28 F, L. LANDACRE 
concerned in the formation of mesenchyme and that any delami- 
nation of cells from the lateral ectoderm is readily explained as 
concerned in the formation of other well-known structures. 
Dorsal and ventral to the oral cavity ectoderm forms the dense 
connective tissue in which the teeth are imbedded, but I find 
no evidence for the derivation of loose mesenchyme from the 
lateral ingrowth of ectoderm. 
THE DIFFERENTIATION OF THE MESODERM AND NEURAL CREST 
IN THE HEAD j 
The possibility of following the differentiation of mesoderm 
into muscles, cartilages, and entodermal mesenchyme, and of 
neural crest into ganglia, ectodermal mesenchyme, and cartilages 
depends, as indicated earlier in this paper, upon easily recognized 
histological differences in ectodermal and entodermal cells. 
Ectodermal cells are smaller than entodermal cells, are usually 
pigmented, containing brown pigmented granules, and usually 
contain few yolk granules, or, if loaded with yolk granules, the 
granules are small and fairly uniform in size. Entodermal cells, 
on the other hand, are large and contain large numbers of large 
granules of varying sizes, and are not pigmented. When an 
embryo is stained with Delafield’s haematoxylin and orange G, 
the contrast between neural crest and mesoderm is very striking. 
These differences between ectodermal and entodermal deriva- 
tives can be followed in the urodeles up to the stage where the 
neural-crest ganglia, branchial muscles and eye muscles, branchial 
and cranial cartilages can be readily identified. One can follow 
the differentiation of the ectoderm and entoderm into mesen- 
chyme with ease where the two types do not overlap. In areas 
of overlap even, the two types can be followed easily in the earlier 
stages when they lie in continuous sheets of cells. In the loose 
mesenchyme where the ectodermal and endodermal mesenchyme 
cells are mingled one can for a long time identify with certainty 
ectodermal and endodermal mesenchyme cells. ‘The loose mesen- 
chyme finally becomes homogeneous, entodermal cells assuming 
the appearance of ectodermal cells, and after this stage it is 
impossible to say which type of cell predominates. 
