GKOWTH OF THE COEPUS CALLOSUM 51 



entire eallosum; that is, the callosum at this age is approxi- 

 mately mature in respect of the number of myehnated fibers in 

 it. The relations shown b}'^ the ratios in chart 4 are in harmony 

 with these events. 



/. Thickness of the corpus callosum 



As to the first appearance of the callosum, Hamilton ('86) and 

 Goldstein ('03) both state that in man it appears in the end of 

 the third or the fourth month of intra-uterine life. 



In the rat, according to Zuckerkandl ('09), the callosum 

 appears in the fetus of 19 mm., which, according to Stotsenburg 

 ('15), corresponds to about the eighteenth day of fetal life. 



M 



^^^^^ 



Fig. 2 Outline of the callosum. Showing at what localities the measure- 

 ments listed in table 5 were taken. 



To follow the growth of the callosum I have measured its 

 thickness in three localities I-I,' II-II,' III-IIT.' Figure 2 

 shows the position of the localities at which the thickness of 

 the callosum was carefully determined. These localities have 

 been selected as fairly representative of the entire callosum. 



Locality I. The maximum diameter of genu on the line of 

 I-L' 



Locality II. The line II-IF represents the maximum thick- 

 ness of splenium. 



Locality III. This gives the maximum thickness of truncus,. 

 which is represented by the line III-III' drawn at the point just 

 before the fornix longus fibers make contact with the callosum. 



The measurements for thickness have been made under the 

 microscope with a low-power lens, but for the measurement of 

 the callosal length I have used a shding calipers reading to a 

 tenth of a millimeter, the entire callosum being too large to 



