E. (Eugen) Korschelt.

Text-book of the embryology of invertebrates (Volume 4) online

. (page 37 of 56)
Online LibraryE. (Eugen) KorscheltText-book of the embryology of invertebrates (Volume 4) → online text (page 37 of 56)
Font size
QR-code for this ebook


development seems to take place within the atrial cavity. As a rule,
the egg is fertilised only after its expulsion, and then surrounds
itself with a homogeneous membrane. This membrane (Fig. 178, m)
which soon rises up from the surface of the egg in such a way
that a space filled with fluid can be seen between it and the egg,



DOLIOLUM CLEAVAGE .



383



has been called the vitelline membrane by ULJANIN, and for a long
time shows, on the external surface, remains of the original follicular
epithelium (/). Since, however, both GKOBBEN and For, (No. 21)
observed within this membrane cells which without doubt correspond
to the test-cells of the Ascidiacea we may perhaps rather regard it
as the equivalent of the chorion of the Ascidian egg.




ch



FIG. 178. Three embryonic stages of Doliolum Miilleri (after ULJANIN). A, gastrula-
stage ; B and O, embryos at later stages of development, ch, chorda ; ec, ectoderm ;
/, follicle-cells ; m, egg-shell (chorion) ; ms, mesoderm ; n, rudiment of the nervous
system.

Cleavage is total and almost equal, and closely resembles that
described above in connection with the solitary Ascidians. The
blastula and invagination-gastrula (Fig. 178 A) observed by ULJANIN
were comparable to the similar stages in the Ascidiacea.

We have very incomplete accounts of the subsequent stages. At
the next stage observed by ULJANIN and depicted in Fig. 178 B,
the embryo is pear-shaped, and within it we can see three distinct
rudiments. A large dorsal cell-mass (n) is regarded by ULJANIN as



384



TUNICATA.



the rudiment of the central nervous system, which here does not take
the form of a tube but of a solid ectodermal growth, vvhile a ventral
cell-mass (ch) is assumed to be the rudiment of the chorda and a
posterior mass the mesoderm (ms). According to ULJANIN, the arch-
enteron is used up in the formation of the chorda and the mesoderm ;
the rudiment of the adult intestine on the contrary owes its origin
to an independent ectodermal invagination which occurs later.

In the next stage (Fig. 178 (7), the embryo appears to be folded
several times within the egg. A dilated anterior region principally



ms'




FIG. 179. Two so-called larval' stages of Doliolum Muileri (after ULJANIN). A,
younger stage ; B, older stage, ch, chorda ; d, atrial cavity ; eb, ectodermal vesicle ;
h, heart ; m, egg-shell ; ms', anterior mesoderm-rudiment ; ms", posterior mesoderm-
mdiment ; n, nervous system ; p, pharynx ; x, mesodermal rudiment of the stolon ;
y, mesoderm-masses.

occupied by the large ganglionic rudiment (n) can be distinguished
from a caudal region bent back upon itself, in which the chorda (ch}
is seen to be already developed. Two lateral mesoderm-bands (ms)
run along the whole length of these two regions of the body. Similar
stages were also observed by FOL (No. 21).

In the stages which follow (Fig. 179 A), the embryo straightens
itself out within the egg-shell and is now able to raise itself from
the bottom of the sea on which the egg rests and to swim about



DOLIOLUM LARVAL DEVELOPMENT.



385



by means of its long caudal region, and is therefore usually called
a him i although it is still enveloped in the much-distended egg-shell
(t/t) in which traces of the follicle-cells can be found. We do not
know for certain when this egg-shell is cast off. During these pelagic
ontogenetic stages in which the Doliolum resembles the Ascidian
larva, the body is elongate (Fig. 179 A) and the middle of it is
occupied by a vesicular ectodermal swelling (eb) caused by the
accumulation of a clear fluid. This vesicle divides the body into a
posterior and an anterior region. The anterior develops into the
young Dnl lot-urn (the first asexual or "nurse" form, the blaatoeooid)
while the ccrodermal vesicle and the caudal region must be regarded

TL




KK;. 180. Young " mirst; " t'onn <>f l)li<,/n m l-'Ji renberyii, with remains of the larval
tail (after ULJANIN). ch, chorda ; rf, so-called dorsal stolon ; e, endostyle ; h, heart
and pericardium ; ///, egg-shell ; /?, nervous system ; r, rosette-shaped organ (rudi-
ment of the ventral stolon).

as provisional larval organs and degenerate later (Figs. 179 B, and
180). The structure of the caudal region corresponds to that of
the same region in the Ascidian larva. It consists of a chorda and,
laterally, of muscle-plates derived from the mesoderm-bands. . At the
anterior end of the caudal region, a part of the mass of mesoderm-
cells (Fig. 1 79, mx") is not transformed into spindle-shaped muscle-
fibres. Two cell-masses (y) are subsequently given off from this into
the ectodermal vesicle, where they break up and change into blood-
corpuscles.

The anterior region of the body contains the very large rudiment
of rhe central nervous system (Fig. 179 A, n) and the anterior

cc



386



TUNICATA.



portion of the lateral mesoderm-bands (ins) which also give off into
the ectodermal vesicle from their posterior ends elements that change
into blood-corpuscles. An ectodermal invagination can also be seen
forming ventrally (p) and from this is derived the whole intestine
of the " nurse " form. This invagination first gives rise by its dilata-
tion to the pharyngeal cavity (Fig. 179 B, p), while the intestine
proper is derived from a solid cone of cells which develops at the
base of the invagination. This cone very soon develops a lumen at
first closed at both ends, and this becomes differentiated into the
oesophagus, the stomach and the intestine, the rudiment of the
-digestive gland also becoming visible. The rudiment of the intestine
opens only later into the atrial cavity (cl). The latter develops later
than the pharyngeal cavity from an independent dorsal ectodermal
invagination (Fig. 181, cl) which, as it enlarges, comes into close
contact with the posterior wall of the pharynx. In this way the
transverse and somewhat diagonally-placed branchial lamella arises,
in which the four pairs of gill-clefts found in this generation (Fig.
245, p. 475) soon appear in the form of small round perforations.
According to ULJANIN, the two pairs that lie dorsally develop before
those that lie ventrally.




JTl'



FIG. 181. Dorsal region of an older larva of Doliolum M tiller i (after ULJAMN). d,
atrium ; fl, ciliated pit ; m, muscle-hoops ; n, ganglion ; nb, branchial nerve.

Only the middle part of the rudiment of the central nervous system
(Fig. 181, n) retains its original massive character, while the anterior
and posterior ends soon become narrower In the anterior narrowed
portion, an irregular cavity develops and breaks through into the
pharynx. At this point, the ciliated pit (fi) appears and a delicate
canal connects it with the sub-ganglionic body (the homologue of
the " glande hypophysaire" of the Ascidians). From the middle
swelling of the neural rudiment, the actual ganglion and the sub-
ganglionic body develop, while the posterior narrowed portion gives
rise to an unpaired nerve which runs backwards (rwvus branchialis, nl,



DOLIOLUM LARVAL DEVELOPMENT.



387



ULJANIN) in which we perhaps have the homologue of the ganglionic
ci-11 -strand discovered by VAN BENEDEN and JULIN in the Ascidians
(p. 370). The peripheral nerves and the sensory organs develop
later, and among these the vesicular auditory organ which belongs
to the left side of the body deserves special mention (Fig. 245 A, ot) ;
the vesicle itself arises as an ectodermal invagination into which a
cell wanders and develops into the otolith. According to ULJANIN,
tlu- auditory orpfan of Doliolum Mulleri remains throughout life a
mere cup-shaped ectodermal invagination.



ec







ms 1



KK-. ivj. Transverse section through two ontogeuetic stages of Doliolum (after
I I..IAMN). .1. section through anterior region of the body at a stage somewhat
holder tan that depicted iu Fig. 179 A ; /?, section through an older stage, d,
paired phuryngeal outgrowth, which takes part in the formation of the ventral
stolon ; ff, ectoderm; en, entoderm ; h, rudiments of the heart and pericardium;
///>. nifsodenii ; //<*'. int'soderin of the ventral stolon; //, rudiment of the nervous
system ; />. pharynx.

The mesoderm of the anterior region of the body gives rise
principally to the muscle-hoops (Fig. 181, ni), the pericardial rudi-
ment (Fig. 182 B, h) and the mesoderm-mass (ms) of the ventral
proliferating stolon of the "nurse" stage (the rosette-shaped organ
of KEFERSTEIN and EHLEKS). Two cell-groups become separated
posteriorly and ventrally from the mesoderm-layer which envelops
the pharyngeal cavity like a mantle ; one of these, in close contact to
the ectoderm, becomes the mesoderm of the ventral stolon (ms),
while the other, near the rudiment of the alimentary canal, changes
into the pericardial vesicle (h), a cavity appearing within it round
which the cells become arranged into an epithelium. As in other
Tunicates, the heart is derived from a dorsal invagination of this



388 TUNICATA.

vesicle.* It should here be mentioned that the dorsal closure of the
cardiac tube is brought about by a histologically differentiated lamella
(the "mittelfeld" of GROBBEN) as to the development of which, how-
ever, we have no detailed information, but we are reminded of the
participation of the epicardial lamella in the formation of the heart-
in the Ascidians (VAN BENEDEN and JULIN, p. 370).

The muscle-hoops develop in the way described by LEUCKART for
Salpa democratica (see p. 431), through the fenestration of the meso-
dermal lamella, these perforations separating one muscle-hoop from
another.

In this way, the general course of the most important systems of
organs occurring in the first " nurse " generation is indicated (Figs.
180, 243, 245, p. 475). To these, two stolons connected with the
formation of buds have to be added. One of these (Figs. 180, r, and
245 A, rs) lies behind the fifth muscle-hoop, close to the posterior
end of the pericardial vesicle ; this we shall call the ventral stolon
(the rosette-shaped organ). The second or dorsal stolon (Figs. 180, d,
245, ds) j- arises from the dorsal surface in the seventh intermuscular
space and forms a geniculate process pointed posteriorly, into the
base of which an open coil of the seventh muscle-hoop extends (Fig.
243, st'). We shall have to enter into the details of the structure
and development of these stolons and of their relation to the forma-
tion of the subsequent generations later (p. 470).

After the young barrel-shaped "nurse" has developed fully, the
provisional larval organs gradually atrophy. While the internal
parts undergo fatty degeneration and the cells become mixed with
the blood, the ectodermal envelope gradually shortens so that the
ectodermal vesicle and the larval tail soon form merely a rounded
prominence on the body of the " nurse ". This outgrowth strikingly
resembles an embryonic organ consisting of reserve ntitrition found
in the Thaliacea, the so-called elaeoblast, a fact which makes the
derivation of the latter from the transformed tail of the Ascidian
larva, attempted by SALENSKY, appear somewhat probable (see below
p. 432)

The first " nurse " generation of Doliolum, at a later period, as
FOL first pointed out, undergoes a remarkable metamorphosis, the

* GBOBBEN'S statements as to the formation of the heart in the larval
Doliolum have been misunderstood and misrepresented by ULJA^IN.

t [This is better termed the dorsal outgrowth, as it does not itself give rise
to buds, but receives those structures in rudiment from the ventral stolon and
only gives attachment to them (pp. 472-476). ED.]



1'YUOSOMA EMBRYONIC DEVELOPMENT. 389

u'ills. the endostyle and the whole of the alimentary canal degenerat-
ing completely, while the muscle-hoops considerably increase in size,
and the nervous system develops correspondingly. The "nurse"
then, like a swimming bell of a Siphonophoran stock, carries out the
locomotory function, while the nutritive and respiratory functions
of the whole stock are fulfilled by certain laterally- placed buds (tropho-
:;/y/>/V/x) on the dorsal outgrowth.

4. Pyrosoma.

The development of Pyr<>x<tnia from the egg resembles in many
respects that of the Thaliacea. Embryonic development takes place,
as in them, within the body of the mother and is consequently direct
or abbreviated. It even takes place, as at first in the Thaliacea,
within the egg-follicle. Pyrosoma is, however, specially distinguished :
(1) by the large amount of food-yolk in the egg, which leads to a
di.M-oidal cleavage and the development of a germ-disc and (2) by
the early asexual multiplication of the embryo. The primary indi-
vidual which develops from the embryo and which has been called
the Cyathozooid by HUXLEY, at an early embryonic stage, gives rise
by a kind of transverse fission to four more individuals, the first
Asrf(Jto.io(}i(.k of the colony (Fig. 193, etc).

\Ve owe our knowledge of the embryonic development of Pyrosama
chietiv to HUXLEY (No. 72), KowALEVSKY (No. 71), and SALENSKY

-N.74).



A. Cleavage and Formation of the Germ-Layers.

( July a single egg matures in the genital rudiment of the Ascidio-
zooid which has arisen through budding, as also is the case in the
Thaliacea. Part of the remaining cell-material of the so-called genital
strand becomes arranged round the egg as the follicle, while another
part is used up in forming the rudiment of the testes and of the
oviduct which appears as an outgrowth of the follicle. The egg
iiT<>ws oreatly by the addition of food-yolk, so that finally the forma-
tive volk and the germ-vesicle within it form a mere prominence
upon the large yolk-sphere (Fig. 183 A). After the oviduct has
become connected with the atrial cavity, spermatozoa pass into it
ami remain in it until the egg is ready for fertilisation, while the
oviduct pa'rtly degenerates. At the same time, an active immigra-
tion of follicle-cells takes place into the space extending between
the Mirface nf the euu and the follicular epithelium (Fig. 183 A, fz).



390



TUNICATA.



These cells, which have been called by KOWALEVSKY inner foHicular
cells and by SALENSKY kalymmocytes, and as to the derivation of
which from the follicle-cells there can be no doubt, are homologous
with the test-cells of the Ascidians and the inner follicle-cells of the
Thaliacea (SALENSKY'S gonoblasts). According to SALENSKY, they
take a certain part in the formation of the embryo here as in the
Thaliacea. The statements on this subject, however, appear to us
somewhat inconclusive.

An epithelial lamella further becomes separated from that part
of the inner surface of the follicle which lies next the oviduct (Fig.
184, ds) ; this covers the germ-disc like a cap and represents a
secondary germ-envelope that takes no further part in the develop-
ment of the embryo. This has been called by SALENSKY the cover-
ing layer.





FIG. 183. A, lateral aspect of the egg of Pyrosoma, showing the first cleavage ; B t
the germ-disc of Pyrosoma at the six-celled stage, viewed from above (after KOWA-
LEVSKY). fz, inner follicle-cells.

The cleavage of the egg of Pyrosoma, first made known through
the investigations of KOWALEVSKY, is discoidal and recalls that of
the Teleostei. The first stages seem to have a fairly regular course,
the germ becoming divided into two, four and eight blastomeres by
the successive appearance of meridional furrows (Fig. 36). The stage
of three blastomeres observed by SALENSKY and that of six found by
KOWALEVSKY (Fig. 183 B) must be regarded as accidental irregu-
larities. We have no further details as to the course of cleavage,
but its result is a so-called morula-stage (Fig. 1 84 B) in which the
germ-prominence is composed of blastomeres apparently irregularly
arranged and already forming several layers.

The numerous inner follicle-cells (kalymmocytes) wander by means of
amoeboid movements into the spaces between the blastomeres (Fig. 184 A, fz)
and are even able to penetrate the cell-substance of the latter. This seems to



PTBO8OMA CLEAVAGE OF THE EGG.



391



be the case, however, only in the first stages of cleavage and to have no further
significance, since the follicle-cells, as it appears, do not remain inside the
blastorneres. Many inner follicle-cells, as cleavage advances are, however,
found scattered between the blastomeres (Fig. 184 B, fz) and, as the distinc-
ion of size between the two kinds of cells disappears when the blastomeres
divide further, and the original histological character of the inner follicle-cells
( an also no longer be recognised, it is finally impossible to distinguish thefollicle-
cells from the actual germ-cells or blastomeres. For this reason, and because




Ku;. 184. Sections of two germ-discs of 1'i/rosoma (diagrammatic after SALENSKV).
A, eight-celled stage ; /*, older stage, b, blastomeres ; do, food-yolk ; ds, covering
layer ; '/-. yolk-cells ; f, egg-follicle ; fz, immigrated follicle-cells ; ov, oviduct.

SALKNSKV \vas unable to find follicle-cells showing the commencement of dis-
integration, this author concluded that the inner follicle-cells take part in the
formation of the embryo, a view resembling that held by him in connection
with the Thaliacea (p. 420). We consider it to be more probable that the
embryo here, as in the Thaliacea, is produced solely by the blastomeres, and
that the follicle- cells which wander in between the blastomeres undergo final
disintegration. [See footnotes, pp. 420 and 421].

'



392



TUNICATA.



Mention must now be made of cells which, in the later stages of
cleavage, are found in large numbers in the yolk, near the point at
which the germ-disc lies on it, and which may be called yolk-cells
(Fig. 184, dz). SALENSKY, who traced back these cells to follicle-
cells that had immigrated into the yolk, has named them the yolk-
kalymmocyteK. Since, however, as we shall see below (p. 395), these
yolk-cells take part in the formation of the intestinal wall, we are
inclined to regard them as blastomeres belonging to the entoderm-part
of the germ-mass. We here have a repetition of the conditions found
in the meroblastic egg of the Vertebrata, in which also yolk-cells (to
be considered as entoderm) are said to take a similar part in the
formation of the intestinal gland-cells.

For information as to the formation of the germ-layers in Pyro*<.nu,n
we are dependent entirely on SALENSKY'S statements. The embryo,

after a number of cell-
divisions, lies on the yolk
as a prominence composed
of uniform polygonal cells
which are irregularly distri-
buted. This prominence
soon becomes bilaterally
symmetrical, the largest
mass of cells collecting in
the posterior half of the
germ-disc, so that the pos-
terior slope is more abrupt
than the anterior (Fig. 185).
According to SALENSKY, the
separation of the germ-layers

takes place through delamination, the most superficial cell-layer (ec)
first becoming arranged into an epithelium (ectoderm) ; the mass that
remains (the meso- entoderm) then undergoes a similar transformation,
the lowest layer, that in contact with the yolk, becoming separated
as the intestinal epithelium (entoderm). Between the ectoderm and
the entoderm the mesoderm extends, being greatly developed in the
posterior half of the germ-disc while, in the anterior half, it is want-
ing or else is represented merely by a few cells (Fig. 186 A and ).

Taking into account the process of formation of the germ-layers in the
meroblastic eggs of the Vertebrata, we may perhaps be allowed to conjecture
that in Pyrosoma also the separation of the germ-layers is not an actual
delamination, but an invagination or infolding of the posterior edge of the
germ-disc, such as, for instance, occurs in the Selachians.




FIG. 135. Median section through a germ-disc
of Pyrosoma (after SALKNSKY). ch, cavities
of the chorda ; do, food-yolk ; dz, yolk-cells ;
ec, ectoderm ; mn, meso-entoderm ; h, pos-
terior ; v', anterior.



PYKOSOMA DEVELOPMENT OF THE CYATHOZOOID.



393



At the time when the separation of the germ-layers takes place
systems of cavities appear in the mesoderm (SALENSKY) these
being connected with imaginations on the lower (entodermal) surface
of the germ-disc. One of these imaginations is rather large and lies
near the posterior edge of the germ-disc (Fig. 185, ch). It is con-
nected with a system of cavities running forward in the median line
of the disc. The two other (paired) imaginations (Fig. 186 A, c)








Fi<;. 1st). Two tnuisvrrsc sections through a young germ-disc of /'///-uxo/tni (after
SAI.ENSKY). . 1 , through the posterior, and B, through the anterior region, c, coelom ;
t'./., cavity of the chorda ; ec, ectoderm ; en, entoderm ; ms, mesoderm ; n, nervous

system.

lie laterally and somewhat in front of the first and probably com-
municate with the lateral system of cavities.* These are regarded
by SALENSKY as the rudiments of the coelomic sacs, and the axial
system of cavities as the equivalent of the chorda. SALENSKY was
unable to decide whether there are here a number of separate spaces
serially arranged or a continuous, but somewhat bent longitudinal
canal.

B. Development of the Uyathozooid.

The next changes to be noticed in the germ-disc are the appearance
of the rudiment of the nervous system of the Cyathozooid and the
development of the peribranchial sacs. The nervous system arises as
an ectodermal thickening in the anterior part of the germ-disc (Fig.
1 s 7, //), which later becomes depressed as a furrow, and in this way

" [SALENSKY figures the paired coelomic imaginations at an earlier stage
than that shown in Fig. 186 A, and further he regards the depression seen on
the right of the swelling containing ch as the coelomic invaginatiou, and in
his tigure, of which the above is a copy, letters it as such. FA).]



394



TUNIC ATA.



orms the vesicular rudiment of the ganglion. In cross-sections this
anterior part of the germ-disc is seen to be bilaminar (Fig. 186 B} r
as the mesoderm of the germ-disc does not extend so far forward.



A




B






FIG. 187.- Two germ-discs of Pyrosoma (after KOWALEVSKY). n, rudiment of the
nervous system ; o, aperture of one of the peribranchial tubes ; p, peribranchial
cavity (tube).

The two peribranchial sacs or tubes appear as ectodermal invagina-
tions (Fig. 187 A, p) directed from before backward, which soon
lengthen (Fig. 187 B) and show, at the anterior end, the original

aperture of invagination (o).
The two anterior ends with
their apertures unite in
front of the rudiment of
the nervous system (n)
(KOWALEVSKY), and thus
give rise to the atrial
aperture (Fig. 189, d) of
the Cyathozooid. Accord-
ing to SALENSKY, on the
contrary, the latter is pro-
duced by an unpaired ecto-
dermal invagination with
which the anterior ends of
the peribranchial tubes
come into contact, the
original apertures of these
tubes having closed before
the formation of the cloaca




FIG. 188. Transverse sections through the germ-
disc of Pyrosoma, at the stage depicted in
Fig. 187 .-I* (after SALENSKY). A, through the
anterior part of the disc, with the rudiment of
the nervous system ; jB, through the middle part
with the peribranchial sacs, dh, enteric cavity ;
dz, yolk-cells ; ec, ectoderm ; en, entoderrn ; n,
rudiment of the nervous system; p, invagina-
tions of the peribranchial sacs.



(SALENSKY).



PYROSOMA DEVELOPMENT OF THE CYATHOZOOID. 395

.In the meantime the germ-disc has separated somewhat from the
surface of the food-yolk (Fur. 188). The cavity thus formed is the enteric
cavity, which originally appears covered by the entodenn (en) only on
its upper surface. At a later staut- the entoderm covers the whole of






FIG. 189. (Jerm-disc of /'i/roxo/tui, with the atrial orifice developed (after KOWA-
LEVSKY). d, cloaca ; >'n, endostyle ; n, nervous system ; p, peribrauchial tubes ;
l><\ pericardial sac ; /><', the posterior tubular continuation of the same.

the cavity, its lateral edges bending downward and growing towards
one another (KOWALEVSKY). According to SALENSKY, the yolk -cells
also take part in this ventral closure of the enteric rudiment (Fig.
188, dz) by coming to the surface of the food-yolk and changing into




Ku;. 190. - Transverse section through the posterior region of a germ-disc at the stage
depicted in Fig. 18P (alter SALKNSKY). dh, enteric cavity ; ec, ectoderm ; en, ento-
derm ; , rudiment of the endostyle : ///*. mesoderm ; pb, peribranchial tubes ; //'.
pericardial tube.

epithelial cells of the entoderm (p. 392). While the enteric rudiment



Online LibraryE. (Eugen) KorscheltText-book of the embryology of invertebrates (Volume 4) → online text (page 37 of 56)