1891.] Recent Studies of the Vertebrate Head. 339 
of these two nerves is very slight. The IX. nerve arises from the 
concavity between the fourth and fifth neuromeres. The X. nerve 
is evidently made up of the fused roots of several spinal nerves. The 
latter arise like the cranial nerves from corresponding concavities 
in the spinal cord. The cranial neuromeres are to be regarded as 
homologous with the neuromeres of the spinal cord. Orr stated 
that the internal ridges projecting into the fourth ventricle cor- 
responded not to the nerve-roots, but to the spaces between the 
nerve-roots. In Acanthias, Platt (’90) describes a pair of head- 
cavities anterior to the premandibular cavities. This observation 
is of great interest in the light of Dohrn’s recent studies on 
Torpedo. : : 
While many observers have noted the relations of the cranial 
nerves to the neuromeres, McClure (89, ’90) seems to be the first 
to attempt to comprehend the entire brain in a schematic, seg- 
mental arrangement of neuromeres. Basing his observations on 
the embryos of Amblystoma, Anolis, and the chick, he concludes 
that the primitive brain consisted of approximately ten neuro- 
meres, which, beginning with the anterior, he calls olfactory, 
optic, oculomotor, trochlear, trigeminal, abducens, facial, auditory, 
glossopharyngeal, and vagus neuromeres respectively. He follows 
closely the observations of Orr on the lizard, and quotes his 
definition of a typical neuromere.® The forebrain is to be con- 
sidered as consisting of two neuromeres and possibly part of a 
third, the midbrain of two, and the hindbrain of six. “The 
olfactory neuromere is connected with the olfactory nerve.” The 
two neuromeres of the forebrain described by McClure are the 
same as those described by Orr in the region of the thalamence- 
phalon of the lizard posterior to the secondary forebrain. But 
Orr says “they never give off any nerves.” As McClure studied 
Orr's preparations, this disagreement is interesting. The segmen- 
tal nerve belonging to the optic neuromere is assumed to have 
degenerated. The midbrain probably consists of two neuromeres, 
since the III. and IV. nerves arise from this brain segment, and 
the view is further strengthened by the fact that Scott figures i> 
Petromyzon an _appearance of neuromeres in the midbrai... 
Orr. Journ. Morph., Vol. I., No. 2, p. 335. 
