86 Journal of Comparative Neurology. 



Hum is that seen in nearly all regions of the amphibian brain 

 where there is no sharp distinction of region, and where the 

 ventricular and cortical layers are not yet distinguished. The 

 number of fibres is, as yet, insufficient to develop a white layer. 

 The pallium, therefore, consists, at that stage, solely of (i) outer 

 neurolgia and fiber layer, (2) cellular layer, and (3) epithelial 

 layer (i. e., layer of the nuclei of the spongioblasts.) Two ele- 

 ments introduce the differentiation; first, the increase of cellular 

 elements adjacent to the ventricle through the process of pro- 

 liferation of neurons to serve as reserves (as well as possibly for 

 nutritive purposes) and, second, the increase of fibres forming 

 distinct tracts and a more or less continuous white layer. 



In many turtles the whole pallium retains much of its 

 primitive simplicity. The pallium is relatively large and of a 

 nearly uniform structure, and its cellular elements are in a com- 

 mon stratum, or at least, there is no marked line of separation. 



We believe that it may prove expedient to distinguish two 

 varieties of cortex. The first or more primitive type is produced 

 by a migration of the whole cellular area to a belt some distance 

 from the ventricle. This is found in almost the entire cortex of 

 turtles and characteristic of the occipital and dorsal regions of 

 serpents and lizards, and is retained in the hippocampal cortex 

 of higher vertebrates. The second type is that which is found 

 in other regions of the pallium where the ventricular belt is re- 

 tained and a second band of cortex is differentiated. Both these 

 forms are derivable from the embryonic structure where both 

 are united in a common stratum about the ventricle. The latter 

 condition is that which is, with slight modification, the perma- 

 nent one in amphibians. The complicated cortex of mammals 

 is a modification of the second type above mentioned. 



We conceive that we are warranted in insisting upon the 

 essential uniformity of the neuron, whether it takes part in the 

 formation of a nerve or of a tract or of a cortical area. It is 

 essentially a cell with processes which form associations, if not 

 anastomoses, with like elements, and thus produces chains of 

 progressive association and translation. The neuroblasts, which 

 arise from the ventricle, migrate from it to a suitable position, 



