318 DEVELOPMENT OF ELASMOBRANCH FISHES. 



of the epidermis, while this layer also has the greater share in 

 forming the olfactory sac. 



In Elasmobranchs the epiblast is at first uniformly composed 

 of a single row of cells. The part of the layer which will form 

 the central nervous system next becomes two or three cells deep, 

 but presents no distinction into two layers ; the remaining 

 portions of the layer remain, as before, one cell deep. Although 

 the epiblast at first presents this simple structure, it eventually, 

 as we have seen, becomes divided throughout into two layers, 

 homologous with the two layers which arise so early in Amphi- 

 bians. The outer one of the two forms the horny layer of the 

 epidermis and the central epithelium of the neural canal. The 

 inner one, the mucous layer of the epidermis and the nervous 

 part of the brain and spinal cord. Both layers apparently enter 

 into the formation of the organs of sense. 



While there is no great difficulty in determining the equiva- 

 lent parts of the epidermis in Elasmobranchs and Amphibians, 

 it still remains an open question in which of these groups the 

 epiblast retains its primitive condition. 



Though it is not easy to bring conclusive proofs on the one 

 side or the other, the balance of argument appears to me to be 

 decidedly in favour of regarding the condition of the epiblast in 

 Elasmobranchs, and most other Vertebrates, as the primitive one, 

 and its condition in Amphibians as a secondary one, due to the 

 throwing back of the differentiation of their epiblast into two 

 layers to a very early period in their development. 



In favour of this view are the following points: (i) That a 

 primitive division of the epiblast into two layers is unknown in 

 the animal kingdom, except amongst Amphibians and (?) Osseous 

 Fish. (2) That it appears more likely for a particular feature of 

 development to be thrown back to an earlier period, than for 

 such an important feature as a distinction between two primary 

 layers to be absolutely lost during an early period of develop- 

 ment, and then to re-appear again in later stages. 



The fact of the epiblast of the neural canal being divided, 

 like the remainder of the layer, into nervous and epidermic 

 parts, cannot, I think, be used as an argument in favour of the 

 opposite view to that here maintained. 



It seems probable that the central canal of the nervous 



