TRANSPLANTATION OF HEMISPHERES 165 



and 5, and 3 and 6). The reduction in size, which is the only 

 apparent distortion, is due to the lack of the ascending fibers. 

 But the dorsal half of the hemisphere, whose functioning is 

 entirely olfactory, is practically completely developed with the 

 exception of the tracts connecting it with the rest of the brain 

 stem. 



Of the three differentiated regions of the central gray, the 

 anterior olfactory nucleus, the nucleus medianus septi, and the 

 primordium hippocampi, the first is the only one normally 

 differentiated. The nucleus medianus septi is reduced in size 

 relatively, as is also the primordium hippocampi. 



In a pre\dous paper (Burr, '16) it was shown that in the absence 

 of the olfactory nerve entering the hemisphere, the latter never- 

 theless showed all the typical differentiation of the central gray, 

 although reduced in size absolutely. The type of operation there 

 reported did not preclude the entrance of ascending fibers from 

 low^er centers in the brain stem. Hence it is reasonable to sup- 

 pose that a part of the differentiation of the nucleus medianus 

 septi and the primordium hippocampi is due to the ingrowth 

 of centripetal fibers. 



Herri-ck ('10, p. 424) suggests that the nucleus medianus septi 

 arises through the wandering into that region of neuroblasts 

 from other areas. From the above it seems more probable that 

 this specialized area of the central gray comes about through 

 proliferation of neuroblasts already present as a result of the 

 stimulus given by the ascending fibers. 



It is evident, then, that the factor which produces the second 

 phase of nervous development in the telencephalon is not the 

 functional activity of the nasal epithelium transmitted through 

 the olfactory nerve, but rather the stimulus afforded by the 

 actual ingrowth of neuraxes into the w^all of the hemisphere. 

 Only in the very young stages of development, however, do the 

 olfactory neurones actually come into intimate relationship with 

 the cell bodies of the secondary olfactory neurones. In the 

 older stages the primary neurones interlace in the glomeruli 

 with the secondary neurones at a considerable distance from the 

 cell bodies from which their dendrites arise. This would seem 



