48 



NOBUHARU SUITSU 



second phase of slower increase which extends through the 

 period of sexual maturity (about seventy to eighty days), and 

 the third phase of slow increase which extends from eighty 

 days on. 



It is clear that the callosal area increases most rapidly before 

 myelination by the growth and addition of new fibers during 

 the first ten days after birth. From ten days up to sexual 

 maturity, the area increases not only by the addition and enlarge- 

 ment of the fibers, but largely by the formation of the myelin 

 sheaths. After sexual maturity is reached, the callosal area 



30 40 50 60 70 80 90 100 110 120 130 140 150 160 



Chart 3 The base line represents the total brain areas from 30 to 155 sq.mm. 

 The corresponding values for the callosal area are measured on the ordinates as 

 shown by a theoretical curve based on formula 1. Albino rat. 



still continues to increase up to 378 days, chiefly by the enlarge- 

 ment of the fibers already myelinated. 



d. Growth of the area of the calloswn on the total brain area 



The calculated values for the callosal and total brain areas 

 on the observed body weight are given in columns C and E, 

 table 2. When the callosal areas are plotted on the total brain 

 areas, the relations of the data are shown in chart 3, in which 

 we may recognize the following three phases in the relative 

 growth of the callosal area. 



