FATE OF NEURAL CREST IN HEAD OF URODELES 33 
chyme, which retains for a long time its entodermal character- 
istics. Entodermal mesenchyme later differentiates into carti- 
lages forming the posterior portion of the trabeculae, the para- 
chordals and base of the cranium, the occipital arch, the auditory 
capsule, and probably the lateral walls of the cranium. 
7. The neural crest and mesoderm, where not concerned in 
the formation of ganglia and muscles, respectively, shift their 
positions extensively, but the net result of their migrations is to 
leave entodermal mesenchyme in the dorsal head regions, ecto- 
dermal mesenchyme in the ventral head regions, and mixed 
mesenchyme in the median longitudinal axis of the head. Where 
this overlapping occurs ectodermal mesenchyme is lateral in 
position, while entodermal mesenchyme is mesial in position. 
The loose axial mesenchyme lying in the median line between 
the brain floor and the dorsal surface of the mouth and pharynx 
is almost pure entodermal mesenchyme. 
8. The lateral ectoderm, aside from the oral region, is not 
concerned in the formation of mesenchyme, but does give rise 
to the profundus ganglion, to lateral-line ganglia, to lateral- 
line organs, and to epibranchial ganglia. 
9. The lateral ectoderm in the oral region gives rise to an 
ectodermic collar, which surrounds the solid entoderm of the 
oral region, and in this collar arise dental ridges and teeth and 
dense connective tissue in which the teeth are imbedded. We 
can find no evidence for the formation of loose mesenchyme 
from this ectodermic collar. 
10. The histological grounds on which the distinctions in the 
behavior of ectoderm and entoderm rest are as follows: entoderm 
cells are larger than ectoderm cells: entoderm cells are rarely 
pigmented, while ectoderm cells usually are. Ectoderm con- 
tains small, fairly uniform-size yolk granules and loses them 
early, while entoderm contains large granules of irregular size 
and retains them much longer. In addition to these histological 
differences, the continuity of sheets of ectoderm and of entoderm 
cells is a valuable aid in following their distribution. The 
distinctions mentioned above can be observed after definitive 
ganglia, muscles, cartilages, and mesenchyme are formed. 
