1 86 Journal of Comparative Neukologv. 



same species should be used (ov comparative resuUs, and that 

 particular attention should h^ pa:d to the season of the year at 

 which the observations are made. Moreover, to i^ive rtliable 

 results, we have tound that the frogs must be free from wounds 

 for the frog in which abrasions of the skin on the feet or nose 

 have been present for some time, react very diffcrenil}' from 

 the normal frog, the spinal cord after death r.ipidly absorbing 

 water to a large amount, and becoming very soft. 



Moreover, many of the frogs of the species which we em- 

 ployed are infected with parasites (Distoma). These sometimes 

 accumulate in the pia of the spinal cord, and may be present 

 there in sufficient quantity to materially affect its weight. The 

 cord therefore, should be examined for the presence of such 

 parasites, an examination which may be carried on under a 

 hand lens. In addition, there is always the normal variability 

 of the individual in weight and length of tlie body, and the 

 development of the nervous system, so that for satisfactory re- 

 sults, we can hardly rely on individual observations, but must 

 use a method of averages. 



As far as the spinal cord is concerned, the principal varia- 

 tion which occurs in the frogs of the sizes here used is in their 

 length, and this usually modifies their weight. In order, there- 

 fore, to get rid of the individual variability in the length of the 

 cord, we have in all cases, divided the total weight of the cord 

 in grams, by its length in millimeters, and thus obtained the 

 weight of an average millimeter of cord, the whole mass of the 

 cord being conceived of as forming a regular geometrical figure, 

 the length of which was equal to the observed length of the 

 cord. 



The weight of the "standard millimeter of cord" thus ob- 

 tained, gave us the numbers which have been used for com- 

 parison. 



Manner of Preparation. 



To obtain the data used in the tables given farther on, the 

 preparation of the frog was conducted as follows : Recently 

 caught specimens of R. virescens were employed. Two 



