DEVELOPMENT OF ASCIDIANS. 



203 



Huxley, merely as a kind of stalk, from which new zooids 

 bud out, and this process, in his opinion, "leads to the still 

 more singular process of development in Pyrosoma, in which 

 the first formed embryo attains only an imperfect develop- 

 ment, and disappears after having given rise to four ascidio- 

 zooids." In ClaveUina and PeropJiora the original parent 

 Ascidian throws off branches or stolons from which develop 

 new individuals. 



The usual mode of development in the simple and com- 

 pound Ascidians (forming the order Ascidiacea) is by fertil- 

 ized eggs. We will give the life-history of an Ascidiau as 

 based on Kowalevsky and Kupffer s researches on Phallus ia 

 mammillata Cuvier, in which the embryonic stages were ob- 



A B 



Fig. 138. Embryo A?cidian. A, a, primitive opening ; h, primitive digestive cav- 

 ity ; c, fegmentation-cavity or primitive body-cavity ; B, i, pharynx ; a, nerve-cavity; 

 t, epithelium forming the body-wall ; a;, rudimentary notochord. After Kowalevsky. 



served, and Ascidia intestinalis, in which the larval and 

 subsequent stages were studied. 



The egg consists of a yolk unprotected by a yolk-skin, but 

 surrounded by a layer of jelly containing yellow cells. The 

 yolk undergoes total segmentation. The next step is the 

 invagination of the ectoderm, a true gastrula state resulting. 

 Fig. 138, A (after Kowalevsky), represents the gastrula ; h, 

 the primitive digestive cavity ; a, the primitive opening, 

 which soon closes ; and c, the segmentation-cavity or primi- 

 tive body-cavity. After this primitive opening (a) is lost to 

 view, sometime before the embryo has reached the stage B, 

 another cavity (n) appears with an external opening. This 

 cavity is formed by a union of two ridges which grow out 



