The Development of the Cartilaginous Skull ete. in Necturus. 421 
Fig. 7 d shows also, in lower magnification, the plate of pro- 
cartilage underlying the auditory vesicle and dorsal to the hyoid 
muscle. In this figure the basicranial plate is narrower but deeper 
than in the preceding figure of the series (7c), and now closely 
invests the lateral surface of the chorda. ‘The section passes through 
the region in which the hyobranchial clefts are united (see fig. 9). 
In the centre of the tissue enclosed between the united hyobranchial 
clefts and the floor of the branchial chamber, the thyroid outgrowth 
is sectioned, dorso-lateral to which are seen the large procartilage 
bars of the glossopharyngeal arch. Attached to the ventral surface 
of these bars, the cerato-hyoideus internus is seen in section, 
while below the thyroid outgrowth the paired muscles genio-hyoidei 
are sectioned. Beneath the hyobranchial cleft lies the wide mylo- 
hyoid muscle. 
The plane of fig. 7 e passes through the procartilage of the occi- 
pital arch. At the base of this arch, the chorda is bounded by 
procartilage from the posterior part of the basicranial plate. The 
large vagus ganglia are sectioned and the second postotic myotome, 
that of the third postotic segment. 
With the description of figs. 7 a—7 e, from the series of sections 
from which the procartilage represented in fig. 7 was modeled, the 
first part of this study closes. 
The purpose of the foregoing pages has been to show, by de- 
tailed comparison of section with section, that from the mesectoderm, 
a tissue surrounding the mesothelium of the branchial arches, and 
bounding the epithelium of the mouth, in the embryo of 11 mn, 
the cartilage of the visceral skeleton and of the anterior part of the 
trabeculae arises, while the remainder of the cartilaginous skull finds 
its origin in the mesoderm. 
It has also been my purpose to show that the mesothelium of 
the branchial arches is converted into muscular tissue. In the third 
part of this paper, I shall show to which part of the mesothelium 
each muscle of the head may be traced, and shall there bring together 
those scattered facts regarding the myology of the head which have 
appeared in the preceding study of successive sections. 
Before passing to the consideration of the cartilaginous stage 
in the development of the skull, I call attention to the fact that 
the origin of the anterior part of the trabeculae from the same kind 
of tissue (mesectoderm) as that which gives rise to the visceral 
