104 ADAM SEDGWICK:. 



of the reticulum. The only exception to this rule is the 

 third nerve of Scyllium and Acanthias (and probably 

 others), which is undoubtedly differentiated from the ciliary 

 ganglion to the floor of the mid-brain ; but this is, perhaps, 

 more an apparent exception than a real one, because the 

 ciliary ganglion belongs to the fifth nerve and the order of 

 fibrous differentiation is normal, viz. from the root of the fifth 

 nerve, through the ciliary ganglion, to the floor of the mid- 

 brain. I commend this observation on the development of the 

 third nerve to the physiologist, with a view to a renewed 

 investigation of its functions. It is rendered the more inter- 

 esting by the fact that in Lepidosiren it is commonly stated 

 that the area of the third nerve is supplied by the ophthalmic 

 branch of the fifth, the third nerve being absent.^ 



I have already, in my ' Notes on Elasmobranch Develop- 

 ment,' stated my reasons for believing that the views put 

 forward by Hensen as to the origin of nerves were nearer the 

 truth than those of any other zoologist. I have, in this paper, 

 shown not only that the network required does exist, but also 

 how it arises, and how it gives rise to the rudiments of the 

 peripheral nerve-fibres. Minot, in his ' Human Embryology,' 

 p. 624, says that Hensen's theory of the origin of nerves 

 "cannot be adopted because the outgrowths of the nerve-fibres 

 have been observed ; moreover, Altmann has pointed out that 

 the fibres seen in the embryonic mesoderm are really pro- 

 cesses of the mesoderm cells, and, as shown in the excellent 

 fig. 2 of his plate, are quite distinct both from the ecto- 

 derm and endoderm." (The italics are mine.) This passage 

 is, according to my work, full of errors ; for I maintain, as 

 the result of long and careful observation, extending over 

 many years, that the outgrowth of nerve-fibres from cells in 

 the ganglia and medullary wall not only has not, but cannot 



> This statement rests on Hyrtl's work. It must, however, be remembered 

 that his specimen was confessedly rotten in its nervous tissues, and by the 

 fact that V. Wijhe ("Nied. Arch. f. Zoologie,' Bd. v, 1882) has found the 

 third nerve in Ceratodus. Parker does not deal with the brain and nerves 

 in his memoir on Frotopterus. 



