DEVELOPMENT OF ASGIDIANS. 



393 



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 Pyroxn/nti, in which 

 the first formed embryo attains only an imperfect develop- 

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

 zooids." In ClaveUin-a and Peropkora the original parent 

 Aseidian 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 Aseidian as 

 based on Kowalevsky and Kupffer's researches on Phallusia 

 mammillata Cuvier, in which the embryonic stages were ob- 



a 



Fig. 386 4 . Embryo Aseidian. A. a. primitive opening: h. primitive digestive 

 cavity; c, segmentation-cavity or primitive body-cavity; B. i, pharynx; u, nerve- 

 cavity; t, epithelium forming the body-wall; .r, rudimentary notocord; .O. sec- 

 tion of a fish embryo: u, nervous tube, open in front and situated dorsally; 

 ch, notocord; bb, mouth; e, alimentary canal; a, place of vent; m, mesoderm. 



served, and Ascidia intestinalis, whose larva was studied. 



The esg 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 

 imagination of the ectoderm, a true gastrula state resulting. 

 Fig. 386", A (after Kowalevsky), represents the gastrula ; h, 

 the primitive digestive cavity ; , 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 y 

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

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



