228 Royal Society :— 
lies between the origin of the nerve / and the eye, answers to all 
that part of the brain which lies between the origin of the seventh 
nerve of Petromyzon and the optic nerve. - Consequently the lateral 
walls of the neural canal in the same region answer to that region 
of the skull in Petromyzon which lies between the origin of the 
seventh and the origin of the optic nerve. Hence, as each myo- 
tome of Amphiovus represents the corresponding portion of a 
protovertebra, it follows that the same region of the skull in the 
Lamprey and other Vertebrata represents, at fewest, six protover- 
tebrx, almost all traces of which are lost, even in the embryo con- 
dition of the higher Vertebrata. 
It may further be concluded that the several pairs of nerves 
which leave the cerebro-spinal axis, between those which answer 
to the portio dura and the optic nerve, in Amphiowus, are repre- 
sented by the third, fourth, fifth, and sixth pairs of cranial nerves 
of the higher Vertebrata. The nerve a, in fact, has the charac- 
teristic course and distribution of the orbito-nasal division of the 
trigeminal ; while, without at present drawing a closer parallel, 
it is easy to see that the nerves 6,c,d,e,f, and g, with their 
respective myotomes, supply the requisite materials for meta- 
morphosis into the oculomotor, pathetic, trigeminal, and abducens 
nerves, with the muscles of the eye and of the jaws, in the more 
differentiated vertebrate types. 
Thus that part of the cerebro-spinal axis of Amphiowus which 
lies in front of the seventh myotome answers to the preauditory 
part of the brain in the higher Vertebrata, and the corresponding 
part of the head to the trabecular region of the skull in them. 
On the other hand, from the seventh myotome backwards, a certain 
number of segments answer to the postauditory, or parachordal, 
region of the skull of the higher Vertebrata. 
The answer to the question, how many? involves sundry con- 
siderations. It must be recollected that though the branchial 
chamber of Amphiovus is the homologue of the branchial chamber 
of other Vertebrata, it does not necessarily follow that the im- 
perfect branchial skeleton of Amphioxus corresponds with their 
branchial skeleton. The branchial skeleton of the higher Verte- 
brata consists of cartilaginous rods, which seem to be developed 
in the somatopleure, and to be homologous with the ribs, while the 
branchial skeleton of Amphiovus consists of fibrous bands appa- 
rently developed in the splanchnopleure. 
The branchial arches of the higher Vertebrata, in accordance 
with their essentially costal nature, receive their innervation 
from the glosso-pharyngeal and pneumogastric nerves, which are 
homologues of spinal nerves; and, in seeking for the posterior 
limits of that region in Amphiowus which corresponds with the 
skull and brain in other Vertebrates, we must only take into 
account as many pairs of those nerves which arise from the 
cerebro-spinal axis as we know are, in the Vertebrata next above 
Amphioxus, devoted to the branchial arches. In none of these 
are there more than seven pairs of branchial arches; so that not 
