BODY TEMPERATURE OF MICE 347 



The young mice which were employed in these experiments 

 belonged both to broods born and kept in the warm room and to 

 broods born and kept in the cold room. For the purposes of the 

 experiments the warm-room individuals were either (1) taken 

 directly from the nest, or (2) completely exposed for varying 

 periods in the warm room, or (3) exposed for varying periods in 

 the cold room. The cold-room individuals were either (1) taken 

 directly from the nest, the mother being present, or (2) taken 

 directly from the nest, the mother being absent, or (3) fully ex- 

 posed for varying periods in the cold atmosphere. 



From tables 13 to 20 the synopsis on the following page has 

 been prepared. Each figure in the latter is the mean of those 

 for all individuals of the same age which were subjected to 

 approximately the same conditions. Only the 'first temperature 

 recorded' has been used for this purpose. In the synopsis, I have 

 thought it expedient to throw together the figures from tables 14 

 and 15, also those from tables 16 and 17, and those from tables 

 19 and 20, since the differences between the groups thus com- 

 bined were not great enough to warrant their separation. 



From these figures curves have been constructed (fig. 3), show- 

 ing the correlation between age and ability to regulate body tem- 

 perature under different conditions of external temperature. These 

 curves show, in a striking way, the development of homothermy 

 in the lifetime of the individual mouse, culminating in a practi- 

 cally perfect power of temperature regulation (within the limits 

 of the experiments) at about the age of three weeks. From the 

 tenth day onward, mice exposed to the temperature of the warm 

 room of my experiments (usually between 20° and 25°) seem to 

 have maintained the same temperature as those taken directly 

 from the nest. This accords well with Pembrey's experiments, in 

 which the young animals were subjected to an atmospheric tem- 

 perature about 10° lower than that of the body. But it is plain 

 that for greater differences, the power of heat regulation is not 

 developed until long after the 10th day. Even at the age of eight- 

 een days, the mean temperature of 3 mice, exposed to an atmos- 

 pheric temperature of about 7°, was 23.3°, or 13° lower than that of 

 mice of the same age kept in the warm room. The temperatures of 



