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 (cz); and the other 
is one of the auditory vesicles (0), 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 16c, the largest section of this series, 
passes through the forebrain in the region of the 
eyes and through the gill clefts. The forebrain (fd) 
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 (Jz), 
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. Only 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 
