FATE OF NEURAL CREST IN HEAD OF URODELES 17 
in the region of the V ganglion and at the ventral border of the 
IX ganglion. In the overlapping areas mentioned the ento- 
dermal mesoderm lies mesial to the neural crest. 
In the second stage plotted, 3 mm. in length (fig. 8), but 
six hours older than the last, the neural crest in the region of the 
gasserian ganglion and anterior to this ganglion has migrated 
ventrally and caudally and now forms two prominent extensions, 
the one anterior to the eye and mesial to the olfactory capsule, 
the other posterior to the eye and extending slightly into the 
mandibular bar ventral to its ganglion-forming region. This 
anterior neural crest is now completely detached from the neural 
crest in the region of the VII ganglion, except for a few scattered 
cells on the dorsal portion of the neural tube. 
The neural crest at the level of the VII ganglion has grown 
ventrally to the region of the dorsal border of the alimentary 
canal, doubling its length as compared with the last stage. The 
IX ganglion has also grown ventrally to the same extent and the 
X is now represented by a conspicuous ventrocaudal extension 
of the same general neural-crest mass from which the IX ganglion 
forms. The neural crest of VII, IX, and X have not, as yet, 
grown ventrally beyond their ganglion-forming regions. The 
neural crest in the region of VII is connected dorsally by a well- 
defined strand of neural crest cells with that of ganglia [X and X. 
The lateral mesoderm lying posterior to the anterior end of 
the alimentary canal at this stage is interrupted by the out- 
growth of the endoderm to form the spiracular and hyoid pha- 
ryngeal pouches. The interruption of the lateral mesoderm at 
the level of the spiracular cleft is not formed entirely. by the 
pharyngeal pocket. Its ventral portion represents the remains 
of the prominent notch shown just caudal to the eye in figure 7. 
The head mesoderm lying anterior to the alimentary canal 
has extended in two directions, dorsally and anteriorly, until 
it has reached the dorsal wall of the brain at the vertical level 
of the anterior end of the optic vesicle. It has also extended 
ventrally and caudally posterior to the optic vesicle. The 
posterior boundary of this extension lies just ventral to the 
spiracular pharyngeal pocket. The ventral limit of this extension 
