A CASE OF CYCLOPIA 227 



scattered fashion and occasionally form irregular groups of 

 various sizes. There are no giant cells in this stratum over 

 this area of the cortex. 



The blood supply of the cyclopian cortex, as evidenced by the 

 size and number of blood vessels, seems to be quite as rich as is 

 the case normally. 



With regard to the atypical course of the thalamic projection 

 fibers it is to be noted that their presence in the plexiform layer 

 is only the result of the altered relations between the pallium 

 and the thalamus. Normally in mammals the thalamic fibers, 

 to reach the pyramidal dendrites, must traverse the cortex from 

 within outwards. Harrison (10) has shown that nerve processes 

 wil develop readily even when the neurone is situated in an 

 entirely strange environment. Under the altered form rela- 

 tions, then, in this case it is but natural that these fibers should 

 still, retain their growth energy and pass ventrad along the only 

 course open to them to gain the cerebrum (figs. 46 and 47). On 

 reaching the latter, they mostly take the shortest way available 

 by which they can reach the dendritic processes of the pyramidal 

 elements, namely, by coursing in the zonal layer. The apparent 

 influence of this contact has already been noted. 



It is of interest to note here that the passage of both efferent 

 and afferent projection fibers in the plexiform or zonal layer of 

 the cortex is the normal condition obtain' ng in Amphibia. In 

 these forms the zonal layer represents the only white layer of 

 the cortex visible in transverse section (4). The cortical neu- 

 rones occupy the space between the zonal layer and the epen- 

 dyma. Their axones curve outwards to reach the peripheral 

 white matter, while their dendrites come into contact with 

 the afferent projection fibers in this layer also. 



In the more caudal portions of the thalamus are found numerous 

 fibers which in view of crowding have been unable to reach the 

 cerebrum. Even under such adverse conditions the growth of 

 these fibers was not arrested. They are here woven together 

 into knots and tangles, and numbers of them have already been 

 described as even piercing the limiting layer of neuroglia and 

 ramifying in the pia {(3 figs. 42 to 44) . 



