222 D. DAVIDSON BLACK 



Cerebral vesicle 



There is no appearance whatever of corpus striatum, rhinen- 

 cephalon, or in fact of any of the structures developed ventral 

 to the recessus neuroporicus in this region under normal circum- 

 stances. The term 'rhinencephalon' is here used to indicate the 

 basal structures of the forebrain in most intimate connection 

 with the olfactory nerve and does not include pallial olfactory 

 centers. It is only for the sake of convenience in the present 

 description that I consider myself justified in employing the term 

 rhinencephalon in this fashion; for when used in the above sense 

 it effectually defines the limits of those areas which are quite 

 absent from this brain. 



Cortex cerebri. The basal portion of the cerebral vesicle varies 

 in thickness from about 10 mm, at its thickest at the point of 

 entrance of the thalamic radiations, to about 5 mm. at its thiij- 

 nest. The cortex shows well developed cell lamination, and the 

 types of lamination vary in different regions. It is impossible, 

 however, to identify these areas as representing any of the his- 

 tologically differentiated regions to be found in the normal cor- 

 tex at birth. 



There is practically no medullated tissue to be found through- 

 out the cortex. Indeed, the only area in which it is prominent 

 at all is at the point of entrance of the thalamic radiations. In 

 this region in figures 46 and 47, the cortex of the left posterior 

 pole is shown cut tangentially. In the second layer of the cortex 

 here, numerous groups of cells (C.isl.) disposed in irregular, more 

 or less circumscribed areas, are to be noted. These islands are 

 made up of two kinds of cells; (a) very numerous, small, embry- 

 onic elements, and (b) large polymorphic multipolar cells. In 

 figure 15 this arrangement is brought out in transverse section. 

 Only over this area of the cortex in this specimen were these 

 large polymorphic elements to be found predominating in the 

 stratum which is normally the layer of small pyramids. It would 

 thus appear that the presence of the fibers of the atypical tha- 

 lamic radiation has exerted an influence upon the growth of these 

 neurones. Just what connection, if any, exists between these 

 cells and the thalamic fibers could not be ascertained. The tis- 



