120 OUTLINES OF CHORDATE DEVELOPMENT 



doderm into the wall of the gut. This leads to the formation 

 of a postero-dorsal protuberance which is the rudiment of the 



V 



ba 









E 



FIG. 38. Posterior ends of a series of young frog embryos, showing the later 

 history of the blastopore, and the relation of the neural folds to it. The embryos 

 are viewed obliquely from the postero-lateral aspect. After F. Ziegler. A. Blas- 

 topore nearly closed; neural folds just indicated. B. Blastopore becoming di- 

 vided into neurenteric and proctodseal portions; neural folds becoming elevated. 

 C. Neurenteric canal forming; neural folds closing together. D. Neural folds in 

 contact throughout. E. Neural folds completely fused; tail commencing to grow 

 out. b, Blastopore, containing yolk plug; 61, rudiment of neurenteric canal (dorsal 

 part of blastopore); bi, rudiment of proctodaeal pit (ventral part ol blastopore); 

 ba, branchial arches; g, neural groove; nf, neural folds; np, neural plate; p, proc- 

 toda?al pit; s, rudiment of oral sucker; t, rudiment of tail; x, neural folds roofing 

 the blastopore and establishing the neurenteric canal. 



tail, but in the stage we are describing this is only just indicated 

 (Fig. 38, D, E}. 



