In Pig. 16, line d., trie section passes through tiie anterior 

 portion of the cloacal cavity, cl, which is seen to connect 

 the lateral sacs; the constriction, which will untirnately 

 completely cut off the U-shaped bag from the median vesicle 

 has proceeded in this region to a considerable extent, and 

 has greatly narrowed the opening between the peribranchial 

 and branchial cavities. In a section further back. Fig. 17, 

 line e, the folds which are foniing the peribranchial sacs, 

 are much less deep and v/ider apart; the connection between 

 the left sac and tne stolonic partition is present in this 

 region. Finally, Fig. IS, line _f, represents a section be- 

 yond the peribranchial sacs, the posterior pouches of which 

 at this stage have not yet begun to grow out; the section 

 passes behind the connection with the stolonic partition, but 

 through the storaach, s^t^, which is seen on the left side. 



The further development of the peribranchial cavity 

 from this stage on merely consists in the completion of tne 

 constriction, whereby the 'J-shaped bag is completely cut off 

 from the branchial sac, and in the extension of tne anterior 

 and posterior pouches of the peribranchial sacs, which final- 

 ly surround the whole pharynx laterally. These relations 

 are illustrated by the series of sections. Figs. 19--21, which 

 are taken from the same bud. Fig. 19 is a section through 

 the anterior end of the bud, and shows tne lateral extensions 

 of the peribranchial sacs surrounding the pharynx. The peri- 

 branchial cavity is now entirely cut off, and its lateral 

 portions are united in the middle region of the bud by the 



-23- 



