154 yournal of Comparative Neurology and Psychology. 



of 72 per cent, in the ventral roots between the body-weights of 

 1.5 grams and iii grams. Hardesty ('00), working in this 

 laboratory, counted the number of medullated fibers in the ventral 

 roots of the sixth spinal nerve in the frog (R. virescens), and 

 found an increase of 53 per cent, between body- weights of 7.16 

 grams and 60.9 grams. 



On comparing the results obtained by Schiller with those of 

 the other investigations here cited, the question at once arises: 

 Is this discrepancy due to a fundamental difference in the manner 

 of development between the cerebral motor nerves, and the spinal 

 motor roots, or does the cat represent a manner of development 

 different from that of the rat and the frog? In this paper we shall 

 offer a partial answer to the above questions by following the 

 increase in the number of medullated fibers in the oculomotor 

 nerve of the white rat and of the cat. 



PART I. THE WHITE RAT. 

 Material and Technique. 



White rats were used, ten males and three females, ranging in 

 weight from 11 grams (eleven days old) to 414 grams (about two 

 years old). Observations were begun at eleven days because at 

 birth none of the fibers is medullated, and it is not until eleven 

 days that the medullation is appreciable. The rats were killed 

 with chloroform, and the nerves immediately removed, and fixed 

 in I per cent, osmic acid. They were then washed in water, 

 passed through the graded alcohols and xylol, and imbedded in 

 paraffin (m. p. 56° C). Sections were made near the origin of the 

 nerve, but after the fila had come together into a single trunk, 

 and were cut in several thicknesses, but those of 4/( were found to 

 be the best, and were chiefly used. The fibers were counted by 

 the photographic method described by Hardesty ('99) except in 

 the case of the smallest nerve, which could not be photographed 

 satisfactorily. This was counted by using a net and an auto- 

 matic counting machine. 



It was noticed that a fairly sharp distinction could be made 

 between the largest fibers at any age, and the other fibers. In this 

 paper the word "large" is used to indicate the largest -fibers 

 found at given age, and the word "small" is used to indicate the 

 other fibers. The photographic apparatus used gave a magnifi- 



