GROWTH OF THE CORPUS CALLOSUM 45 



Phase 3 (from thirty-sixth to 378th day). The initial dif- 

 ference is 0.66 mm.2, and from this point on the difference value 

 decreases very slowly till it becomes zero. 



To explain the slower growth of the callosal area compared 

 with that of the entire brain, it will be necessary again to touch 

 on, 1) the change in histological structure; 2) the development 

 of myelin; 3) the later growth of the myelinated fibers as a whole. 



1. Since the entire brain is composed of both white and gray 

 matter, while the callosum represents white matter only, the 

 growth of the total brain depends on the growth of both the 

 cell bodies and the fibers, while that of the callosum depends 

 on the growth of fibers alone. According to Sugita's ('18) 

 studies on the development of the cerebral cortex in the rat, 

 the thickness of the cortex, the total number of the nerve cells 

 in it and the size of these cells have all attained nearly their 

 full values at about the age of twenty days. The further devel- 

 opment after this age is represented principally by growth 

 of the fibers and their myelin sheaths. The growth of the 

 callosal area during the first twenty days after birth is more 

 rapid than the process of myelination in it. 



There must therefore be a steady addition of the new unmy- 

 elinated fibers. Although the general form of the growth curve 

 of the callosal area is similar to that of the total brain area, 

 nevertheless the rate of the former is slower for about twenty 

 to thirty days after birth, a period at which the nerve cells grow 

 very rapidly, as the observations of Sugita show. Thus we 

 conclude that the difference here noted between these two curves 

 is due in a large measure to the presence of the cell bodies in the 

 brain and their absence from the corpus callosum. 



2. So far as the appearance of the myelinated fibers in the 

 callosum is concerned, these fibers myelinate from about the 

 tenth day on rather slowly, but at about twenty days of age, 

 the myelination process becomes more rapid, and at the thirtieth 

 to the thirty-fifth day the myelinated fibers in the callosum are 

 present in large numbers. 



During and after this phase the cortical area increases chiefly 

 by the deposition of the myelin sheaths. The fact that the 



