The Development of the Cartilaginous Skull ete. in Necturus. 419 
fuse with one another, so that the external opening is one wide 
cleft extending from side to side of the head. The opening of the 
branchial chamber to the exterior by means of the hyobranchial clefts 
is still retained in the embryo of 15mm, although the thin walls 
' that bound the clefts are closely applied to one another, and form 
in section hardly more than a line through the surrounding tissues. 
Fig. 7d shows a transverse section through the region in which the 
hyobranchial clefts are united in one wide cleft. 
The »horns« of the bar of the glossopharyngeal arch come from 
beneath the hyobranchial pockets, between these pockets and those 
of the third branchial clefts. The ventral margins of the third clefts 
now meet the external opening of the united hyobranchial clefts. 
Between the third and fourth clefts lie the bars of the first vagus 
arch, which pass forwards ventral to the point where the third clefts 
meet the hyobranchial, and here fuse with the bar of the glosso- 
pharyngeal arch. The fourth cleft lies between the two vagus bars. 
The fifth cleft, which is not included in the figure, lies between the 
second vagus bar and the trunk of the embryo. This cleft is small, 
and still lies in a plane nearly transverse to the long axis of the 
embryo. 
In fig. 7a, which represents a section through the mesectodermic 
part of the trabeculae, I would call attention to a closer grouping 
of the cells in the neighborhood of the trabeculae, between the eye 
and the dorsal wall of the mouth. These cells are mesectodermic, 
and ultimately give rise to the antorbital process. 
Fig. 75 shows a section throught the anterior extremity of the 
basicranial plate. Although at this stage of development the tissue 
which composes the plate seems homogeneous, true cartilage forms 
only in the posterior part of the plate and in the lateral margins 
of the anterior part. That part of the plate which surrounds the 
point of the chorda disappears without giving rise to skeletal tissues. 
‘The procartilage of the quadrate and of Mecker's bar is seen in 
fig. 75, and between these two elements of the mandibular arch lies 
the mandibular muscle. The ventral part of the section passes through 
the anterior part of the mylohyoid muscle. A diverticulum from the 
floor of the branchial chamber is formed by the united bases of the 
hyomandibular pockets (compare fig. 9), and the median space thus 
surrounded by endoderm is the anterior part of the hyoid arch. 
In fig. 7¢ a section is seen, which passes through the anterior 
part of the auditory vesicle. Here the basicranial plate is divided 
28* 
