42 



NOBUHARU SUITSU 



after its thickness is attained, is largely caused by the thickening 

 of the fibers and the formation of the myelin sheaths (Sugita, '17). 

 According to the measurements of Donaldson and Nagasaka 

 ('18), which were made on the largest fibers in the dorsal and 

 ventral roots of a spinal nerve, all such fibers enlarge to the same 

 extent, and the axis-sheath relation in the fibers was found 



C^=^ 



D 



B 



c^ 



C F 



Fig. 1 Outline drawings of the cross-section of the corpus callosum. Albino- 

 rat. Enlarged ten times. A, Just born; B, 10th day; C, 20th day; D, 30th day; 

 E, 100th day ; F, 378th day. 



to be such that the area of the axis in the youngest groups was 

 about 40 per cent of the area of the section of the entire fiber, 

 but this increases with the advancing age of the rat until it 

 becomes about 50 per cent in the old animals. Certainly, my 

 preparations suggest that after the rather rapid increase in the 

 area, the number of fibers is not much increased during later 

 growth. 



