436 Julia B. Platt 
still unformed. The membranous ear, the ductus endolymphaticus, 
and the auditory ganglion are sectioned. 
The plane of fig. 29 passes through the posterior part of the 
auditory capsule. The eartilaginous floor of the capsule is seen in 
section at the left, and is found to be no longer connected with the 
basal plate seen at the right of the figure. A double row of cells 
however connects the basal plate with a bar of cartilage 
which lies between the membranous ear and the brain, and 
which, in later development, becomes part of the postero-median 
wall of the capsule. The cartilage of the basal plate is here »mesotic<, 
and the region is that in which the posterior of the two primary 
connections between the basal plate and the auditory capsule occurs. 
A section through the occipital arch is shown in fig. 30. At 
the left of the figure is seen the vagus ganglion above which mus- 
cular tissue from the second myotome is sectioned. Attention need - 
hardly be directed to the striking similarity between the cartilaginous 
arch seen in fig. 30, and the incomplete arch of fig. 29, or to the 
manifest difference in the relation of the basal plate to the otic 
capsule demonstrated by comparison of fig. 29 with fig. 28. 
The occipital arch, like the following neural arches, lies prima- 
rily in a myoseptum, and in fact in the myoseptum which separates 
the second myotome from the third (the third postotic somite from 
the fourth). In the next posterior myoseptum lies the first neural 
arch of the trunk, and in the plane which separates the first myotome 
from the second (the second postotie somite from the third) lies that 
anterior rudimentary arch which is taken into the median wall of 
the ear. We have therefore in the skull of Necturus traces of two 
cartilaginous arches shown by their relations to the myotomes to be 
serially homologous with the neural arches of the trunk. The anterior 
of these arches becomes completely incorporated in the median wall 
of the auditory capsule, while the posterior, or occipital arch fuses 
with the auditory capsule dorsally and ventrally, but remains medianly 
separate, allowing thus the exit of the glossopharyngeal and vagus 
nerves. The two arches are connected from side to side by an 
interoccipital cartilage, which, like the cartilage connecting dorsally 
the neural arches of the trunk, arises from paired Anlagen. 
Since that part of the inner wall of the auditory capsule with 
which the cartilage roofing the posterior part of the brain becomes 
connected, is derived in Neeturus from a praeoccipital arch, the 
appellation tectum interoecipitale seems more adequately deserip- 
> a. oe 
