582 ALICE JOHNSON AND LILIAN SHELDON. 
that, in Elasmobranchs, all the main branches of the nerve 
except the post-branchial and the part between the ganglion and 
the brain are split off from the epiblast. Van Wijhe holds 
that the epiblast takes some share in the formation of the 
ganglion at least, while Froriep expresses doubt as to this 
point, comparing the fusion to the similar fusion of nerve-cells 
and epithelium cells in the ear. We are strongly inclined to 
the last view. Professor Marshall (16) has shown how very 
early the nerve-cells of the ear become indistinguishably fused 
with it, and there seems no reason why this should not be the 
case with other sense organs. As to the splitting off of the 
nerve-trunks from the skin, Mr. Beard’s observations and 
deductions seem to us inconclusive. 
In Elasmobranchs Professor Balfour mentioned and figured a 
fusion between the mucous canals of the head and the nerves 
supplying them, no line of demarcation existing between the 
two structures (v. loc. cit., pp. 144, 145, plate xi, fig. 7). He 
describes this as occurring first in his Stage P, but it is possible 
that it may take place rather earlier in the Elasmobranchs, as 
it certainly does in the Newt. Mr. Beard seems to have de- 
tected the earlier fusion in Elasmobranchs, and to be unaware 
that the fact of the fusion was described by Professor Balfour, 
who found that the nerves were all derived from the brain out- 
growths, as we believe to be the case in the Newt. It appears 
to us that the epiblast in this animal takes no part in the 
formation of the ganglion or nerve branches, and that the 
special nerve to the sense organ is an outgrowth from the 
ganglion, advancing pari passu with the withdrawal of the 
latter from the surface, so that there is at no time any break 
in the connection between the sense organ and its nerve supply. 
The withdrawal of the ganglion and formation of the nerve 
is only a result of the differentiation of the nerve supply intoa 
ganglionic and a fibrous part. 
The disposition of these sense organs seems to us a very 
insecure guide to the segmentation of the head. Mr. Beard 
considers that the relations of the sense organs to the gill- 
clefts shows them to be of segmental value, since they are in 
