BRANCHIAL SENSE ORGANS IN ICHTHYOPSIDA. 129 
ment is practically that of a typical segmental nerve in which 
post and pre-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. 2,3and4). Even with the 
limited amount of material at the writer’s 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 Dr. Marshall (fig. 5), it is seen 
that the state of affairs here 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 seg- 
mental nerve, ganglion, and thickening of a gill-bearing segment, 
is the absence in the olfactory segment of any pre- or post- 
branchial nerves. 
Fig. 2 shows us a_ ganglion fused with an _ epiblastic 
sensory thickening and connected with the brain by a short 
nerve stalk. In fact it is the picture of a branchial sense 
organ and its associated ganglion. 
The facts of development here given, which accord so marvel- 
lously with the development of the other cranial segmental 
nerves, certainly render necessary a modification of Marshall’s 
view as to the nature of the olfactory organ, and in fact a 
modification in the sense of the above passage, in which the 
nose is regarded not as a gill-cleft, but as the sense organ of a 
gill-cleft. 
Marshall based his views firstly on the correspondence in 
_ anatomical and histological structure between the nose and 
other gill-clefts, secondly, on the frequent occurrence of two 
branches of the olfactory nerve, one on each side of the sup- 
VOL, XXVI.—NEW SER. 1 
