No. 3.] MUSCLES AND NERVES [IN AMIA CALVA. F274 
two ventral occipital roots and the nervus vagus thus issue, at 
this age, through the same foramen. The occipitale laterale, 
at this age, may therefore be considered as a dorsal occipital 
arch that has inclined forward against the hind end of the 
auditory vesicle, or as parts of three such arches that have 
fused with each other and are about to fuse with the auditory 
vesicle, as later stages show. In the former case, the arch, 
inclining forward, has pushed before it, against the vagus, the 
dorsal roots of two occipital nerves; in the latter, the two 
dorsal occipital roots, if they do not abort, have, in joining and 
fusing with the vagus, cut through the ventral ends of the 
«“Anlage”’ of two dorsal arches, so retarding or deranging their 
development that no trace of their ventral ends is found in 
12 mm. larvae. The vagus, under either supposition, would lie 
immediately behind the hind wall of the auditory vesicle, and 
would, if the dorsal occipital roots have not entirely disappeared, 
be formed by the fusion of the dorsal roots of three or more 
postauditory nerves. 
That the occipitale laterale represents three, rather than one, 
dorsal arch is shown by its relations to the muscle segments 
and intermuscular septa. The first septum, the one that lies 
between the first and second segments, has its attachment, in 
I2 mm. larvae, near the upper, outer end of the occipitale 
laterale ; the second has its attachment near the middle of 
that piece, and the third, to the elevated portion of its lower 
end. The fourth septum is attached to the first occipital arch, 
the fifth to the second occipital arch, the sixth to the first 
spinal arch, etc. 
The dorsal portion of the first muscle segment, the seg- 
ment lying in front of the first septum as above defined, 
arises, at its anterior end, from the hind wall of the auditory 
capsule, and extends across the upper, unformed end of the 
occipitale laterale. The ventral portion of this same segment 
arises from that thickened portion of the base of the skull 
that forms the hind boundary of the eye-muscle canal, and lies 
along the ventral portion of the lateral surface of the hind 
end of the skull, appearing, in vertical sections, below the 
notochord. It extends nearly, if not quite, to the anterior end 
