30 F. L. LANDACRE 
only maintains its continuity, but grows extensively, surrounding 
the branchial muscles and filling the areas between muscles and 
epithelial structures. In this rather dense and continuous sheet 
of neural-crest cells there can be identified the still more con- 
densed areas where the cartilages, such as Meckel’s and the 
branchial cartilages, will form. Coincident with the formation 
of procartilage, there is going on a disintegration of the remaining 
neural crest to form loose ectodermal mesenchyme in much the 
same way as loose entodermal mesenchyme is formed from the 
mesoderm in more dorsal regions. Consequently, in the ventral 
head and branchial regions where the mesoderm gives rise to the 
muscles, most of the loose mesenchyme is derived from neural 
crest and is therefore ectodermal, while in the dorsal head regions, 
from which neural crest has disappeared as a continuous sheet, 
it is chiefly entodermal (mesodermal) mesenchyme. 
Our conclusions concerning the metamorphosis of neural-crest 
cells into cartilage (excepting Miss Platt’s conception of the 
behavior of lateral ectoderm) are so closely in accord with those of 
Platt, Dohrn, and Brauer that it is not necessary to give a detailed 
discussion of the process. The branchial cartilages all differ- 
entiate out of the ventralmost extension of the neural crest. 
They all show uniformly a stage where the neural crest is 
in the form of a sheet of cells surrounding the corresponding 
mesodermic branchial muscle primordium. Since this sheet of 
neural-crest cells forms a syncytium, it is true mesenchyme; 
although somewhat more compact than entodermal mesenchyme. 
Throughout the whole ventral head region and branchial region 
the differentiation of this primitive mesenchyme shows two 
types of behavior: a) Where no cartilages form its cells become 
detached from the sheet of neural-crest cells and form loose 
mesenchyme surrounding muscles and cartilages. ‘This is prac- 
tically pure ectodermal mesenchyme. The few entodermal cells 
found may prove usually to be growing blood-vessels or blood- 
cells; b) where a cartilage forms the neural-crest cells first increase 
in number, then condense, passing through typical procartilage 
stages, and finally form true cartilage. We find, in agreement 
with Miss Platt, that the anterior portion of the trabecular 
