458 Julia B. Platt 
c) The second postotie somite is divided into a dorsal and ventral 
part by the vagus ganglion. Each part gives rise to musele fibres. 
The ventral fibres atrophy, while the dorsal fibres become attached, 
without myoseptum, to the fibres formed in the dorsal part of the 
third postotic somite, and are permanent. 
d) In the plane separating the second postotic somite from the 
third, or the first myotome from the second, lies the praeoccipital 
arch. In the myoseptum between the third and fourth postotie 
somites, the second and third myotomes, lies the occipital arch. In 
each following myoseptum, an arch of the vertebral column is found. 
There are consequently three occipital somites in the head of Nec- 
turus. 
e) Unless SEWERTZOFF mistook, as seems to me probable, the 
muscle formed in the ventral part of the second somite for a muscle 
formed in the first somite, there is one more segment in the head 
of Necturus than in Siredon, and this segment is the first postotie. 
f) The first and second vagus roots lie respectively median to 
the second and third postotic somites. The two hypoglossus nerves 
lie respectively median to the fourth and fifth somites. The anterior 
hypoglossus nerve innervates the muscle fibres derived from the 
second, third, and fourth postotic somites. 
g) The ventral hypoglossus musculature arises by buds from 
the second, third, and fourth myotomes (third, fourth and fifth post- 
otic somites). The second myotome gives rise to the muscle genio- 
hyoideus. The third myotome gives rise to the muscle sterno- 
hyoideus. 
h) The myotome which gives rise to the muscle genio-hyoi- 
deus lies above, not posterior to, the fifth visceral (second vagus) 
arch, and the bud which formes this musele consequently grows first 
posteriorly around the fifth visceral cleft, thus indicating that the 
somite in question has migrated forwards from a position originally 
posterior to the branchial arches. The vagus somites should conse- 
quently be regarded as elements foreign to the segments in which 
they lie, and the sixth somite (v. WisHE) is probably not an integral 
part of the primitive vertebrate head, to which the branchial arches 
and the three posterior cranial nerve roots, one glossopharyngeal 
and two vagus, belong. 
i) The second postotie somite in Necturus is possibly the 
homologue of the third postotie somite in Acanthias. 
j) The branchial clefts are intersegmental in original position. 
