The former of these becomes more and more elongated, following the outline of, and fold ing itself around, the body, which is included in the ovum, as in many reptiles.
In Amaroucium, according to Milne-Ed wards' observations, the tail is formed by the marginal portion of the yolk being condensed, at the same time that the body is flattened, and then separated from it throughout its length. Van Beneden, however, remarks, that in the simple Ascidians, as we have stated above, the tail is rather an elongation of one part of the and that, like the exterior organs of all other animals, it is formed by extension and not by division or separation.
The body part of the embryo has an in ternal cavity, and is formed of the yolk; it is surrounded by the membrane, which becomes the skin of the body and tail. The caudal appendage is hollow, and communicates with the central cavity ; it does not in Ascidiw pre sent the spiral or zigzag cavity observed in the Bar yllidw.
There is soon formed within the embryo another layer distinct from the exterior, and which is destined to become the parietes of the intestinal tube. There exist then two layers, internal and external, from which all the organs are derived. In the thickness of the external layer, near the middle of the body, and rather superiorly, occurs a black pigment cell, which Van Beneden regards as a rudi mentary eye. In Anzaroucium, Milne-Edwards has observed one or two blackish oculiform points towards the posterior extremity of the body.
The chorion is now ruptured, and the larval embryo appears with a pellucid membrane covering its body and tail (fig. 781. F). This membrane has been already mentioned as in vesting the ovum at a late stage of the first period of its development, immediately after the vitellus lost its mulberry aspect, and when the albumen was formed. It closely envelopes the caudal appendage, and termi nates beyond it by a tapering prolongation : subsequently it becomes the test of the ani mal. The larva, resembling in appearance a frog-tadpole, still remains a while in the cloacal pouch, but is soon carried out by the current leaving the animal : it is very active, wriggling about for hours. After a
brisk locomotive existence for about twelve hours, the embryo fixes itself to a foreign body. The tail then begins to disappear, being slowly absorbed; its pellucid membrane remains entire for a little time, but is ultimately detached from the body. At the same time other appendages spring from the body, the principal one of which arises from the an terior extremity, elongates itself more and more, attains a trumpet-like shape, and may be considered as the future mouth (fig. 781. G). In Amaroucium, Milne-Edwards ob served generally five appendages arising from the anterior extremity of the body, two of which soon disappear, and the three others, which are persistent for a while, terminate in button-shaped, dilated suckers, touching the parietes of the external integument. In Cynthia ampulla other processes or appendages occur all over the body ; these are somewhat similar to the anterior appendage, but are never constant, either in direction or number; one specimen had seven appendages on one side. Sometimes the numerous appendages resemble tentacles, giving the embryos the appearance of young These processes are hollow, communicating with the central cavity. None of them appear to open externally, or to be at all like suckers; and a considerable space frequently exists between them and the enveloping membrane.
The young animals are attached to solid bodies by this external tunic, on a different side to that at which the orifices make their appearance. " If," says Van Beneden, " the appendages served as suckers for attachment, the animals would have the mouth on the side by which they are attached." It is at this stage of growth that the walls of the internal cavity are distinctly seen through the external membranes. The contents of the digestive tube are seen to consist of a rather opaque mass, distinct from the surrounding clear space.
As soon as the animal is fixed, the trans parent membrane that has remained floating since the absorption of the caudal appendage, is completely detached, and the integument and the test begin to be formed.