canal by its side walls growing up, arching over, and coalescing in
the median dorsal line (fig. 13, D). This union of the laminae
dorsales to form the neural canal commences at the posterior end
behind the blastopore and gradually extends forwards. Consequently
the blastopore comes to open into the posterior end of the
neural canal (fig. 13, D), while the anterior end of that cavity remains
open to the exterior. In this way the archenteron communicates
indirectly with the exterior. The short canal leading from the
neural canal to the archenteron is known as the neurenteric canal
(fig. 13, D, nec). Previous to this stage some of the hypoblast cells
at the front edge of the blastopore and forming part of the dorsal
wall of the archenteron (fig. 13, C, ch) have become separated off,
and then arranged to form an elongated band, two cells wide, underlying
the posterior half of the neural canal (fig. 13, D, E, ch). This is
the origin of the notochord. Outgrowths from the sides of the
archenteron give rise to laterally placed masses of cells, which are
the origin of the mesoblast. These masses show no trace of metameric
segmentation. The cavities (reproductive and renal vesicles)
which are formed later in the mesoblast represent the coelom.
Consequently the body cavity of the Tunicata is a modified form
of enterocoele. The anterior part of the embryo, in front of the
notochord, now becomes enlarged to form the trunk, while the
posterior part elongates to form the tail (fig. 13, E). In the trunk
the anterior part of the archenteron dilates to form the mesenteron,
the greater part of which becomes the branchial sac; at the same
time the anterior part of the neural canal enlarges to form the
cerebral vesicle, and the opening to the exterior at the front end of
the canal now closes. In the tail part of the embryo the neural
canal remains as a narrow tube, while the remains of the wall of the
archenteron-the dorsal part of which becomes the notochord—are
converted into lateral muscle bands (fig. 13, G) and a ventral cord
of cells, which eventually breaks up to form blood corpuscles.
As the tail grows longer, it becomes bent round the trunk of the
embryo inside the egg-membrane. About this period the epiblast
cells begin to form the test as a cuticular deposit upon their outer
surface. The test is at first devoid of cells and forms a delicate
gelatinous investment, but it shortly afterwards becomes cellular
by the migration into it of test cells formed by proliferation from
the epiblast.[1]
| ||||||||||||||||||||||||||||||||||
| (After Kowalevsky.) | ||||||||||||||||||||||||||||||||||
| Fig. 13.—Stages in the Embryology of a Simple Ascidian. | ||||||||||||||||||||||||||||||||||
|
A to F, Longitudinal vertical sections of embryos, all placed with the dorsal surface uppermost and the anterior end at the right. A, Early blastula stage, during segmentation. B, Early gastrula stage. C, Stage after gastrula, showing commencement of notochord. D, Later stage, showing formation of notochord and of neural canal. E, Embryo showing body and tail and completely formed neural canal. F, Larva just hatched; end of tail cut off. G, Transverse section of tail of larva. | ||||||||||||||||||||||||||||||||||
|
The embryo is hatched about two or three days after fertilization, in the form of a tadpole-like larva, which swims actively through Larval Stage. the sea by vibrating its long tail. The anterior end of the body is provided with three adhering papillae (fig. 13, F, adp.) in the form of epibiastic thickenings. In the free-swimming tailed larva the nervous system, formed from the walls of the neural canal, becomes considerably differentiated. The anterior part of the cerebral vesicle remains thin-walled (fig. 13, F), and two unpaired sense-organs develop from its wall and project into the cavity. These are a dorsally and posteriorly placed optic organ, provided with retina, pigment layer, lens and cornea, and a ventrally placed auditory organ, consisting of a large spherical partially pigmented otolith, attached by delicate hair-like processes to the summit of a hollow crista acoustica (fig. 13, F, au). The posterior part of the cerebral vesicle thickens to form a solid ganglionic mass traversed by a narrow central canal: this becomes the ganglion of the adult Ascidian. The wall of the neural canal behind the cerebral vesicle becomes differentiated into an anterior thicker region, placed in the posterior part of the trunk and having a superficial layer of nerve fibres, and a posterior narrower part which traverses the tail, lying on the dorsal surface of the notochord, and gives off several pairs of nerves to the muscles of the tail. Just in front of the anterior end of the nervous system a dorsal involution of the epiblast breaks through into the upturned anterior end of the mesenteron and thus forms the mouth opening. Along the ventral edge of the mesenteron, which becomes the branchial sac, the endostyle is formed as a narrow groove with thickened side walls. It probably corresponds to the median portion of the thyroid body of Vertebrata. A curved outgrowth from the posterior end of the mesenteron forms the alimentary canal (oesophagus, stomach and intestine), which at first ends blindly. An anus is formed later by the intestine opening into the left of two lateral epiblastic involutions (the atria), which rapidly become larger and fuse dorsally to form the peribranchial cavity. Outgrowths from the wall of the branchial sac meet these epiblastic involutions and fuse with them to give rise to the first formed pair of stigmata, which thus come to open into the peribranchial cavity; and these alone correspond to the gill clefts of Amphioxus and the Vertebrata.
|
|
Fig. 14.—Sketches of Ascidian Larvae. |
Fig. 14 shows a few characteristic forms of Ascidian “tadpoles,” or free-swimming larvae. A and S are typical simple Ascidians; M is the aberrant tailless form found in some Molgulidae; and C is the larva of a typical compound Ascidian.
After a short free-swimming existence the fully developed tailed larva fixes itself by its anterior adhering papillae to some foreign Metamorphosis to Adult Form. object, and then undergoes a remarkable series of retrogressive changes, which convert it into the, adult Ascidian. The tail atrophies, until nothing is left but some fatty cells in the posterior part of the trunk. The adhering papillae disappear and are replaced functionally by a growth of the test over neighbouring objects. The nervous system with its sense organs atrophies until it is reduced to the single small ganglion, placed on the dorsal edge of the pharynx, and a slight nerve cord running for some distance posteriorly (van Beneden and Julin). Changes in the shape of the body and a further growth and differentiation of the branchial sac, peribranchial cavity and other organs now produce gradually the structure found in the adult Ascidian.
The most important points in connexion with this process of development and metamorphosis are the following: (1) In the
- ↑ Some of the first test cells are also probably derived from the epithelium of the egg follicle.
