cated by a great increase in thickness of the ectoderm at a 

 point opposite the extreme anterior end of the branchial sac. 

 This thickened area becomes invaginated until the bottom of 

 the pit touches the endodermal wall, and a complete fiision of 

 the two soon takes place: Pigs. 23, a and h, and Fig. 5. The 

 cells in the centre break down, and the cavity of the pharynx 

 is put into conmunication with the outside. As is shown in 

 Pigs. 23, a_ and h, many mesodermal cells attach themselves 

 to the inner surface of the ectodermal depression, become 

 greatly elongated, and are eventually transformed into muscle 

 fibres . 



In Botryllus , according to Pizon (22), it is the 

 branchial wall which thickens and evaginates to fuse with the 

 ectoderm, while the latter plays but a small part in the pro- 

 duction of the orifice. The irocess, as it occurs in P e r o - 

 phora virldis , is quite similar to that described by Kov/alev/- 

 sky (13) for Phallusia , and by Van Beneden and Julin (33) for 

 Clavelina . 



The cloaca 1 orifice is formed in exactly the same 

 manner by the union of an ectodermal invagination v;ith the 

 wall of the cloaca at the anterior end of the latter, cl .o , 

 Fig. 5, and Pig. 20. It will not be necessary to speak here 

 of the various appendages and ciliated grov/ths of the pharynx 

 which arise later, namely, the tentacles, papillae, languets, 

 dorsal lamina, and peripharyngeal bands, as these are merely 

 differentiations of the pharyngeal epithelium and have been 

 sufficiently described by numerous authors. 



