774 THE RESPIRATION AND [pt. m 



was the next investigator who occupied himself with this problem. 



Giaja defined the "summit metabolism" as the maximum energy 



expenditure which the homoiothermic animal can rise to in its 



struggle against cold, and spoke of a "metabolic quotient" as 



follows: ^ . ^ 1 r 



Summit metabolism , _ 



— n z r-rr-p = metabolic quotient. 



Basal metabolism ^ 



For the mouse and the rat the metabolic quotient is approximately 

 3*5, for birds 4-0. Before hatching, the chick embryo, according to 

 Giaja's measurements, which were quite comparable with those of 

 Pembrey, could be said to have a negative metabolic quotient, but 

 by 6 hours afterwards the value of this constant was i-g, and, 5 days 

 after, it rose to 2-0, and at 48 days to 2-6. Giaja observed the 

 intermediate state spoken of by Pembrey, i.e. the condition before 

 hatching, in which the chick maintains its heat-production when the 

 temperature is lowered but cannot raise it to compensate for a fall. 

 Exactly the same results were obtained on the rabbit immediately 

 after birth; its metabolic quotient, 1-3 at 12 hours, rose to 2-4 a 

 fortnight later. 



A certain further insight into the ontogeny of heat regulation is 

 obtained by examining Fig. 83 b, which has already been described. 

 It shows Brody & Henderson's work on the effect of temperature 

 on the growth-rate of the chick embryo at different stages. The 

 reason why the curve X^ is not a curve but a straight line is, in their 

 opinion, that between the 13th and i8th days of incubation the 

 heat-regulating mechanism of the embryo has probably developed 

 sufficiently to enable it to keep its body-temperature constant within 

 the limits of 37-2° and 40-6°. Thus at this stage the chick embryo 

 is not strictly cold-blooded. These experiments would authorise us 

 to suppose that the development of heat regulation is not quite 

 sudden, as it seemed at first to be from Pembrey's experiments, but 

 the zone of temperatures within which it can accurately adjust itself 

 widens continually as development progresses. The most recent in- 

 vestigations of this are those of Kendeigh & Baldwin. They state 

 that "irequently in the literature of avian physiology and life-history, 

 statements are to be found that the body-temperatures of nestling 

 birds are extremely variable and similar to those of cold-blooded 

 animals". Their own work on the house wren, Troglodytes aedon, 

 showed clearly that full temperature regulation was attained by the 



