14 F, L. LANDACRE 
derived from entoderm, and of course this is equivalent to meso- 
derm and therefore superfluous. There is in the head region, 
however, a good deal of loose tissue quite similar to mesenchyme 
found in the trunk and like that found in the trunk, derived from 
mesoderm, which is itself a derivative of endoderm. Now, since 
it is necessary, in describing the behavior of mesenchyme in 
the head, to distinguish between that derived from the neural 
crest, which retains most of its ectodermal characters, and that 
derived from mesoderm, which for a long time retains its endo- 
dermal characters, I have ventured to suggest the terms ‘ento- 
dermal mesenchyme’ and ‘ectodermal mesenchyme’ for mesen- 
chyme in the head where the two types of cells can be distin- 
guished. When head mesenchyme becomes homogeneous, that 
is when we can no longer distinguish two types of cells, it will 
be referred to as mesenchyme, with the implication, however, that 
it sometimes contains both ectodermal and entodermal cells. 
While both types of mesenchyme have passed through inter- 
mediate stages, the entoderm through a mesodermal stage and 
the ectoderm through a neural crest stage, each retains the char- 
acter of its more remote rather than of its immediate ancestor. 
This seems to justify the terms ectodermal mesenchyme and 
entodermal mesenchyme rather than neural-crest mesenchyme 
and mesodermal mesenchyme. The term ectodermal mesen- 
chyme is substituted for mesectoderm, which has become general 
in the literature. The tissue we are dealing with is not a derm 
or layer, but a true mesenchyme quite similar to mesenchyme in 
the trunk, but coming from ectoderm rather than from mesoderm 
as is in the trunk. 
The use of the terms ectodermal mesenchyme and endodermal 
mesenchyme is further justified by the fact that in the head all 
branchial muscles come from the mesoderm, and show through- 
out their earlier stages definite somatic and splanchnic layers, 
indicating their relation to the lateral mesoderm of the body, 
while the mesenchyme of the head is more or less loose and of a 
syncytial character, like that derived in the body from the 
sclerotomes and the lateral and splanchnic mesoderm. As the 
mesoderm of the anterior trunk region grows forward into the 
