ON THE CELLULAR THEORY OF DEVELOPMENT. 95 



puted by many morphologists, but they are easy of proof by 

 the simple inspection of good preparations to minds not warped 

 by the cellular theory as ordinarily taught. In fact, had it not 

 been for the undue persistence of this hypothesis beyond the 

 time of its fruitful life, they would have been recognised long 

 ago, and much needless waste of labour in trying to make the 

 facts of nerve-development conform to the theory would have 

 been saved. 



The nerve-crest in Selachians (Scy Ilium, Acanthias, 

 Raia, and Pristiurus) is, as I pointed out some time ago 

 (^^ Notes on Elasmobranch Development,'' ' Quart. Journ. of 

 Micr. Sci.,' vol. xxxiii), from its first appearance, in three 

 pieces.! Tj^g gj-st of these pieces reaches from the region of 

 the fore-brain to the hind brain. The posterior limit of it is 

 marked in older embryos by the root of the trigeminal nerve. 

 It gives rise to the reticulum of the front part of the head, 

 and contributes to that of the mandibular arch. The 

 following nerves are formed within its limits : — The trigeminal 

 and its branches, which include the so-called ramus ophthal- 

 micus profundus with the ciliary ganglion and the third 

 nerve (see below). Very possibly other nerves, viz. the 

 fourth, the sixth, and the olfactory, may be also developed 

 from this part of the reticulum, but I have no observations on 

 this point. 



The manner in which these nerves are laid down may be 

 described as follows : — When the nerve-crest, which in this 

 region of the head very early spreads ventralwards on each 

 side of the brain, is breaking up into the reticulum, certain 

 tracts of it remain unaltered and characterised by a greater 

 density of nuclei. These tracts mark the course of the future 

 nerves and the sites of the future ganglia. They them- 



coUection of the nuclei at the surfaces, as it may be described — I hope to con- 

 sider in another place. Now, I may merely hint that it is probably due to the 

 impress of some well-marked larval phase in earlier stages of evolution (see my 

 article on " von Baer's Law, &c.," in ' Quart. Journ. Micr. Sci.,' vol. xxxvi). 

 ' Goronowitsch (' Morph. Jahrb.,' Bd. xx) has recently found the same fact 

 for the bird, but he makes no reference to my results on this point. 



