2 F. L. LANDACRE 
Only a relatively small portion of the neural crest in the head is 
involved in the formation of cerebral ganglia. 
In the urodeles the neural crest forms in the anterior head 
region an almost continuous sheet of mesenchyme, lying lateral 
to the axial mesoderm which is derived from the endoderm, and 
is interrupted for a time by the olfactory bulb and optic stalks 
only. In the region of the mandibular and branchial bars, the 
neural crest migrates ventrally into these bars to the extreme 
ventral level of the body and is prevented from forming a junc- 
tion with the crest of the opposite side by the presence of the 
heart and ventral aorta. 
The possibility of determining the fate of neural crest cells 
that do not enter into the composition of cerebral ganglia depends 
upon conditions that exist in some types and are apparently 
absent in others. It is the almost unanimous opinion of workers 
that it is extremely difficult and often impossible to determine 
the fate of all neural crest cells in the anterior head region. In 
the branchial region of the lower vertebrates particularly, the 
situation is very different. A number of reliable workers have 
given detailed descriptions of the fate of the neural crest cells 
and of the manner in which they enter into the composition of 
structures usually considered as mesodermal in origin. As to 
the migration of the neural crest beyond ganglion-forming regions 
in all vertebrates, there is apparently little doubt. } 
In the urodeles the determination of the extent of the migra- 
tion of the neural crest and the differentiation of the neural 
crest and its derivatives from those of the endoderm are com- 
paratively easy except in the anterior region. The contrast 
between the large, heavily yolk-laden, light staining endoderm 
cells and their derivatives on one hand and the smaller, dark 
staining, pigmented, slightly yolk-laden ectoderm cells and their 
derivatives on the other hand is very striking. In addition to 
these distinctions based upon size of cells, staining reaction, 
pigmentation, and number of granules, there is also an actual 
difference in the size of yolk granules carried by endoderm cells 
as compared with those carried by ectoderm cells, those carried 
by endoderm cells being larger. These distinctions, furthermore, 
