SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 699 



of oxygen and 2-0 ex. of nitrogen were given off by fertile eggs 

 during the first week of incubation. He tried many methods in his 

 efforts to get to the bottom of it; thus he studied the gaseous exchange 

 of the whole yolk in vitro, the composition of the air in the air-space, 

 and of that extractable from eggs by bringing them into a high 

 vacuum. His results left him with the conviction that there was an 

 output both of oxygen and nitrogen, and he suggested various possible 

 mechanisms which would account for it. Typical experiments are 

 shown in Table 81. It is probable that they simply represent the 



Table 8 1 . Hasselbalch's experiments. 



Cubic centimetres of gas 



adjustment of the shell, the yolk, and the white, as they gain 

 gaseous equilibrium with the external air. Hasselbalch unfortunately 

 did not state how long a time elapsed between the laying of the eggs 

 and his respiratory experiments upon them. In spite of the fact — 

 plain from his tables — that unfertilised eggs gave off as much oxygen 

 as fertilised ones, he persisted in maintaining that "the condition 

 for physiological oxygen-production in the first hours of develop- 

 ment is the presence of living cells". By means of his in vitro experi- 

 ments with yolks in salt solutions he showed that the osmotic pressure 

 had an influence on the gases given off; thus hypotonic solutions 

 (i per cent, sodium fluoride, etc.) led to a decrease in the oxygen 

 generated, but hypertonic solutions set up oxidation processes in the 

 yolk which abolished it altogether by causing an oxygen uptake. 



