720 AIMELS. [Vou. XII. 
In addition to these possibilities of some difference in the 
developmental composition of the parts concerned, there is the 
strong probability, already explained, that the dorsal arches 
may, in early stages of development, shift somewhat, with their 
septa, relatively to their vertebrae. Can a dorsal arch, then, 
be definitely assigned in every case, without proper investi- 
gation, to the vertebra to which it has, in the adult, its apparent 
attachment? It seems to me not, and yet it is evidently of the 
first importance to know, definitely, to which of two adjacent 
vertebrae a dorsal arch should be assigned, when the vertebral 
composition of the occipital part of the skull is under consider- 
ation. It is also of first importance in the naming or number- 
ing of the spinal and occipital nerves. 
The spinal nerves are usually assigned to the vertebrae that 
bear the dorsal arches next in front of them, and named in 
consequence. This does not, however, apply to the com- 
plete serial numbering of the nerves, for the nerve in front of 
the first vertebra, between that vertebra and the hind end of 
the skull, is usually counted as the first spinal nerve. Contrary 
to this generally accepted method Willis considered it in man 
as the last cranial nerve, and Froriep (No. 34, p. 230) considers 
the corresponding nerve in the chick as the last occipital nerve, 
or nervus postoccipitalis. That the nerve, in Amia, is in reality 
the first spinal nerve, and not the last occipital one, seems 
certain if Ihering is correct in his statement (No. 61) that 
in the tail of Amia, the nerve of each segment issues in 
front of the arch of that segment. It is evident that the 
general distribution of the nerve itself can give no definite 
solution of the problem, for the nerve belongs, in that distri- 
bution, to the muscle segment between two vertebral septa, 
and not to the vertebrae to which those septa were primarily 
attached. 
Unable from my own work to arrive at any satisfactory 
conclusion on this subject, I follow, for the present, Schmidt 
in the serial numbering of the arches, and Ihering — that is, the 
generally accepted method — in the numbering of the nerves. 
The first spinal arch in all my figures and descriptions is, there- 
fore, the arch that lies between the first and second vertebrae, 

