294 The Alligator and Its Allies 



through the pharyngeal wall. The outlines of the 

 visceral folds, especially of the mandibular, begin 

 to be apparent. The nasal pit (;?) now shows as a 

 round depression in front of the more definitely 

 outlined eye (e). The auditory vesicle (o) is so 

 deep beneath the surface that it may be seen only 

 by transmitted light. 



Figures i ^a-e represent transverse sections of an 

 embryo of about this general state of develop- 

 ment, except that the gill clefts are not so definitely 

 open as in the surface view. 



Figure i^a, the most anterior section of the series, 

 passes through the forebrain {fb) in the region of 

 the eyes, and through the hindbrain (hb) anterior 

 to the auditory vesicles. The forebrain is here 

 a large cavity with a dense wall of a comparatively 

 even thickness. Owing probably to the section not 

 being exactly in the transverse plane, the eyes are 

 cut in different regions, that on the left {oi') being 

 cut through its stalk, while that on the right (oc) 

 is cut near its middle region and hence does not 

 show any connection with the forebrain. The 

 almost complete obliteration of the cavity of the 

 optic vesicle to form the optic cup by the invagin- 

 ation of the outer wall of the vesicle is shown on 

 the right side of the section (oc) . The lens vesicle 

 (Iv) is completely cut off from the superficial ec- 

 toderm (ec), which is comparatively thin. The 

 hindbrain (hb) has the usual shape for that struc- 

 ture. Its ventral wall is dense and thick, while its 



