MYODOME AND TRIGEMINO-FACIALIS CHAMBER 227 
tinuation of the walls of the aortal groove, must be of similar 
origin. Why the aorta has been excluded from this prootic 
portion of the myodome is not apparent, but it would seem as 
if it might be related to the development of the hypochorda. 
According to Stohr (’95), the hypochorda of Rana, when first 
formed, is attached to the dorsal wall of the alimentary canal 
by a series of tubular bridges, which persist longer in the ante- 
rior than in the posterior region of the trunk, and, for a time, 
there prevent the lateral dorsal aortae from fusing with each 
other in the median line excepting between the bridges. In the 
head region the hypochorda is said to develop later than in the 
trunk, and the related bridges would hence there also, while they 
persisted, prevent the lateral dorsal aortae from fusing with each 
other excepting between the bridges. It may then be that, 
such a bridge persisting in the prootic region, the lateral dorsal 
aortae could not there fuse with each other, and before this 
bridge had disappeared they had become fixed in position by 
the early development of the anterior aortic arches. Anterior 
to the spinal region of the cranium they, however, fused with 
each other, in certain fishes, that point either representing an 
interval between two hypochordal bridges, or lying anterior to 
the anterior bridge, as the case may be. This would then not 
only explain the formation of the circulus cephalicus, but also 
account for its position external to the ventral processes of the 
prootics. 
In further support of the assumption that the ventral proc- 
esses of the prootics are formed by ventrolateral vertebral proc- 
esses 1s the fact, possibly significant, that these processes, like 
the neural processes in the spinal region, enclose a large 
canal between their proximal portions and a smaller one be- 
tween their distal ends, the two cavities being separated from 
each other by a horizontal partition. In my 5l-mm. specimen 
of Hyodon this partition is partly of cartilage and partly of 
membrane. In all the other fishes examined it is wholly of 
membrane, excepting as that membrane may have’ undergone 
ossification as part of the parasphenoid, a median longitudinal 
opening thus being left, when the parasphenoid is removed, be- 
