The Development of the Cartilaginous Skull ete. in Necturus. 409 
length of the muscles, which increases as the embryo grows, until, 
as muscles genio-hyoidei, they extend to the anterior extremity of 
the lower jaw. 
The mesectoderm, which in fig. 1 % is evenly dense in the space 
between the mesothelium of the mandibular and hyoid arches, is 
seen in fig. 5 d, to be more dense in the neighborhood of the man- 
dibular muscle, while the cells about the hyoid muscle have become 
more scattered. The mouth is still entirely closed by endoderm. 
The mesectoderm cells on the anterior and posterior surfaces of the 
invaginated oral ectoderm are more closely grouped than in the 
younger embryo. In the plane of fig. 5 d, there is no doubt that 
the cells bounding the antero-lateral margin of the oral epithelium 
are mesectodermic. 
The plane of fig. 5 e passes through the dorsal wall of the nasal 
epithelium. If the section be compared with fig. 1 2, it is seen that 
the mesectoderm cells, which in fig. 17 are closely grouped in the 
angle where the superficial ectoderm turns inwards to form the 
anterior (dorsal) epithelium of the mouth, compose in the older 
embryo, a comparatively dense tissue in the same relative position. 
Posterior to the mouth the relation of the tissues is somewhat 
changed. The united mesothelial tissues of the hyoid and mandi- 
bular arches, seen in fig. 1 7, have been converted into the muscle 
mylo-hyoideus in the older embryo, and the mesectoderm cells which 
surround the mesothelium in fig. 1 7, are now grouped more densely 
in the antero-lateral region of the mandibular arch, while the median 
cells of this arch, and those of the hyoid arch, have become more 
scattered. A slight constriction in the lateral surface of the section 
indicates the plane of separation between the hyoid and mandibular 
arches. Fig. 1 7’ shows that in the younger embryo, the mesecto- 
derm in the anterior part of the mandibular arch had already begun 
to become more dense than in the posterior or hyoid part of the 
‚section. 
I trust that comparison of the two series of sections, figs. 1 a—1i 
and 5 a—5e, suffices to show that the mesothelial tissue of the 
branchial arches has given rise to muscle cells, and that where the 
surrounding cells are most closely grouped in the older embryo they 
may be traced by their relative position to mesectoderm cells of the 
younger embryo, which are separated sharply from cells of the 
mesoderm by the absence of yolk granules. The origin of these 
cells which are free from yolk, I have described in a former study 
