344 FRANCIS B. SUMNER 



3. OBSERVATIONS UPON VERY YOUNG MICE 



It has long been known that the young of many species of 

 mammals and birds behave, in respect to their body temperatures, 

 like the so-called 'cold-blooded' or poikilothermous animals. W. 

 F. Edwards,^- as early as 1832, recognized that "the young of 

 mammalia appear to be distinguished into two groups in relation 

 to animal heat. Some are born, as it were, cold-blooded, others 

 warm-blooded." A similar distinction was drawn among birds. 

 Among the mammals which are born in a warm-blooded con- 

 dition he mentions the guinea-pig; among those born in a cold- 

 blooded condition he includes cats and dogs. These two groups 

 of animals differ likewise in the degree of their structural develop- 

 ment at the time of birth, those which are born in a 'warm- 

 blooded' condition being much farther advanced. Edwards cites 

 experiments of his own in which the body temperature of kittens 

 became reduced to 17° or 18°, and of puppies to 13° or 14°C. 



The general accuracy of Edwards's observations is still recog- 

 nized. Subsequent investigators have found similar wide fluctu- 

 ations in the temperature of various young mammals, following 

 changes of atmospheric temperature. For example, Pembrey" 

 determined the temperatures of young mice at the ages of one to 

 ten, days. At the former age, the temperature of three young 

 mice was 32° when taken from the nest, but after forty minutes 

 at a temperature of 23°, the body temperature fell to that of the 

 air.^^ In the case of a mouse ten days old, the (groin) temperature 

 was 33° at the beginning of the experiment, 32.25° after thirty 

 minutes at about 22°. Pembrey concludes that for mice the power 

 of heat regulation "appears to be well developed by the tenth day 

 after birth" (p. 375). It will be shown below that this conclusion 

 is not warranted except for the comparatively small differences of 

 air temperature which were employed by Pembrey 



« Op. cit. 



« Journal of Physiology, vol. 18, 1895, pp. 363-379. 



■''' Pembrey's methods of determining body temperature were extremely crude. 

 According to his own statement, "with such small animals as young mice and rats 

 the temperature was only roughly measured by covering the bulb of the thermom- 

 eter with the bodies of two or three young animals, or by placing it in the groin 

 when the animals were bigger" (p. 364). 



