198 JOHN BEARD. 



As a consequence, post-branchial and £>rce-branchial nerves were absent, 

 and the whole segmental nerve was reduced to a ganglion and a supra- 

 branchial sensory nerve, this nerve, as its name implies, being con- 

 nected with the innervation of the still-existing branchial sense organs. 

 Of course the main stem of the nerve connecting ganglion and brain 

 was also present. 



A very similar condition of things exists in the nose. The early 

 development has its exact parallel in the development of the nerve of 

 the second segment. The sole difference is that the sense organs of 

 the nose have not, as in the case of those of the second segment, 

 undergone further development in a linear direction (fig. 46), but have 

 confined that development to a somewhat circular area; that is, they 

 have developed in many directions, but to a limited extent in each. 

 A change of function has also probably occurred. In higher forms 

 this, of course, is certain. 



A glance at the diagram (fig. 46) will illustrate the meaning of the 

 above remarks. The supra-branchial nerve of the second segment 

 (s.b.n.) is represented by a line. In the nose (olf.o.) a supra-branchial 

 nerve can hardly be said to be present. The sense organs have 

 developed within an enclosed figure. 



For the rest, the development of the nerve of the first segment is 

 practically that of a typical segmental nerve in which post- and prae- 

 branchial branches are aborted. 



The nerve grows down from the brain to a thickening of epiblast, it 

 fuses with this thickening (fig. 1), and a ganglion is formed at the 

 point of fusion (figs. 2, 3, and 4). Even with the limited amount of 

 material at my disposal, it can fairly well be shown that the ganglion 

 is formed from the skin. 



When the nerve first fuses with the skin, just as in other cases, no 

 ganglion is present (fig. 1). 



The ganglion first develops after the fusion, and from the inspection 

 of figs. 2, 3, and 4, which are camera drawings of actual sections, it 

 will be plain that there are strong reasons for believing that, as in 

 other cases, the ganglion is proliferated from the sensory thickening. 

 At any rate, in a later stage, which has also been figured by 

 Marshall (fig. 5), it is seen that the state of affairs exactly resembles 

 that in the ciliary ganglion and thickening (fig. 8), Gasserian 

 ganglion and thickening (fig. 17), &c. The only difference between 

 the olfactory ganglion and thickening and the complete segmental 



