SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 701 



In one respect, at any rate, Hasselbalch's experimental findings 

 were subsequently reversed, for a few years later Krogh, who was 

 undertaking an extended test of the question whether animals ex- 

 crete small amounts of nitrogen or not, took occasion to examine 

 the hen's egg from this point of view. His experiments, which were 

 conducted with irreproachable technique, led to quite negative con- 

 clusions. For the most part he occupied himself with later stages than 

 Hasselbalch, but even on the ist day of development he could find 

 no evolution of nitrogen which was outside the small experimental 

 error. His conclusion was that certainly not more than 0-003 c.c. 

 per hour, or 2-5 mgm. of nitrogen for the whole 3 weeks of develop- 

 ment, was excreted, and so far the 2-5 mgm., if indeed they do leave 

 the egg, have not been missed by chemists. Krogh's experiments 

 were fully supported by some work of Tangl & von Mituch. "Hassel- 

 balch found", said Krogh, "by evacuating egg-contents in the 

 mercury pump, that fresh eggs contain a considerable surplus of 

 dissolved gases above that which could be taken up by a corre- 

 sponding quantity of pure water. The surpluses are, according to 

 him, confined to the yolk and I venture to suggest that it is the fatty 

 substances which dissolve the gases. In an egg of 60 c.c. about 1-2 c.c. 

 of nitrogen is contained whereas 60 c.c. of water saturated with air 

 at 38° contain only 0-55 c.c. Hasselbalch found that after two days' 

 incubation the surplus had sensibly diminished and there can be no 

 doubt that the whole of it will be given off during development as 

 the substances of the chicken dissolve less of the gas than pure water." 



Pott had claimed that much more carbon dioxide was given off 

 by an egg in pure oxygen than in ordinary air, but his technique 

 was inferior. Hasselbalch found that whatever the effect was, it 

 was very variable; thus in one experiment in 82 per cent, oxygen 

 the carbon dioxide output was half the normal, and the oxygen 

 uptake only a quarter the normal, but in 79 per cent, oxygen the 

 carbon dioxide output was slightly raised, while the oxygen uptake 

 was three times the normal. These effects could only be due to 

 toxic action of high oxygen concentrations (see Riddle's work in 

 Section 18-9) alternating with true accelerating effects. Hasselbalch 

 also concluded that the excretion of gaseous nitrogen from the eggs 

 could take place at these high oxygen concentrations to a far greater 

 extent than under normal conditions, as much as 2-268 c.c. being 

 given off per hour in one experiment. Krogh's work, which did not 



