448 Julia B. Platt 
the branchial clefts are from the first strictly intersegmental, and 
that what seemed a partial division of the vagus somite (second and 
third postotic), is actually a partial fusion of the two vagus somites. 
I call these two somites, vagus somites, for the reason that they lie 
above the vagus arches, respectively external to the two vagus roots. 
The anterior part of the neural crest cells of the vagus Anlage 
extends outwards over the first vagus somite, in the typical manner 
of cranial nerves, while the posterior part of the vagus Anlage, passes 
directly downwards, median to the second vagus somite, in the manner 
of a typical spinal nerve, and becomes attached to the brain in the 
second vagus root. While the first vagus somite appears to be cut 
horizontally into dorsal and ventral parts by the anterior vagus nerve, 
the vagus fibres from the second, posterior, root pass forwards and 
join those of the main root before passing outwards, so that the 
second vagus somite remains entire. These relations were noted in 
my former study, and similar relations have been of late so often 
noted in other Vertebrates that the separation of cranial from spinal 
nerves on the ground of their different relations to the axial meso- 
derm is no longer tenable. The vagus bridges the line of separation 
and unites the two groups. 
The first and second hypoglossus nerves lie median to the fourth 
and fifth postotic somites. In the plane separating the second postotie 
somite from the third, lies the rudimentary neural arch, which is 
taken into the median wall of the auditory capsule (page 436). The 
first postotic somite does not give rise to muscle fibres, hence the 
rudimentary neural arch — the praeoccipital arch — lies between 
the first two myotomes!. In the myoseptum between the third and 
fourth postotic somites lies the occipital arch. In each of the following 
myosepta an arch of the vertebral column is found. The primary 
relations of the occipital arch to the myotomes appears to me to 
1 IT use the word »somite« to designate a primary segmental division of 
the axial mesoderm. NEAL (97), following v. WIJHE (82), uses »myotome« as 
synonymous with »somite«, thus counting with the myotomes, a segmental 
division of the axial mesoderm, such as the fourth somite of v. WIJHE, in which 
no muscle fibres appear. To my thinking, muscle fibres belong to the making 
of a »myotome«, and muscle fibres in a primitive relation similar to that of 
the segmental divisions of the continuous longitudinal muscle of the body. I 
count the most anterior segment of this longitudinal muscle in Necturus, the 
first myotome, although this myotome arises from the second postotic somite, 
while in Petromyzon (NEAL 97) the first postotic somite also contributes a 
muscle segment to the longitudinal muscle. 
