Donaldson, American and European Frogs. 141 



For the foregoing comparison of the central nervous system 

 and its parts, together with the determination of the percentage of 

 water, data on all three species have been available. But before 

 commenting on the results just given, I wish to present some obser- 

 vations based on the comparison of two species only. 



These additional observations are on the peripheral nervous 

 system and relate first, to the number of medullated fibers in the 

 spinal nerve roots; comparing R. esculenta with R. pipiens (there 

 being no corresponding observations on R. temporaria). Second, 

 to the length of the internodal segments; comparing R. temporaria 

 with R. pipiens (there being no corresponding observations on 

 R. esculenta). 



The number of medullated Jterve fibers in the spinal nerve roots 

 of R. pipiens compared zuith the number in R. esculenta. — In a 

 female R. pipiens weighing 48.2 grams, Hardesty ('99) reported 

 14,582- medullated nerve fibers in both roots of the ten spinal 

 nerves of one side. This was a much larger number than had 

 been found by Birge ('82) in a specimen of R. esculenta of greater 

 body weight. To reduce Dirge's figures for the specimen of 

 R. esculenta, weighing 63 grams, to those for a specimen weighing 

 only 48.2 grams, we have proceeded as follows: 



The smallest frog in Birge's series, with a body weight of 

 1.5 grams, in which he enumerated 2992 motor fibers in the ven- 

 tral spinal roots of one side, was selected as one limit, and to this 

 frog the same proportion of sensory fibers as was found in the 63 

 gram specimen, was allotted, a concession which probably makes 

 the number of sensory fibers somewhat too large. 



The number of fibers corresponding to each gram of body 

 weight between 1.5 grams and 63 grams was then determined. 

 By this method, it was found that when the number of fibers in the 

 spinal nerves of the 63 gram frog was reduced to the number for a 

 48.2 gram frog, it amounted to 92.8 per cent of that found in the 

 63 gram frog, or 8925 fibers. Thus the difference between the 

 two species is (14,582 — 8925) 5657 fibers, or put in another way, 

 R. esculenta has only about 61 per cent as many medullated nerve 

 fibers in the spinal nerve roots as has R. pipiens. On reducing 

 the original observations of Birge for the number of fibers in the 



2 By a clerical error the number was printed on p. 78 (Hardesty '99) as 14,783. It should be 14,582, 

 and consequently I shall use the corrected number subsequently, even when referring to Hardesty's 

 paper. 



