3 5^ Journal of Comparative Neurology and Psychology. 



fore, the tracts of medullated fibers become more densely me- 

 dullated in the older animals. 



The area of the cross section of the encephalon has in- 

 creased in size, and incipient sulci are distinctly more marked 

 in the adult than in the newborn brain. At the temporal mar- 

 gin of the cortex a sulcus, scarcely indicated at birth, has be- 

 come well marked in the course of dev^elopment. The increase 

 in size of the temporal lobe seems, as indicated in the draw- 

 ings, not to have been a progressively symmetrical growth ; up 

 to thirty days the ventral portion has developed most, after 

 that the ventro-lateral portion undergoes the greatest change. 



Cerebellum. 



The sections from which the drawings were made were 

 taken in the median sagittal plane, passing therefore through 

 the vermis. The changes in this part of the encephalon are 

 readily appreciated from comparison of the drawings. 



As to general contour, it will be seen that the folia are 

 numerous at birth, and in the course of development become 

 larger and more pronounced. The Purkinje cell layer is marked 

 in the drawings by a white line (the cells not staining by this 

 method) separating the molecular and granular layers. 



At birth the molecular la}'er is free from medullated fibers. 

 The granular layer contains fibers radiating from the white lam- 

 inae ; these fibers are especially numerous at the apex of the 

 laminae. In the granular layer very many fibers are seen run- 

 ning more or less parallel to the white laminae. Further out 

 in the granular layer such fibers are shorter and finer. Almost 

 all the fibers seem to be medullated in the white laminae even 

 at birth, and are densely packed together. 



There is a great increase in the number of fibers found in 

 the granular layer from birth to maturity. Such an increase is 

 particularly marked in the fibers at the junction of the laminae 

 and granular layer. It can be seen that the folia, present at 

 birth and reaching the surface of the vermis, tend to divide as 

 the animal becomes older, and that the folia deep-seated at birth 

 push their way towards the surface. 



[64] 



