COMPARISON OF THE GERMINAL LAYERS. 301 



decidedly in favour of regarding the condition of the epiblast in 

 the larger group as primitive, and its condition in the smaller 

 group as secondary, and due to the throwing back of the 

 differentiation of the epiblast to a very early period of de- 

 velopment. 



In favour of this view may be urged (i) the fact that the 

 simple condition is retained in Amphioxus through life. (2) 

 The correlation in Amphibia, and the other forms belonging to 

 this group, between a closed auditory pit and the early division 

 of the epiblast into two strata; there being no doubt that the 

 auditory pit was at. first permanently open, a condition of the 

 epiblast which necessitates its never having an external opening 

 must clearly be secondary. (3) It appears more likely that a 

 particular genetic feature should be thrown back in develop- 

 ment, than that such an important feature, as a distinction 

 between two primary layers, should be absolutely lost during 

 an early period of development, and then re-appear 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 system arose phylogenetically 

 as an involution from the exterior, and that the epidermis 

 lining it is merely part of the original epidermis, which has 

 retained its primitive structure as a simple stratum, but is 

 naturally distinguishable from the nervous structures adjacent 

 to it. 



Where the epiblast is divided at an early period into two 

 strata, the nervous stratum is always the active one, and takes 

 the main share in forming all the organs derived from the 

 layer. 



Formation of the central nervous system. In all 

 Chordata an axial strip of the dorsal epiblast, extending from 

 the lip of the blastopore to the anterior extremity of the head, 

 and known as the medullary plate, becomes isolated from the 

 remainder of the layer to give rise to the central nervous axis. 



According to the manner in which this takes place, three 

 types may, however, be distinguished. In Amphioxus the axial 



