a2 F. L. LANDACRE 
The same explanation is offered for the double composition 
of the trabeculae. The anterior portions of the trabeculae form 
in a region dominated by ectodermal mesenchyme. ‘This region 
lies between the two dorsal spurs just over the eye in figure 11, 
while the caudal portions of the trabeculae and the parachordals 
form in a region containing axial entodermal mesenchyme. 
SUMMARY 
1. The neural crest in the urodeles is incorporated in the 
neural canal at first and later erupted and then grows ventrally 
as a continuous sheet of cells. This ventral migration carries 
it into the ventral region of the anterior portion of the head and 
into the ventral portions of the mandibular and branchial bars. 
2. The dorsal portion of the neural-crest sheet gives rise to 
the general cutaneous and general visceral portions of ganglia 
V, VII, IX, and X cranial nerves. The dorsal portion of the 
neural crest not concerned in the formation of ganglia disinte- 
grates to form mesenchyme, which becomes mingled with ento- 
dermal mesenchyme in the dorsal head region. 
3. The ventral portion of the continuous neural-crest sheet, 
after migrating into the ventral head region and into the mandib- 
ular and branchial bars, surrounds the primordia of branchial 
muscles growing from their lateral surfaces around to their 
mesial surfaces. This ventral portion of the neural crest later 
differentiates into loose ectodermal mesenchyme and into dense 
cartilage primordia, which pass through procartilage into typi- 
cal cartilages. 
4. The greater portion of the mesenchyme in the ventral 
head region and in the branchial regions is consequently ecto- 
dermal mesenchyme. 
5. The cartilages arising from ectoderm. are the anterior 
portion of the trabeculae, Meckel’s cartilage, the palatoquadrate 
bar, and all the branchial cartilages except the second basi- 
br Sneinal or urohyal. 
6. The mesoderm in the head gives rise to the beeen 
muscles, to the eye muscles, and to loose entodermal mesen- 
