GROWTH OF THE CORPUS CALLOSUM 55 



to the thirtieth day the growth rate of the callosal area is faster 

 than before. This is very interesting because, according to 

 Watson ('03), the first phase of psychical maturity is reached 

 at this early age. 



As part of our second question was the querj^ whether there 

 is any correlation between the increasing complexity of the 

 psychical life and the number of the myelinated fibers in the 

 callosum during its postnatal growth. In approaching this 

 problem it is necessary to consider, 1) the method most suitable 

 for myelin staining and, 2) the age at which the myehnated 

 fibers first appear in the callosum. 



After a number of experiments on the most suitable myelin 

 stain, I chose the Kultschitzky-Wolter's method. This method 

 seems to be the best for showing the very fine fibers and succeeds 

 when the Pal-Weigert method almost fails, especially in the 

 cross-sections. The material thus treated even at birth comes 

 out quite clearly, and numerous unmyelinated fibers can be 

 seen as brown rings. 



At five days of age the callosum is almost unmyelinated. 

 Here and there we find scattered blue points which at most 

 indicate incompletely myelinated fibers. 



An examination of the sections at seven days of age showed 

 individual variations. The sections from two rats of this age 

 showed almost no myelinated fibers, while the sections from the 

 third rat at the same age showed a few scattered myelinated 

 fibers. At this age the myehnated fibers begin to appear mainly 

 in the dorsal portion of the callosum. 



At the tenth day a few myehnated fibers can be seen here and 

 there comparable to those found in the best-developed rat at 

 seven days of age. Sections of this region from rats at twelve, 

 fifteen, seventeen and twenty-two days of age showed a steady 

 increase in number of myelinated fibers. Small ('99) says that 

 memory is developed soon after ten days of age. At nineteen 

 days after birth memory is well developed, while from birth up 

 to twelve days of age the instinctive reactions are characteristic 

 (Watson, '03). 



