OTIC AND OPTIC PRIMORDIA IN MAN 221 
one can observe every stage in the slipping out of the epithelial 
cells from the neural fold and Veit’s (’18) excellent description 
of his ‘craniale Ganglienleiste’ can be verified in detail. In 
‘Du Ga’ and H392 and the N. Y. U. No. 4 (fourteen somites) 
there is a continuous crest anlage in the midbrain and the pre- 
otic hindbrain. In the first it reaches almost to the acoustico- 
facial ganglion, while in H392 (eleven somites) it cannot be 
recognized below the trigeminal level. 
Early history of optic vesicle 
We may turn now to the mass movements in the rostral divi- 
sion of the anlage which produce the optic vesicle. The general 
features of the process may be gathered from a survey of the 
sections reproduced in figure 9. The first is taken from H87 
(eight somites). On the left we have a slightly more caudal 
level than on the right as the head end curved somewhat to the 
left. The level of the section is indicated on figures 3 and 4. 
The thickening of the neural fold, viz., the optic anlage, is indi- 
cated by the solid color. The optic sulcus and the bulging of 
the anlage into the mesenchyme stand out clearly. From the 
lateral portion of it the optic vesicle will arise, as we have deter- 
mined by the study of the corresponding region in the older 
embryos. This is an earlier stage of the optic primordium 
than has been described for any mammal. 
Figure 9b is from the twelfth section of Eternod’s nine-somite 
embryo ‘Du Ga.’ The rostral limb of the cranial flexure appears 
below; above is the caudal mesencephalic limb. Only the optic 
anlage is indicated by the solid color; it shows the characteristic 
thickening and the optic suleus. Above in the figure is the 
mesectodermal proliferation with its corresponding ventricular 
sulcus. The optic anlagen in this specimen begin four sections 
(40) behind the rostral end of the neural folds and can be recog- 
nized in five or six sections. 
The next stage we have is in the fourteen-somite embryo of the 
New York University collection. In this case the forebrain 
appears to have lagged behind the rest of the embryo in develop- 
