COMPARISON OF THE GERMINAL LAYERS. 303 



cata, it becomes several cells deep before the completion of the 

 process. In other types the epiblast is several cells deep even 

 before the differentiation of a medullary plate. In the Ariura, 

 the nervous layer of the epidermis alone is thickened in the 



V 



FIG. 191. SECTION THROUGH AN EMBRYO OF LEPIDOSTEUS ON THE FIFTH DAY 



AFTER IMPREGNATION. 

 MC. medullary cord; Ep. epiblast; Me. mesoblast; hy. hypoblast; Ch. notochord. 



formation of the central nervous system (fig. 72) ; and after the 

 closure of the medullary canal, the epidermic layer fuses for a 

 period with the nervous layer, though on the subsequent forma- 

 tion of the central epithelium of the nervous canal, there can be 

 little doubt that it becomes again distinct. 



It seems almost certain that the formation of the central 

 nervous system from a solid keel-like thickening of the epider- 

 mis is a derived and secondary mode ; and that the folding of 

 the medullary plate into a canal is primitive. Apart from its 

 greater frequency the latter mode of formation of the central 

 nervous system is shewn to be the primitive type by the fact 

 that it offers a simple explanation of the presence of the central 

 canal of the nervous system ; while the existence of such a canal 

 cannot easily be explained on the assumption that the central 

 nervous system was originally developed as a keel-like thicken- 

 ing of the epiblast. 



It is remarkable that the primitive medullary plate rarely ex- 

 hibits any indication of being formed of two symmetrical halves. 

 Such indications are, however, found in the Amphibia (fig. 192 

 and fig. 72) ; and, since in the adult state the nervous cord 

 exhibits nearly as distinct traces of being formed of two united 

 strands as does the ventral nerve-cord of many Chaetopods, it is 



