chial cavity from the inner vesicle of the bud-rudiment, but 

 the manner in which it arises is not the same in all species 



of Ascidians. 



e 



In Perophora , Didennum, and Ama ro uiu m, according 



to Kowalewsky (12 and 13) two parallel longitudinal furrows 

 appear on the outside of the inner vesicle, and by gradually 

 deepening finally divide the latter completely into three 

 portions. The two lateral divisions which are thus cut off, 

 grow up over the middle one, and fuse to form the median por- 

 tion of the peribranchial cavity, which now surrounds the 

 branchial sac dorsally and laterally. 



Seeliger (29) has described a different method of 

 formation of the peribranchial cavity in Glavelina . Accor- 

 ding to him the inner vesicle is not divided into three por- 

 tions, but into two, one of which, the posterior, gives rise 

 to the branchial sac and the gut, while the other forms the 

 whole peribranchial cavity. These results were contradicted 

 by Van Beneden and Julin (33), who maintained that in Glave - 

 lina , the process is the same as that described by Kowalev/sky- 



As Seeliger's view was not founded on an investigation of 

 an unterrupted series of stages, and as there were wide gaps 

 in his observations at periods which are especially concerned 

 in the formation of the peribranchial cavities, the supposi- 

 tion that his results are wrong is very probable. In Dis - 

 taplia Salensky (27) has shown, and his results have been 

 confiiined by Hjort and Prl . Bonnevie (10), that the inner 

 vesicle gives rise to two lateral evaginations , which become 

 completely constricted off as separate vesicles, the peri- 



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