96 The American Naturalist. [February, 
and Selachians. But the fourth seems to be wanting, and corres- 
ponding in position to the third myotome are two small cellular 
masses, not connected with each other, out of which are devel- 
oped the muscles externus rectus and retractor bulbi. Between 
the vagus nerve and the first cervical spinal nerve are four myo- 
tomes, the cephalic of which is rudimentary. The oculomotor, 
trochlear, and abducens nerves are not described in their earliest 
stages. The III. with a broad origin springs from the base of 
the midbrain, and innervates the muscles derived from the first 
head-cavity. The IV. arises as a large cellular outgrowth from the 
place where the roof of the midbrain passes into the hindbrain, 
and’ resembles in every respect the “ Anlagen” of the dorsal 
cranial nerves, sending an extension to the epidermis. The 
absence of a trochlear ganglion in the serpent, bird, and Selachians, 
and its presence in the lizard, gives rise to the query whether the 
trochlear nerve may not primarily have been the motor nerve of 
the protective organs of the parietal eye. The VI. springs by 
10-12 fine fibres from the base of the medulla oblongata, and 
innervates the muscles derived from the two cellular masses that 
appear to belong to the third head-cavity. The V., VIJ—VIIL, 
[X.,and X. nerves take their origin from the neural ridge, ina 
manner similar to the dorsal roots of spinal nerves, and their 
respective ganglia unite with the epidermis above the branchial 
arches. Between the V. and VII—VIII. the neural ridge early 
aborts. The ophthalmic ganglion of the V., from its development 
on a dorsal root and its anastomosing with the III. nerve, # 
ventral root, is regarded as homologous to a dorsal ganglion ; 
The ganglion of the VII—VIII. nerve divides into two portions, 
the anterior part being the proper ganglion of the facial nerve, 
the other forming the auditory ganglion. The accessorio-vagus : 
nerve arises by a broad base extending from the IX. nerve to the 
second cervical spinal nerve. Later the neural ridge loses its 
connection with the brain, and becomes a commissure between the 
second cervical nerve and the caudal vagus root, so the X: 
-then arises by 5-6 roots. The hypoglossus originates by fo 
roots, the caudal root being a branch of the first cervical spinal n 
nerve. Anterior to the roots of the hypoglossus are two rudi- oe 
