146 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



case with Protoplerws, the buccal roof in front, that is to say in the 

 neighbourhood of the mesial plane, passes without interruption into 

 the external skin : in other words the maxillary ridge is not con- 

 tinued to the mesial plane so as to meet its fellow. In later stages 



'■ — Sagittal sections through Polypterus. 



A stage 14 ; B stage 17; C, stage 20 ; D, stage 23 ; B, stage 24 +. a, anus ; ar, archenteron • evd 

 entoderm ; ,nt, entenc cav.ty ; f. h , cavity of forebrain ; /.,, foregut ; N, notocl orfl ; ^primary fold 

 of bram floor ; ptn, pmeal rud.ment ; s.c, cavity of spinal cord ; „, yolk. Y 



the roof of the mouth would be hidden in a view from the ventral 

 side owing to the forward growth of the lower jaw 



The anterior portion of the buccal cavity in Urodele Amphibians 

 arises ma manner essentially similar to that described above 



In the Gymnophiona and the Amniota a characteristic modifica- 

 tion of the mouth margin is brought about by the fact that as 



