2 NAKAGAIVA. [Vol. IV. 



PART I. THE CEREBRAL CORTEX. 



1. Amphibia. — In Amphibia the gray matter is mainly con- 

 fined to the ventricular area, extending not more than one-third 

 of the way out toward the surface (Fig. i, vg). But at the inner 

 corner of the cortex we find a ujimber of cells zvhick 7mist be 

 considered as constituting the rudimentary cortical layer. My 

 first reason for taking this to be the rudiment, is the fact that 

 the portion of the hemisphere where these cells are found coin- 

 cides with the portion in the reptilian brain, where the differen- 

 tiation of the layers is most complete ; and the inference is, that 

 the scattered cortical cells must be developed first in this portion 

 before they make their appearance continuously in the mantel, 

 in the form of a cortical layer. It is highly probable, therefore, 

 that we shall find in the reptilian embryo a stage corresponding 

 to the amphibian structure as just described. This statement is 

 further justified by my observations in mammalian embryos, 

 which, at a certain stage, present the reptilian structures as de- 

 scribed subsequently ; in short, by Von Baer's law, by which the 

 higher forms repeat the structure of the lower in their ancestral 

 history. The second and conclusive argument is, that these cells 

 are situated superficially to the fibres of the corpus callosum, and 

 consequently cannot be regarded as ventricular gray substance 

 (Fig. I, eg). In Rana catesbiana and MenobrancJius these cells 

 are irregularly arranged ; but in Spelerpes ruber, from which the 

 figure has been taken, a decided tendency to arrangement into a 

 layer can be seen. Rows of from four to six cells are joined in 

 lines parallel to the surface, making a sort of disconnected layer. 



2. Reptilia. — In Tropidonotus (Fig. 2) the cortical layer is 

 distinctly developed, and in close examination it will be seen 

 tJiat there are two cell-layers in the cortex, specially marked at 

 the inner corner. Thus making, in all, four layers for the cortex 

 of Tropidonotus, viz. : first, the superficial white layer (Fig. 2,f') ; 

 second, the first gray layer {eg') [Edinger's] ; third, the second gray 

 layer, or the layer of cells found among the fibres of the corpus 

 callosum {eg") ; and lastly, the ependyma {ep). This disposition 

 of the two gray layers is especially interesting, because it corre- 

 sponds to the typical structure of the cortical gray substance 

 in Aves as well as in Reptilia, and thus appears to be the charac- 

 teristic feature of the cerebrum of Sauropsida (Fig. 2, cg\ eg"). 



