276 The Alligator and Its Allies 



shown in any figure of the series except the first. 

 In this figure the forebrain (fb)and midbrain (mb) 

 are widely separated instead of being connected, as 

 in the preceding figure, where the section passed 

 through the actual bend of the cranial flexure. 

 The anterior and ventral part of the cranial 

 cavity, the forebrain (fb) , is nearly circular in out- 

 line. It exhibits on one side a well-marked optic 

 vesicle (ov), which is sufficiently advanced in 

 development to show a rudimentary optic stalk. 

 The outer wall of the optic vesicle is in close con- 

 tact with the superficial ectoderm, which shows 

 as yet no sign of the formation of a lens vesicle. 

 The plane of the section being probably not quite 

 at right angles to the long axis of the embryo, 

 the optic vesicle of one side only was cut. The 

 wall of this part of the forebrain is of about the 

 same thickness and appearance as in the preceding 

 stage. The other cerebral cavity (mb) of this 

 section is probably the hinder part of the midbrain, 

 though it may be the anterior part of the hind- 

 brain; there is no sharp line of demarcation be- 

 tween these regions of the brain. This cavity (mb) 

 is much smaller in section than the forebrain; its 

 walls are of about the same thickness. 



Ventral to the midbrain is the anterior end of 

 the notochord (;//), surrounded by the mesoblast. 

 At various places throughout the mesoblast irregu- 

 lar open spaces may be seen; these are blood- 

 vessels. The ectoderm (ec) and amnion (a) have 



