FATE OF NEURAL CREST IN HEAD OF URODELES 29 
It is a rather striking fact in the distribution of ectoderm and 
endoderm in the head that, in general, ectoderm gives rise to 
mesenchyme and cartilages located ventrally, that is, farthest 
from its source of origin, while mosoderm gives rise to mesen- 
chyme in the dorsal regions of the head, which makes its location 
farthest from its source of origin. The loose entodermal mesen- 
chyme of the head is derived in part from the mesoderm, which 
also gives rise to branchial muscles ventrally and to eye muscles 
dorsally. The actual formation of mesenchyme in these loca- 
tions consists in the detachment of individual cells from the 
mesial surface of the more anterior and ventral muscle primordia, 
such as the masseter and temporalis, as well as from the more 
dorsal mesoderm from which eye muscles arise. 
That portion of mesoderm which migrates beyond the muscle- 
forming area breaks down completely into loose mesenchyme. 
The mesoderm of the head, therefore, shows two types of tissue, 
a) dense masses of cells which can be followed into definitive 
branchial and eye muscles, retaining all the time their ento- 
dermal character up to the time muscle fibrils appear in the case 
of branchial muscles, and, b) loose mesenchyme which gradually 
loses its entodermal character and assumes the form of ectodermal 
mesenchyme. 
The formation of ectodermal mesenchyme and cartilages from 
neural crest is really easier to follow than the mesodermal deriva- 
tives, except in the dorsal head regions from which the ectodermal 
mesenchyme withdraws as a continuous sheet. The neural 
crest, in its ventral migration, forms a continuous sheet of cells 
readily distinguished from mesoderm and lying lateral.to the 
mesoderm. This continuous sheet of neural-crest cells gives 
rise, in its dorsal region, to the neural-crest ganglia and in other 
dorsal regions disintegrates into mesenchyme, which disappears 
as a continuous sheet in certain regions as indicated in figures 
9, 10 and 11. 
That portion of the neural crest which migrates into the ventral 
head region, that is, dorsal and ventral to the oral region and 
into the branchial bars, presents a quite different history. This 
ventral portion of the neural crest does not disintegrate. It not 
