FATE OF NEURAL CREST IN HEAD OF URODELES Sik 
bars, the palatoquadrate and Meckel’s cartilages, all the branchial 
cartilages except the second basibranchial are formed from ecto- 
dermal mesenchyme, while the posterior portion of the trabecular 
bars, the parachordals and basal plate of the chondrocranium 
along with the occipital arch arise from entodermal mesenchyme. 
The roof of the chondrocranium arises very late and is assumed 
to come from entodermal mesenchyme on the basis of the distri- 
bution as shown in figure 11, although at the stage when the 
lateral walls of the cranium appear the mesenchyme is homo- 
geneous. I am unable to determine the composition of the brain 
membranes and their homologues, the choroid and _ sclerotic 
coats of the eye, on account of the lateness of their formation, 
which occurs after the mesenchyme from which they form has 
become homogeneous. ‘The cartilages listed above as ectodermic 
or entodermic in origin show, in addition to the fact that their 
development from ectoderm and entoderm can be followed, 
quite definite ectodermic and entodermic characters after they 
have assumed their definitive forms and after their cells are 
surrounded by hyaline material so dense that cell division within 
the cartilage has practically ceased. 
The formation of the second basibranchial from entoderm 
needs a word of explanation. The second basibranchial of Miss 
Platt is the urohyal of some authors. It is the most caudal of 
the median branchial cartilages and lies ventral to the other 
branchial cartilages, except at its anterior attached extremity. 
It develops from mesoderm along with the paired geniohyoid 
muscles which are attached to its extremity. The mesoderm 
from which these three structures develop is continuous at first 
with the lateral borders of the alimentary canal from which other 
muscle primordia form. It retains for a long time its ento- 
dermal type of cell, so that there can be no question as to its 
origin. No explanation is offered of this curious fact further than 
that suggested by its position, which, as shown by a reference 
to figure 11, is at a point where ectoderm does not extend, that 
is, on the midventral line ventral to third, fourth, and fifth 
branchial bars. 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 33, NO. 1 
