56 NOBl/HARU SUITSU 



By the twenty-fifth to the twenty-seventh day after birth 

 there has been a distinct advance over the condition just de- 

 scribed. The callosum now has myeUnated fibers evenly dis- 

 tributed over its entire cross area, but these fibers are not yet as 

 closely packed nor as darkly stained as those found in the mature 

 callosum. 



Although I have not counted the exact number of myelinated 

 fibers in the callosum at different ages, it may be said safely 

 that the increase of the callosal area is accompanied by an 

 increase in the number of the myelinated fibers with increasing 

 age. 



We may conclude, therefore, that the increasing complexity 

 of psychical response is associated with an increasing number of 

 myelinated fibers in the callosum at least during the first two 

 phases of its grow^th. After the thirtieth day of age all the 

 myelinated fibers in the section stain as darkly as those in the 

 sections from the adult rats. 



Passing to the sections of the callosum from the adult rats, 

 we find that in them the fibers are very closely packed together 

 and darkly stained. Just when new fibers cease to appear in 

 the callosum of the rat is difficult to determine. 



I am particularly interested in the fact that the psychical 

 development of the white rat runs about parallel with the 

 increase in the number of the myelinated fibers in the sagittal 

 callosal area, as is given in table 8 — although of course I recog- 

 nize that this is but one system of neurons and only a fraction 

 of the entire brain — in which corresponding changes are also in 

 progress. 



3. Our third question related to the growth of the area of 

 the cross-section of the callosum in the rat as compared with 

 the corresponding growth change in man. 



Unfortunately, a comparison between the rat and man in 

 regard to the area of the callosum in sagittal section can hardly 

 be made, owing to the absence of suitable data for man. The 

 human callosal area at thirty-three years (mean age of ten 

 ordinary men) has a mean value of ca. 563 sq.mm. (Spitzka, '07), 

 but no record is given for the area at birth. 



