200 Transactions of the 



corneous layer in the form of two not very prominent elevations, 

 so that a slight depression or groove results. We observe further 

 that from the nervous layer a process extends deeply, and touches 

 the chorda dorsalis, which has differentiated itself of the third layer, 

 or motoric-germinative layer. 



It is clear without more explanation that this median process 

 of the nervous layer represents the central nervous system. (Com- 

 pare Fig. 16.) 



If we return to an earlier stage, or if we compare the sections 

 which are made from nearer the caudal extremity, then we are con- 

 vinced that the earliest indication of the central nervous system is 

 always to be recognized as a solid process of the nervous layer, 

 which is directed deeply, which process pushes before it the layer 

 (lying under it in the middle line) more towards the yolk, and also 

 becomes elevated towards the surface in the form of two elevations, 

 which are, however, but slight. 



Whether these elevations grow against each other so as to 

 transform the slight groove between them to a canal, that is to say, 

 whether the mode of establishing the central canal of the central 

 nervous system is the same as in Batrachiaj and cheek, or whether 

 the central canal, which is to be found in somewhat later stages in 

 the central nervous system, develops as a split in the solid nervous 

 process, I am not able to say with certainty; although a great 

 number of sections from different periods were more favourable to 

 the latter view than to the former. 



