724 



THE RESPIRATION AND 



[PT. Ill 



and were proportional to the square roots of the specific weights of 

 the respective gases. All his tables showed that they did not follow 

 this rule, but some more complicated one, being doubtless affected 

 by the complex conditions in the material. Hiifner found, as has 

 been said, that hydrogen penetrated most easily, then carbon dioxide, 

 then nitrogen, and lastly oxygen. The hen's egg-shell was less easily 

 penetrable than the goose's egg-shell, and Hufner suggested that 

 this was necessary for it, since its surface was smaller in proportion 

 to its weight. More recently S. Ancel has shown that the penetration 

 of chloroform vapour into the hen's egg exactly follows Graham's law. 

 Aggazzotti's experiments with eggs incubated 3000 metres above 

 sea level showed (as may be seen from Fig. 1 59) that the composition 

 of the air in the air-space was 



almost identical with the nor- '°r Air space increase 



91- 



mal sea-level values. The 



percentage of oxygen and of 



carbon dioxide was constantly 



lower by a small amount, so 



that there was a certain degree 



of acapnia and anoxaemia of 



the embryo, due, of course, to 



the fact that at the Col d'Olen 



the barometric pressure was 



only a third of what it was at 



Turin. These high-level experiments have no great value, for all the 



embryos incubated at the Col d'Olen died before hatching, and were 



more or less abnormal. 



No very extensive figures seem to exist in the literature for the 

 change in volume which the air-space undergoes during develop- 

 ment, though qualitatively, as is well known, it markedly increases. 

 A curve can, however, be constructed from the data given by 

 Aggazzotti; it is shown in Fig. 160^. It will be remembered that the 

 knowledge of this fact was one of the bases of Mayow's theory of 

 embryonic respiration. The elasticity of the air contained in the air- 

 space acted, he thought, like a kind of piston, compressing the yolk 

 and white into the solid tissues of the chick, but we now regard the 

 expansion of the air-space as the effect of development rather than 

 its cause, and as arising from the evaporation of the water and the 



^ Romanov also gives data for this. 



J 



