The Development of the Cartilaginous Skull ete. in Necturus. 403 
following fig. 1 2, the mesectoderm of one side of the head unites 
with that of the other. 
At the dorsal margin of the branchial arches the mesectoderm 
of each arch is separate from that of the neighboring arches, in 
the stage of development which the model represents. This separa- 
tion is transitory, and more or less unimportant since the fusions 
taking place between the eranial ganglia are in fact unions of the 
dorsal elements of the mesectoderm, while the later union of the 
branchial cartilages above the gill clefts is not less a fusion of 
mesectodermic elements. Moreover, at the stage given in fig. 1, the 
mesectoderm above the hyoid arch is continuous with that above 
the mandibular arch. Below the hyomandibular cleft the mesecto- 
derm of the mandibular arch fuses with that of the hyoid, both in 
the outer and inner part of the arch (see fig. 1 A), while the mes- 
ectoderm in the posterior and median part of each half of the hyoid 
arch unites with the corresponding mesectoderm of the opposite side 
forming the bridge of tissue seen in fig. 1 g. The hyobranchial cleft 
completely separates the mesectoderm of the hyoid and third arches, 
although the mesectoderm forms a continuous tissue at the ventral 
margins of the posterior clefts. In the slightly older embryo from 
which sections 1 a—17 were made, the hyoid mesectoderm is united 
with that of the third arch below the hyobranchial cleft. Primarily 
the mesectoderm of each arch is separate until in its downward 
migration the ventral margin of the gill cleft has been passed. 
The central cavity or core of each arch is filled by mesothelium 
which joins the pericardial wall where the mesectoderm of the se- 
veral arches ends. The anterior wall of the pericardium lies between 
the transverse bridges of mesectoderm connecting the right and left 
halves of the hyoid and mandibular arches. I have mentioned that 
the core of mesothelial tissue ultimately leaves its central position, 
and breaks through the surrounding mesectoderm at the postero- 
lateral part of the branchial arches, where, on the posterior arches, 
the external gills are borne. This change in the position of the 
mesothelium had not occurred in the embryo modeled. 
Above the vagus arches lie the anterior myotomes of the body, 
while the mesoderm above the remaining arches consists chiefly of 
mesenchyma surrounding the notochord, brain, and cranial ganglia. 
Fig. 3 shows the outer surface of the endoderm of the head 
from a lateral and somewhat ventral aspect. The unshaded strips 
at the mouth and branchial clefts indicate the fusions between ecto- 
27* 
