oe F, L. LANDACRE 
continuous sheet in the region of the midbrain except for the two 
dorsally directed spurs shown in figure 11 and between the V 
and VII ganglia, and as a continuous sheet disappears in the 
region immediately dorsal to the hypophysis. The identifica- 
tion of the crest in these regions rests on certain histological 
characters of its cells, but to some extent on the continuity of 
the sheet of cells which everywhere grows continuously in a 
ventral direction morphologically. The continuity of this sheet 
of ectodermal mesenchyme derived from the neural crest needs 
to be emphasized, since it facilitates greatly the identification 
of the progressive changes in extent of the crest. At points 
where the neural crest is diminishing in extent the problem of 
describing its boundaries is much more difficult. | 
The behavior of the head mesoderm, in addition to the facts 
shown by the plots, consists in the formation of branchial muscles 
from the lateral mesoderm, which do not lose their entodermal 
characters, the formation of eye muscles from the dorsal or 
somatic region of head mesoderm, and the formation of mesen- 
chyme, particularly in the anterior and dorsal head regions, which 
mingles with detached cells derived from the neural crest to 
form later homogeneous head mesenchyme. 
THE DERIVATIVES OF THE LATERAL ECTODERM 
Among authors who derive mesenchyme from ectoderm, one 
of the most debated points is concerned with the origin of mesen- 
chyme from the lateral surface ectoderm of the head as compared 
with the origin from the neural crest. Miss Platt, in her earlier 
paper (’94), derived ectodermal mesenchyme entirely from 
lateral ectoderm, but in her later paper seems to include the 
neural crest also a source of this tissue. Kupffer (’95) also, in 
Petromyzon, derives branchial cartilages and mesenchyme from 
the lateral ectoderm, which he designates as branchodermis, 
and later as neurodermis. Lundborg (’99) and Koltzoff (’02) 
both derive mesenchyme from lateral ectoderm. Dohrn (’02), 
however, working on Torpedo ocellata, and Brauer (’04), working 
on the Gymnophiona, can find no evidence that anything but 
