138 Messrs. W. K. Brooks and G. Lefevre on 



Botryllus in so far as tliey show that the peribranchial sacs 

 do not arise separately as closed vesicles, which later unite 

 dorsally to form the cloacal cavity, indicate that the process 

 is not so simple as that which occurs in Botryllus. 



If a transverse section of a young bud be examined at about 

 the time when the elongation spoken of above is just beginning, 

 it will be found that the wall of the inner vesicle is no longer 

 of uniform thickness, but that it is much thicker on the right 

 side than elsewhere. This is the first indication of a marked 

 change which now takes place in the internal relations of the 

 bud. 



By a peculiar process, which may be described as a trans- 

 verse or rotatory growth, affecting the inner vesicle, the 

 thickened right wall is carried or pushed down until it comes 

 to lie on the ventral side, that is, the side next the stolon. 

 The connexion with the stolonic partition remains stationary, 

 and the rotatory shifting of the vesicle takes place on this as 

 a fixed support. The actual right side of the vesicle at the 

 beginning is therefore morphologically the ventral side, as it 

 ultimately forms the floor of the pharynx, and several organs 

 which have a definitive ventral position make their appear- 

 ance first high up on the right side. 



As the displacement of the inner vesicle proceeds the differ- 

 ence in thickness between what w^as at first nearly the whole 

 right side and the rest of the vesicle becomes more marked. 

 The cells composing the entire vesicle, except in the thicker 

 region, which remains uniform, are seen to be growing more 

 and more flattened. It would therefore seem most probable 

 that this shifting or displacement is brought about by the 

 rapid growth and drawing-out of the cells which make up the 

 greater portion of the wall of the vesicle. 



Very soon after this process begins the wall of the vesicle 

 on the ventral side is folded in in such a way as to divide oflf 

 a part of the vesicle on the left side, which is united to the 

 stolonic partition by the original connexion ; this is the left 

 'peribrancliial sac. The fold begins somewhat in front of the 

 middle of the vesicle, and, deepening rapidly in this region, 

 gradually extends posteriorly. 



The first indication of the right peribranchial sac is a slight 

 longitudinal folding-in of the wall of the inner vesicle some 

 distance up on the right side, and appears after the shifting 

 of the vesicle has begun. As in the case of the left fold, the 

 right one also begins near the middle of the bud, and as it 

 deepens and extends towards the posterior end it is carried 

 down ventrally with the displacement of the vesicle. 



As the shifting continues the inner vesicle tends more and 



