36 NOBUHARU SUITSU 



determine the g;ro\vth between birth and maturity of the area of 

 the eallosum measured on the median sagittal section of the 

 albino rat brain. The object in undertaking such a study was 

 to throw some light upon the following questions: 



1. In what manner does the white substance of the mammalian 

 brain increase after birth? 



2. What is the relation in the rat between the increase in 

 the area of the eallosum, the myelination of the fibers in it, and 

 the psychical development of the animal? 



3. How does the growth of the area of the eallosum in the 

 rat compare with the corresponding growth change in man? 



For the psychical development of the rat the studies made 

 by Watson ('03) and Lane ('17) have been mainly consulted. 

 On looking into the literature, nevertheless, I found many 

 references concerning the presence of mental associations in 

 rodents, as indicated by the formation of habits, though Flechsig 

 ('96) denies the presence of association centers as defined by 

 him. Although the callosal fibers form only one system of 

 commissural fibers in the br^in, yet they constitute such a large 

 fraction of this group that changes in them must be of the 

 .greatest importance for the psychical life of the rat. 



In connection with this study I desire to express my sincere 

 thanks to Doctor Greenman, Director of The Wistar Institute, 

 for putting the facilities of the laboratory at my disposal, and 

 also to acknowledge my obligations to Professor Donaldson, 

 under whose direction this research has been made. 



MATERIAL AND TECHNIQUE 



a. Fixation 



For the measurement of the eallosum it is desirable to have 

 material which has suffered the least possible change in volume 

 as the result of fixation and also to employ a uniform technique. 

 When the rat brain is fixed in formalin for sixteen hours followed 

 by Weigert's rapid mordanting fluid for five days, the shape and 

 weight of the total brain suffer but little as the result of the 

 treatment. 



