fig. 206, vn\
The ventral cords are at first composed of as many ganglia
as there are segments. The foremost pair, belonging to the
segment of the chelicerae, lie immediately behind the stomodaeum,
and are as independent of each other as the remaining ganglia.
Anteriorly they border on the supra-cesophageal ganglia. When
the yolk sack is formed in connection with the ventral flexure of
the embryo, the two nerve-cords become very widely separated
(fig. 206, vn) in their middle region. At a later period, at the
stage represented in fig. 201 B, they again become approximated
in the ventral line, and delicate commissures are formed uniting
A. Stage when about three segments are formed. The mesoblastic plate is not
divided into two bands.
B. Stage when six segments are present (fig. ?oo B). The mesoblast is now
divided into two bands.
C. Stage represented in fig. 200 D. The ventral cords have begun to be formed
on thickenings of the epiblast, and the limbs are established.
ep. epiblast ; me. mesoblast ; me.s. mesoblastic somite ; 7>n. ventral nerve-cord ;
yk. yolk.
the ganglia of the two sides, but there is no trace at this or any
other period of a median invagination of epiblast between the
two cords, such as Hatschek and other observers have attempted
to establish for various Arthropoda and Chaetopoda. At the
stage represented in fig. 201 A the nerve ganglia are still present
in the abdomen, though only about four ganglia can be distinguished. At a later stage these ganglia fuse into two continuous
cords, united however by commissures corresponding with the
previous ganglia.
The ganglia of the chelicerae have, by the stage represented
in fig. 20 1 B, completely fused with the supra-oesophageal ganglia
and form part of the oesophageal commissure. The cesophageal
commissure is however completed ventrally by the ganglia of
the pedipalpi.
The supra-cesophageal ganglia are formed independently of
the ventral cords as two thickenings of the procephalic lobes (fig.
205). The thickenings of the two lobes are independent, and
each of them becomes early marked out by a semicircular groove
(fig. 200 D) running outwards from the upper lip. Each thickening eventually becomes detached from the superficial epiblast,
but before this takes place the two grooves become deeper,
and on the separation of the ganglia from the epiblast, the
cells lining the grooves become involuted and detached from
the skin, and form an integral part of the supra-oesophageal
At the stage represented in fig. 201 B the supra-oesophageal ganglia
are completely detached from the epiblast, and are constituted of the
following parts : (i) A dorsal section formed of two hemispherical lobes,
mainly formed of the invaginated lining of the semicircular grooves. The
original lumen of the groove is still present on the outer side of these
lobes. (2) Two central masses, one for each ganglion, formed of punctiform tissue, and connected by a transverse commissure. (3) A ventral
anterior lobe. (4) The original ganglia of the chelicerae, which form the
ventral parts of the ganglia 1 .
The later stages in the development of the nervous system have not
been worked out.
The development of the nervous system in the Scorpion is almost
identical with that in Spiders, but Metschnikoff believes, though without
adducing satisfactory evidence, that the median integument between the
two nerve cords assists in forming the ventral nerve cord. Grooves are
present in the supra-cesophageal ganglia similar to those in Spiders.
The mesoblast. The history of the mesoblast, up to the
formation of a ventral plate subjacent to the thickened plate of
epiblast, has been already given. The ventral plate is shewn
in fig. 203 A. It is seen to be formed mainly of small cells,
1 For further details vide self, No. 440.
B. II. 29
but some large cells are shewn in the act of passing into it
from the yolk. During a considerable section of the subsequent development the mesoblast is confined to the ventral
The first important change takes place when about six
somites are established ; the mesoblast then becomes divided