54 S. II ATTA. THE FATE OF TUF. PERISTOMAL 



proctodaeum is widened to a bursa, which is subserved as the best 

 criterion for the established prectodaeum and anus. 



Photogram ia. Median sagittal section Photogram iY/. Median sagittal section • 



through an embryo with 12 through an embryo with 17 



mesodermic somites. mesodermic somites. 



n. anus. /'. blastopore. J. dorsal lip of blastopore, m. ventral part of peristomal 

 mesoderm, p. proctodaeum. /. tail-bud. v. ventral lip of blastopore. 



The changes standing in question are observable in an embryo of the 

 fourth day, which is provided with about seventeen somites and can exter- 

 nal!}' be distinguished from younger embryos by the following character- 

 istics (Photogram 1). The dorsal portion of the hind end of the embryo, 

 beyond which, in foregoing stages, the ventral portion protrudes (<•?), shows 

 by this stage a characteristic feature in being produced posteriorly, so as 

 to overhang the ventral portion and the anal opening, looking hindwards 

 (/>). The produced hind extremity of the embryo, ivhich is resulted from 

 the coalescence of the tail lobes, constitutes the commencing postanal section 

 of the body, i.e. the rudiment of the tail. 



