100 ADAM SEDGWIOE. 



munity of origin which renders their adult relations perfectly 

 intelligible. Further, I have no hesitation in saying that His' 

 descriptions of the development of nerve-fibres as processes of 

 central or ganglionic nerve-cells, does not apply to Selachians; 

 inasmuch as nerves are laid down long before any 

 trace of nerve- cells can be made out. The neuroblasts 

 of His and of other authors are nuclei lying in a substance 

 which, after death caused by the ordinary reagents, has usually 

 a fibrous structure. This substance is continuous with, and 

 therefore a part of, the reticulum outside. The cell-processes 

 which have been described as growing out from the neuroblasts 

 are merely parts of this reticular substance, the fibres of which 

 become arranged more or less in the direction of the long axis 

 of the nuclei, and the meshes correspondingly drawn out and 

 narrowed. Many of His' drawings even show that this is so, 

 and an inspection of the specimens leaves no doubt at all about 

 the matter. In short, the development of nerves is not 

 an outgrowth of cell-processes from certain central 

 cells, but is a differentiation of a substance which 

 was already in position; and this differentiation 

 seems to take place from the medullary walls out- 

 wards to the periphery, both in the anterior and 

 posterior roots, and to precede, or to proceed pari 

 passu with, the development of other tissues. The 

 nerve crest is, then, to be regarded as a centre for the growth 

 of nuclei, which spread into the body of the embryo and be- 

 come concerned in the formation of many tissues, nervous 

 tissues amongst the rest. There are many other such centres 

 for the production of nuclei ; for instance, I may mention the 

 walls of the coelom, the caudal swellings, and in the Amniota 

 the primitive streak. All these centres of growth are in 

 so-called epithelial tissues. This is, of course, necessi- 

 tated by the fact that Selachian embryos are at one stage 

 composed entirely — or almost entirely — of these so- 

 called epithelial tissues; as are many embryos, e.g. those 

 of Peripatus and of Amphioxus.^ These facts will be dis- 

 ' The significance of this epithelial structure of the yoiiDg embryo — this 



