66 COMPARATIVE ANATOMY 



"anus" develops usually by a similar process. The blastopore rarely 

 persists as a definitive posterior aperture although it does so in cyclostomes 

 and possibly in some urodele amphibians. Otherwise, exactly as in 

 Amphioxus, it becomes roofed over by the neural folds and thus converted 

 temporarily into a neurenteric canal (Fig. 6i) connecting the hind ends 

 of neural tube and enteric cavity. In reptiles and birds the formation of 

 an actual neurenteric passage is more or less obscured by the presence 



NT NC EN 



Pig. 6i.- — Frog: median longitudinal sections of embryos; A, just before conversion 

 of blastopore into neurenteric canal; B, just after formation of neurenteric canal and 

 perforation of proctodeum to form cloacal aperture. B, brain; BP, blastopore; C, 

 cloacal aperture; EC, ectoderm; EN, endoderm; H, hypophysis; HT, heart; MES, 

 mesoderm; NC, notochord; NE, neurenteric canal; NT, neural tube; O, region where 

 mouth will perforate; P, proctodeum; PH, pharynx; R, rectal region of enteron; Y, 

 yolk cells of endoderm; i, fore-brain; 2, mid-brain; 3, hind-brain. A, X24; B, X19. 

 (Redrawn from Marshall, Vertebrate Embryology.) 



of the massive yolk but definite traces of the canal may be recognized. 

 An ectodermal pit, the proctodeum, situated just below the neurenteric 

 canal, perforates into the hind end of the enteric cavity to form the 

 definitive hind aperture, either anal or cloacal (Fig. 61). As result of 

 the mode of development of the enteric apertures, the lining of more or 

 less of the mouth cavity is derived from stomodeal ectoderm and that of 

 the posterior region from proctodeal ectoderm. The remaining and 

 by far greater part of the adult enteric tube is lined by endoderm which 



