302 The Alligator and Its Allies 



other. The walls of these cavities are somewhat 

 wrinkled and irregular and their constituent cells 

 are beginning to show slight differentiation, though 

 this is not shown in the figure. On the left side 

 are seen a couple of darkly stained masses; one is 

 the origin of a cranial nerve (en) ; and the other 

 is one of the auditory vesicles (o), which is still 

 more irregular in outline than it was in the pre- 

 ceding stage. The only blood-vessels to be seen 

 are a few very small ones that lie close to the wall 

 of the brain. The ectoderm is quite thin at all 

 points. 



Figure i6c, the largest section of this series, 

 passes through the forebrain in the region of the 

 eyes and through the gill clefts. The forebrain {fb) 

 exhibits on the left a marked thickening of its wall 

 (ch), the edge of the cerebral hemisphere of that 

 side, which is just beginning to develop; on its right 

 side the lower part of the forebrain is connected 

 by a well marked optic stalk (os) with the optic 

 cup (oc), in whose opening lies the lens vesicle (Iv), 

 now reduced to a crescentic slit by the thickening of 

 its posterior wall. The mesoblast is more dense 

 in those parts of the section adjacent to the pharynx 

 than in the more distant regions, and the ectoderm 

 thickens in a marked way as it approaches the 

 borders of the pharynx and gill clefts. Onl}^ a 

 few small blood-vessels (bv) are to be seen in the 

 region of the forebrain. 



Parts of three pairs of clefts (g) are shown in the 



