100 THE EPIBLAST. 



the nervous layer of the epidermis, while this layer also has 

 the greater share in forming the olfactory sac. 



In Elasmobranchs the epihlast 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 remain- 

 ing 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 Amphibians. The outer one of the two forms the homy 

 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 

 equivalent parts of the epidermis in Elasmobranchs and Am- 

 phibians, 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 develop- 

 ment. 



In favour of this view are the following points ; (1) 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 development, 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. 



