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 (/&) with thick, dense walls. 

 Attached to each side of the forebrain is an optic 

 vesicle (oi>), 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 (/?') ; 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 (bi>). Several other blood-vessels may be 

 seen at various places in the mesoblast, four of 

 them near the pharynx being especially noticeable. 

 The hindbrain (lib} is wider than, but not so deep as, 

 the forebrain; its walls are very thick laterally, but 



