6 Weight of the Central Nervous System of the Frog 



explanation or correction. The formula is constructed so as to express the normal 

 changes in the weight and length of body as related to the weight of the central nervous 

 system — changes which are taking place as the frog grows larger. But it remains to 

 be determined, first, how early in the history of the frog the formula can be applied, 

 and, second, whether sex, season, and nutritive conditions are able to affect the result ; 

 the nutritive condition including not only those changes which may occur from day 

 to day, but those which occur from spring to autumn. 



Repeated examination shows that the formula does not apply to frogs until they 

 have attained a body -weight of approximately 5 grams. For example, in a bullfrog 

 with a body- weight of 3.53 grams the observed weight of the central nervous system 

 was 56 milligrams, whereas the calculated weight was only 49 milligrams. A similar 

 result is obtained when the test is applied to the leopard frogs under 5 grams of body- 

 weight; hence, for frogs of this size, the calculated weight of the central nervous 

 system is too small. 



The failure of the formula to apply to the smallest frogs is probably due to the 

 precocious enlargement of the central nervous system — a character of all young verte- 

 brates, and one still evident in frogs when less than 5 grams in weight. We conclude, 

 therefore, that the relations found in the mature frogs are not established until they 

 have attained a body-weight above 5 grams. From this point on the formula applies 

 to all normal specimens. 



For the consideration of other sources of error it will be most advantageous to 

 examine the two species of frogs separately. We begin with the bullfrogs. 



The original table for the bullfrogs (Donaldson, 1898) contains fifty-two cases. 

 The first five cases (Nos. 1-5) are from frogs below 5 grams in body weight, and for 

 this reason are excluded. Among the remaining forty-seven cases, six (Nos. 7, 32, 

 34, 37, 40, 47 in the original table) are marked "dry," which means that through 

 drying their body-weight had been reduced below the normal. These also are 

 excluded. No. 9 in this table is plainly abnormal, as is seen by comparing the body- 

 weight with the length (body-weight, 8.75 grams; length, 127 millimeters), and 

 for this reason is also excluded. For the foregoing exclusions no explanation is 

 required, as, under the circumstances, one could not expect the formula to apply to 

 them. There are, however, six more cases to be excluded, namely Nos. 10, 11, 43, 

 because the body weight had been increased by the absorption of water ; and Nos. 45, 

 48, and 49, because long captivity had produced a loss of weight through starvation. 



The absorption of water by frogs whose vitality is much reduced is a familiar 

 reaction, and the effects of starvation have been reported in earlier observations from 

 this laboratory (Donaldson and Schoemaker, 1900, p. 112). 



In this series no correction for season is required, as the records are all from 

 July and August frogs, and hence comprise midsummer frogs only. 



The final table contains, therefore, thirty-four records of approximately normal 

 frogs to which the formula had been applied. These are presented in Table I. 



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