4 NAKAGAWA. [Vol. IV. 



are slightly larger than those of the II, and they are very much 

 less in number. Next is the layer of ordinary cells, through 

 which pass the fibres of the corpus callosum. 



It seems very likely that the first three (II-IV) of the four 

 gray layers have been developed from the first cortical layer of 

 the Sauropsida {c^ ; in other words, I am led to homologize 

 these with each other, and also the layer V with the second cor- 

 tical layer (r^"). I find in the young squirrel embryo {Schirus) 

 a stage corresponding to the sauropsidan structure; the first 

 gray layer is very thick, and the second is not so rich in cells as 

 the first. I was not fortunate enough to obtain a stage in which 

 the first gray would be in the process of differentiation into 

 three, but the general disposition was such as to justify, in 

 my opinion at least, the conjecture stated above. 



To summarize : we find in the cells of the gray matter sur- 

 rounding the ventricle a tendency to migrate toward the super- 

 ficies. This process, as we have seen, is in progress in the 

 Amphibia, and initiates the reduction of the ventricular gray, 

 from its condition as the main cellular element of the hemi- 

 spheres to its entire absence in the adult condition of the 



higher forms. 



PART II. OPTIC LOBES. 



I. Amphibia. — The mesencephalon in the Urodela is a tub- 

 ular structure, — gray substance lining the central cavity, the 

 mesocoele, and showing, in most cases, no differentiation. In 

 some salamanders {Spelerpes), however, the cells are arranged 

 in concentric rows, and at places these rows, leaving little spaces 

 between them, anticipate the more complex structure of the 

 Anura. In the latter the mesencephalon assumes a bilobular 

 structure and forms a prominent portion of the brain. Professor 

 Osborn (2, p. 82), in his " Internal Structure of the Amphibian 

 Brain," has described eight distinct layers in the tectum opticum 

 of Rana, — four white layers alternating with the four gray 

 layers, and also traced the optic nerve-tract into the first and 

 second white layers. 



These layers in Rana are so beautifully differentiated that 

 they may be taken as a standard of comparison for those of 

 the Sauropsida. Taking them in order from without inwards, the 

 characters of these layers are as follows : The layer ^A] (Fig. 6) 



