be pointed out belov>f, inaijitain that the ectodem is concerned 

 in the formation of the nervous system. 



As to the derivation of the inner wall, of t.ie ves- 

 icle, the case is not so simple, for in different Ascilians 

 this layer arises from entirely different parts of the parent, 

 coming in some forms from an endodermal, in o triers from an 

 ectodermal structure. In Perophora and Glavelina it is de- 

 rived from the parti t ion-v;all of the stolon, whicn in the 

 latter, and presumably in the former, is of endodermal origin; 

 in Dider nnurn and Dis taplia from the wall of the gut, and in 

 the Polyclinidae from the endodermal wall of the post-abdomen. 

 In all the above mentioned species, then, th« inner ves- 

 icle of the bud-rudiment is derived from an endodermal struc- 

 ture. In Botryllus , however, this inner vescicle is fonned 

 directly as an evagination of the outer v^all of the peribran- 

 chial space, whose origin in the Ascidian larva has been a 

 question of much dispute. Kowalewsky (11) first showed that 

 in the larva of simple Ascidians the peribranchial cavity is 

 formed from two symmetrical ectodermal invaginations, which 

 later fuse together dorsally and surround the branchial sac. 

 Delia Valle (4), however, contradicted these results, and 

 maintained an endodennal origin for the peribranchial sacs in 

 both simple and compound Ascidians, and Pizon (22) has re- 

 cently described the sacs as arising from two divertecula 

 from the branchial sac in the larva of Botryllus. Kowalev/- 

 sky's account is borne out by all the later investigation of 

 the subject, with the exception of that of Pizon. Seeliger 



