FATE OF NEURAL CREST IN HEAD OF URODELES iy 
THE ORIGIN OF THE NEURAL CREST 
There are in the vertebrates three modes in which the neural 
crest is related to the neural tube and overlying ectoderm. Har- 
rison (’01) has described and compared two of these. In the 
first type, the neural-crest cells—selachians and other types— 
represent the dorsal portion of the lateral walls of the neural 
tube which is at first continuous with the ectoderm. The 
neural crest is incorporated in the neural tube, forming a wedge- 
shaped mass in its dorsal portion. This wedge-shaped mass 
later becomes detached from the tube and migrates laterally 
and ventrally. In the second mode the neural crest (teleosts) 
lies between the dorsal border of the neural tube and the ecto- 
derm, not being included strictly in either, but forming later a 
cap over the dorsal border of the neural tube. In the third mode 
(Ameiurus and other types (Landacre, ’10)) the neural crest or 
cells homologous to the neural-crest cells remain in the ectoderm 
lateral to the neural canal and are later detached from this 
position to form parts of cranial ganglia and other structures. 
The three series of urodeles studied correspond closely in the 
behavior of the neural crest to the first type mentioned above 
and do not require an extended description. The neural crest 
is completely incorporated in the dorsal border of the neural 
tube, but can be distinguished from the tube, usually, by its 
looser structure. The following description is based on the 
behavior of the crest at the level of the VII ganglion. 
When the medullary plate can be identified first, it is very 
broad and its lateral borders include the two portions of the 
neural crest. Just before the closure of the neural canal (fig. 1) 
the superficial layer of the ectoderm and much of what will 
become neural crest are invaginated and included within the 
limits of the neural groove. The superficial heavily pigmented 
layer of the ectoderm forms the inner lining of the greater portion 
of the neural groove. The looser texture and greater pigmenta- 
tion of the dorsomesial portions of the walls of the neural groove 
indicate roughly the position of the neural-crest cells. 
