The Development of the Cartilaginous Skull ete. in Necturus. 435 
nerves leave the brain case through a foramen formed by the union 
of the base and dorsal extremity of the oceipital arch with the otie 
capsule. The opercular cartilage now closes a large part of the 
fenestra ovalis. The abducens nerve passes through the trigeminal 
ganglion and onwards with the ramus ophthalmicus profundus. 
The relations of the facial part of the lateral line system are as in 
the younger embryo. 
A bridge of cartilage now roofs the posterior part of the brain, 
and is laterally continuous with the cartilage formed by the union 
of the dorsal extremity of the occipital arch with the postero-dorsal 
part of the inner wall of the otic capsule. This interoccipital bridge, 
as previously described, arises independently from paired Anlagen. 
In an earlier stage of development the neural arches become 
connected with one another at their dorsal extremities by a pair of 
parallel, longitudinal bars of procartilage, which begin to chondrify 
in continuity with the cartilage of the arches. Before the chondri- 
fication is complete, a bridge of procartilage connects the two 
longitudinal bars between the successive arches. This bridge also 
chondrifies and the continuous longitudinal bars of cartilage break 
into pieces that articulate with one another posterior to each trans- 
verse bridge. Thus the spinal cord becomes partly roofed, and the 
articulating processes of the vertebrae are formed. 
I believe that in the paired Anlagen of the interoccipitale, 
we find the equivalent of the longitudinal bars connecting the dorsal 
extremities of the neural arches of the spinal column, and that the 
union of these Anlagen to form the interoccipital cartilage corre- 
sponds to the transverse union of the longitudinal bars of the trunk. 
Moreover, it seems that in Necturus the interoccipital cartilage 
actually connects two arches of which the occipital is the posterior. 
Figs. 28, 29, and 30 show three sections from an embryo in 
which as yet but one cartilaginous bridge — that immediately posterior 
to the facial ganglion — connects the auditory capsule with the 
basal plate. This bridge is seen in fig. 28, where it is found that 
the cartilage of the basal plate is directly continuous with the 
cartilaginous floor of the auditory capsule. The continuity exists 
but for a few sections, and is almost immediately interrupted at 
the point s. The tissue at the left of this point belongs to the ear, 
while the cartilage at the right is from the trabecular part of the 
basal plate. The figure also shows that in the plane of section, 
the median and dorso-lateral walls of the cartilaginous capsule are 
29* 
