164 'Journal of Cojnparative Neurology and Psychology. 



case of the rats, the interpretation is doubtless the same: The 

 "large" fibers constitute a special group which medullates early, 

 while the "small" fibers have grown in subsequently. Table V 

 shows also that most of the "large" fibers have attained a greater 

 size in two months (body-weight 984 grams) than any "small" 

 fiber attains in thirteen years. The explanation previously given 

 is here applicable: The small fibers have come in after the 

 period of most rapid growth, and so never "catch up." By com- 

 paring the percentage of increase in the diameters of "large" and 

 of "small" fibers at different ages (as shown in Table V) it will 

 be seen that the increase in diameter is not dependent upon the 

 size which the fiber has attained, but that after the early period 

 of rapid growth, when the "large" fibers are differentiated, the 

 increase in diameter progresses at the same rate for both "large" 

 and "small" fibers; thus leaving the two classes always clearly 

 distinguished. 



4. Table VI shows that, as in the case of the rats, the area 

 of the sheath in the fully medullated fiber is nearly equal to the 

 area of the axone. 



CONCLUSIONS. 



1. In cats between the ages of one day and six months there is a 

 nearly regular increase in the number of medullated fibers in the 

 oculomotor nerve. This increase amounts to 157 per cent. 



The conclusion of Schiller that nearly all the medullated 

 fibers found in the oculomotor nerve of the cat at maturity are 

 present in the nerve at birth, is therefore not correct. 



2. After the oculomotor nerves in the cat and in the rat have 

 reached the same stage of medullation (the cat of one day corre- 

 sponding to the rat often days), the addition of medullated fibers 

 during subsequent growth in the case of the cat is relatively twice 

 as great as in the case of the rat. 



3. The medullated fibers continue to increase in size during 

 the life of the animal, but the newer fibers, classed as "small," 

 never attain the size of the older ones. This is due to the fact that 

 they come in after the period of most rapid growth. 



4. After the age at which the two groups of "large" and 

 "small" fibers can be distinguished, all the fibers are increasing in 

 diameter at the same rate. 



