220 G. W. BARTELMEZ 
on the right side of figure 10; on the left the section passes through 
the rostral end of the optic primordium (op. prim.). The more 
medial portion of the thickened primordium in this figure shows 
the indefinite lower boundary which is characteristic of a mesec- 
todermal proliferation. It would seem, then, that the optic 
anlage arises only from the lateral part of the area indicated by 
stippling in figure 3, at levels corresponding to the caudal part 
of the forebrain, i.e., the future diencephalon. 
H 197 is the only embrya we have seen which clearly shows 
mesectoderm formation in the forebrain, nor has it been observed 
in any other mammal. It can hardly be an abnormal activity 
in this particular case, for as a whole the embryo fits perfectly 
into the series. More than that, there are hints of such a process 
in another undoubtedly normal specimen, as we shall see. It 
is in better condition histologically than most young human 
embryos, as the photomicrographs (figs. 8 and 10) demonstrate. 
The optic anlage of Eternod’s nine-somite embryo ‘Du Ga’ 
(fig. 9b) is no farther along than in the eight-somite H87. In 
the eleven-somite H637 it is but little younger than in H 197 
and the neural fold medial to it is greatly thickened. ‘There are 
two spots where cells seem to be preparing to leave the primor- 
dium, and this evidence. from an absolutely normal specimen 
affords the best evidence for regarding the extensive prolifera- 
tion in H 197 as normal. Unfortunately, the mechanical in- 
juries to the head in H 637 make modeling practically impos- 
sible. Inthe other embryo of this group, H392 (eleven somites), 
the optic sulcus is decidedly deeper than in H 197, as may be 
seen by comparing figures 9d and 10. Here, as in ‘Du Ga,’ 
the progress of the cranial flexure has bent the rostral end of the 
neural folds ventrally so that in the transverse series the first 
sections pass tangentially through the midbrain. Because of 
the oblique sections of the neural folds, it is impossible to get 
convincing pictures of neural-crest formation. 
So far as the more caudal part of the optic-crest primordium is 
concerned, it is clear that in H637 (eleven somites) the optic 
anlage is continuous with the neural-crest proliferation of the 
mid- and hindbrain. In the beautiful 5y sections of this series 
