OTIC AND OPTIC PRIMORDIA IN MAN 215 
other components are probably derived from the neural crest, 
and there is abundant evidence in our series for such a prolifera- 
tion at the levels corresponding to these nerves. 
It should be said in conclusion that the acousticofacial ganglion 
in man is manifestly derived from the wall of the neural tube 
before its closure. The ganglion cells separate off, leaving the 
dorsal part of the fold thinner here than elsewhere as is well 
shown in figure 8. Even the few sections here figured leave no 
doubt of this. The evidence clearly excludes the possibility that 
the ganglion is formed from cells intermediate between neural 
and somatic ectoderm because the cells lying dorsal as well as 
medial to the ganglionic anlage enter into the formation of the 
definitive neural tube. 
It is obvious from what has been said that the origin of the 
more rostral cranial ganglia in man must be studied largely in 
neural-fold stages. Much of the confusion in the literature is 
due to the failure to recognize this, to ignorance concerning 
the functional components of the cranial nerves, and to the fact 
that distinct components may have different origins. 
THE OPTIC PRIMORDIA 
The earliest anlage of the optic vesicle is so intimately asso- 
ciated with the cranial neural crest that the two must be dis- 
cussed together. We will confine our attention to the extent 
and relations of the latter, as the details of this proliferation will 
be described elsewhere. The transformations which lead to the 
formation of the primary optic vesicle will be considered after 
we have sketched the development of the ‘optic-crest primordium’ 
as a whole. 
The rostrally expanded neural folds in the younger members 
of the series show slight isolated thickenings which are difficult 
to interpret, if indeed they have any significance. When we come 
to the eight-somite embryo No. 391 (Dandy), we can identify 
definite primordia. Here, as we have seen, the otic plate and 
acousticofacial ganglion have appeared. Rostral to them we 
find the lateral third of each fold is thickened, the neural epithe- 
lium protrudes slightly toward the underlying mesenchyme and 
