FATE OF NEURAL CREST IN HEAD OF URODELES 19 
Much of the optic vesicle, aside from the region of the optic 
stalk, is separated from the brain wall by this continuous sheet 
of ectodermal mesenchyme. 
The second change in this region is regressive and is indicated 
by the absence of ectodermal mesenchyme in the region lying 
vertically over the olfactory capsule. The question as to whether 
this is due to a withdrawal by migration or to a disappearance 
of the cells will be taken up in the next section. Althoughthe 
anterior limit of the ectodermal mesenchyme is hard to determine 
on account of the scattered neural-crest cells in this region, there 
is no doubt that the process of disappearance of neural crest at 
this point has taken place. It is much more marked in the 
next stage plotted (fig. 11). Posterior to the eye, the change 
in the extent of the ectodermal mesenchyme is slight as seen 
from the lateral surface. It is almost coextensive with the pri- 
mordium of the mandibular muscle mass. It surrounds this 
mass completely, however, so that the endodermal. derivative 
of the mandibular bar is inclosed by a sheet of ectodermal mesen- 
chyme except at its extreme posterior tip. 
In the posterior head region at the levels of the VII, IX, and 
X ganglia the most marked changes are the ventral growth of 
the neural crest into the hyoid and first two true branchial 
bars, with the accompanying process of surrounding the endo- 
dermal derivatives (branchial muscles) in these bars by ecto- 
dermal cells. This last process is, of course, not indicated on 
the plot. Posterior to the pharyngeal pouch of the second 
true gill in the region of X the neural crest has not only grown 
ventrally, but also caudally as a broad sheet which will be pierced 
by the third true branchial pouch, thus setting off the second 
branchial ganglion of X from the first. In this process the mode 
of development is altered somewhat, since in the more anterior 
gill bars the neural crest grows ventrally into the bars after they 
are formed by the pharyngeal endodermic evagination, while 
here the migration of the neural crest ventrally precedes the 
formation of the pharyngeal evagination and must be displaced 
by it. As in the more anterior ganglia, the dorsal portion of the 
broad mass extending caudally from X forms ganglion, while 
