FATE OF NEURAL CREST IN HEAD OF URODELES 15 
head, it gives rise to a) eye muscles from its dorsal or somitic 
portion; 6b) branchial muscles from its ventral portion, and, c) 
mesenchyme which retains it endodermal characters. That por- 
tion of head mesoderm which gives rise to branchial muscles 
does not become loose or syncytial in character as does the 
mesenchyme, but retains its integrity to such an extent that 
two layers, somatic and splanchnic, can for a long time be 
identified. 
The neural crest, on the other hand, gives rise to specific 
cerebral ganglia such as V, VII, IX, and X and then migrates 
ventrally into the ventral head region and into the branchial 
bars and differentiates into cartilages and loose mesenchyme. 
The neural-crest ectoderm furnishes three specific derivatives, 
a) ganglia, b) cartilages, c) mesenchyme. Since the two types 
of mesenchyme overlap and are histologically distinct for a 
long time and since the ventral portion of the neural crest passes 
from a mesenchymal stage to a cartilaginous stage, the terms 
entodermal mesenchyme and ectodermal mesenchyme seem not 
only to be justified, but to be absolutely necessary to accurate 
description. 
We shall first follow the migration of the neural crest and 
mesoderm. 
In the first stage plotted, which is 3 mm. in length (fig. 7), 
the neural crest is continuous along its dorsal border throughout 
the whole head region, beginning anteriorly at the level of the 
middle of the eye horizontally and extending caudally into the 
spinalmneural crest. It presents three conspicuous enlargements. 
The anterior enlargement extends caudally to the vertical level 
of the posterior border of the eye. The gasserian ganglion 
differentiates out of the posterior portion of this enlargement 
and can be identified at this stage by the slight condensation 
of the cells. This enlargement is a ventral extension of the neural 
crest, but owing to the flexure of the head it seems to extend 
caudally. 
_. The second enlargement or ventral extension is at the level 
of the VII ganglion, the third at the level of the IX ganglion, 
‘and the fourth inconspicuous enlargement at the level of the 
