58 NOBUHARU SUITSU 



SUMMARY 



Employing sagittal sections, the corpus callosiim from seventy- 

 six albino rats, prepared from material fixed and stained by a 

 uniform technique, we have obtained the following results: 



1. In the albino rat the callosum at birth contains no my- 

 elinated fibers. These first appear from the seventh to tenth 

 day and then increase rapidly in number and size. 



2. The area of the callosum as shown in cross-section in the 

 sagittal plane increases between birth and maturity about 

 3.5 times. 



3. This increase occurs in three "phases^ — an early phase, 

 comprising the first ten days of life, in which new unmyelinated 

 axones are added, while those already present increase in d'r.rn- 

 eter. A second phase from the tenth to the thirtieth day, dur- 

 ing which the formation of new myelin sheaths is the main 

 factor, and a third phase after the thirtieth day, during which 

 the increase in the diameter of the fibers is the most important 

 change. Like the peripheral nerve fibers, these central fibers 

 appear to increase in diameter so long as the brain increases in 

 weight. 



4. WTien during growth the thickness of the callosum is meas- 

 ured at the genu, truncus, and splenium, it is found that the 

 proportional growth is greatest at the genu and least at the 

 splenium. Thus the growth change is most marked at the 

 frontal end of the callosum and diminishes toward the occipital 

 end. 



5. This series of growth changes represented by the three 

 phases correlates very well with the psychical development of 

 the rat^ — which is naturally a consequence of the maturing of 

 the entire brain — of which the callosum is a part. 



6. With the present data it is possible to compare the develop- 

 ment of the callosum in man and the rat only at maturity. The 

 comparison shows that the relative area of the callosum in man 

 is distinctly greater than that in the rat — man 4.44 per cent, 

 rat 3.29 per cent. 



7. Using the method of equivalent ages as a basis for com- 

 parison, we should expect to find the fibers in the callosum of 

 man well myelinated at the age of twenty months. 



