SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 693 



times as much carbon dioxide was given out at 28" as at 14°, so 

 that the Q^^q for respiration in this egg closely approached that for 

 developmental rate (see p. 508). The metabolic rate values worked 

 out as follows : 



C.c. carbon dioxide per 

 kilo per hour 



SO that there was a very obvious decline in the respiration intensity 

 with increasing age. In view of the fewness of the figures, no stress 

 can be laid on the shape of the curve, but it does not seem to follow 

 the course suggested for metabolic rate curves by Murray, i.e. slow 

 decline at first, followed by greater rapidity of fall. The table shows 

 also that the rule of decline appHes to eggs incubated at 15° as well 

 as at 28°. Here we have a case like that of the chick, where the 

 earliest stage which it has so far been possible to examine has the 

 highest metabolic rate of all. If figures could be obtained for 

 the snake or the chick at about the time of gastrulation a great 

 advance would have been made. The respiratory quotient of the 

 snake's egg was shown by Bohr to remain in the close neighbour- 

 hood of 0*9 throughout development, so that he concluded there was 

 a dominant complete combustion of carbohydrate material. 



4-11. Respiration of Avian Embryos in General 



I have spoken already about the earlier researches on the respira- 

 tion of the avian embryo. The modern period in this subject begins 

 in 1900, when Bohr & Hasselbalch published the first of their series 

 of classical papers on the evolution of carbon dioxide, the absorption 

 of oxygen and the heat-production of the hen's egg. Their first paper 

 was concerned exclusively with the production of carbon dioxide 

 during the incubation period, for they wished to study the relation 

 of metabolic rate to age and weight, a correlation the importance of 

 which none of the earlier workers, such as Baumgartner or Pott 

 & Preyer had appreciated. Their apparatus consisted of a thermo- 

 stat chamber in which the egg was placed connected with a chain 

 of absorption-tubes, etc. Experiments with empty egg-shells from 



