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 (7) now shows asa 
round depression in front of the more definitely 
outlined eye (e). The auditory vesicle (0) is so 
deep beneath the surface that it may be seen only 
by transmitted light. 
Figures 15a-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 15a, 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 (ov) 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 (0c). The lens vesicle 
(lv) 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 
