COMPARISON OF THE GERMINAL LAYERS. 251 



Calberla maintains the same view for Teleostei (fig. 32), hut further 

 observations on this subject are required. In the Teleostei a very 

 shallow depression along the axis of the keel is the only indication of 

 the medullary groove of other forms. 



In Amphioxus (fig. 180), the Tunicata, Petromyzon (?), Elasmo- 

 branchii (fig. 182), the Urodela and Mammalia (fig. 187), the epiblast 

 of the medullary plate is only formed of a single row of cells at the 

 time when the formation of the central nervous system commences ; 

 but, except in Amphioxus and the Tunicata, it becomes several cells 

 deep before the completion of the process. In other types the epi- 

 blast is several cells deep even before the differentiation of a medul- 

 lary plate. In the- Anura, the nervous layer of the epidermis alone 



MC 



iB&a^ 



JEp 



V 



Ch 



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



AFTER IMPRF.GNATION. 



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



is thickened in the 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 for- 

 mation 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 epidermis 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 011 the assumption 

 that the central nervous system was originally developed as a keel- 

 like thickening 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 Chsetopods, it is quite possible that the 



