of tne stolonic partition. 



2. At an early stage the right side of the inner 

 vesicle is found to be much thicker t/ian the remaining por- 

 tion, and by a peculiar process of rotatory growth or dis- 

 placement of the vpsicle, is carried dovm to the ventral side 

 of the bud, where it forms the floor of the pharynx. This 

 process seems to be due to tne growth and flattening out of 

 the cells composing the whole wall of the vesicle except in 

 the thickened region. I am at a loss to explain why the dis- 

 placement of the vesicle should occur, and to discover the 

 phylogenetic significance of it, if it have any. 



3. The peribranchial sacs arise asymaetrically . 

 As tne displacement proceeds, the wall of the inner vesicle 

 is folded in, at the point where the right side of the vesi- 

 cle joins the stolonic partition, to form the le_ft peilV°^?.5." 

 chial sac. The connection of the latter with the partition 

 of the stolon is retained until a much later stage. A lon- 

 gitudinal furrow appearing high up on the right side of the 

 inner vesicle, separates off tne right peribranchial sac, and 

 is gradually borne ventrally, as the shifting of the vesicle 

 continues. The constriction of the whole peribranchial cav- 

 ity eventually takes place in such a way as to cut off from 

 tne peribranchial sac a :)-shaped bag, composed of the median 

 dorsal connecting piece or cloaca, and the two lateral por- 

 tions of the cavity. Anterior and posterior extensions of. 

 the latter gro\7 out and surround respectively the anterior 

 and posterior ends of the pharynx. The posterior prolonga- 



