SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 773 



the dog, the cat, and the rabbit, the adult condition is reached 

 15 days after birth. Edwards found that these phenomena were not 

 due to the surface/volume ratio of the small animal, and showed that 

 skin covering had little to do with it, for an adult sparrow was able 

 to maintain and regulate its temperature perfectly in the absence 

 of all feathers. Many years later Raudnitz, from a study of the 

 temperature of newly born human infants, concluded that the chief 

 cause of variable temperature was the imperfect development of 

 homoiothermicity. 



Babak, continuing the analysis of the problem, divided heat-regula- 

 tion into two types, ' ' chemical ' ' and ' ' physical ' ' . The human newborn 

 infant, for example, possesses the former function, for it can counter 

 a fall of environmental temperature by increasing its combustions ; 

 but it cannot resist a rise and immediately goes into hyperthermia, 

 having an imperfect control of its capillaries and its transpiration. 

 Plant subsequently extended this point of view to other mammals 

 such as the cat and dog. 



The bird embryo was first investigated in this connection by 

 Pembrey, Gordon & Warren. They demonstrated that, during the 

 incubation period in the chick, changes of external temperature 

 invariably determine, after a longer or shorter period, changes in the 

 same direction in the respiratory exchange. The period required for 

 response did not seem to depend on whether the shell was removed 

 or not. On the other hand, when the recently hatched chick was 

 examined, it reacted to temperature changes exactly as a warm- 

 blooded animal would, a fall of 20° in external temperature raising 

 the expired carbon dioxide to twice its previous amount in 15 minutes. 

 The chick embryo, then, was cold-blooded up to the 19th day of 

 incubation. The physiological transition was observed by Pembrey 

 and his associates to take place on the 21st day of incubation during 

 the time immediately prior to hatching. Sometimes there was an 

 intermediate condition, transient but neutral, in which a fall of 

 temperature did not result in a fall in carbon dioxide output, nor, 

 on the other hand, in a rise. This intermediate condition may give 

 way to the cold-blooded or the warm-blooded condition, according 

 as to whether the chick is feeble or strong and healthy. 



Pembrey later reported that the transition to full powers of heat 

 regulation did not take place in the pigeon (a nidicolous bird) till 

 well after hatching, i.e. about the 6th day of post-natal life. Giaja 



