284 The Alligator and Its Allies 
thickest areas being on each side of the forebrain; 
it is more compact than in the earlier stages, and, 
owing to the low magnification under which it is 
drawn, it is represented here by a single heavy line. 
Under this magnification only the nuclei of the 
mesoderm cells (mes) can be seen, so that this tissue 
is best represented by dots, more closely set in 
some places than in others. The forebrain is an 
elongated cavity (fb) with thick, dense walls. 
Attached to each side of the forebrain is an optic 
vesicle (ov), which is considerably larger than in 
the preceding stage. The connection between 
the cavity of the forebrain and that of the optic 
vesicle is not seen in this section; it is a wide 
passage that may be seen in several sections pos- 
terior to the one under discussion. The beginning 
of the invagination of the optic vesicle to form the 
optic cup may be seen on both sides, but more 
plainly on the right. On the right side also is 
noticed a marked thickening of the ectoderm, 
which is invaginated to form a small pit, the lens 
vesicle (Jv); on the left side the section is just 
behind the lens vesicle. Above the optic stalk on 
each side, in the angle between the optic vesicle 
and the side of the forebrain, is a small blood- 
vessel (bv). Several other blood-vessels may be 
seen at various places in the mesoblast, four of 
them near the pharynx being especially noticeable. 
The hindbrain (hd) is wider than,but not so deep as, 
the forebrain; its walls are very thick laterally, but 
