504 ARTHUR ROBINSON AND RICHARD ASSHETON. 



portion of the primitive streak (PSC). The anus has not yet completely 

 perforated the ventral part of the primitive streak. 



Fig, 26. — 4^ mm. Tadpole. A semi-diagraramatic sagittal section through 

 the posterior end of an embryo in which the tail has just begun to grow out. 

 The central nervous system has entirely separated from the skin, and with it 

 that portion of the primitive streak which was folded up with it (P*?"). By 

 this means that portion of the primitive streak (P^S"'') comes to lie now within 

 the embryo. The ventral moiety of the primitive streak has ceased to exist 

 as such, and the portions derived therefrom are shown by cross-shading ; while 

 the dorsal portion, or still functional portion, if it may be so termed, is indicated 

 as in the two immediately preceding figures by diagonal shading. The bracket 

 P S includes the full extent of primitive streak or its derivatives. 



