Ill 



BUCCAL CAVITY 



147 



already mentioned, the maxillary ridge is cut across by the olfactory 

 groove and so divided into the outer maxillary process and the 

 inner median nasal process, the latter of which is continuous with 

 its fellow across the mesial plane, forming with it the so-called 

 fronto-nasal process (see Chap. X.). 



pan. P an 



pin.-... 



Fig. 80a. — Sagittal sections through Polypterus. 



F, stage 2(j ; G, stage 29; H, stage 32. a, anus; a.c, anterior commissure ; vh, optic chiasma ; 

 I, cloaca ; ent, enteric cavity ; H, heart ; h.e, habenular commissure ; li, liver ; m.r, mandibular ridge ; 

 p, pituitary involution ; ji.c,.posterior commissure ; pa.j/, postanal gut ; pan, pancreatic rudiment ; pin, 

 pineal rudiment ; st, stomach ; y, yolk. 



It is of interest to notice that in various Vertebrates the buccal 

 opening is at first elongated in an antero-posterior direction instead 

 of from side to side. Such is the case with Scyllium (Sedgwick, 

 see Fig. 81) and Torpedo amongst Elasmobranchs. In these cases 

 the slit-like mouth is bounded on each side by a longitudinal ridge. 



