704 THE RESPIRATION AND [pt. hi 



reserve of the egg-contents would affect the values obtained for 

 gaseous exchange, for this source of error was neglected altogether 

 by the Danish workers. From the careful investigations of Aggazzotti 

 on the pH, and of Healy & Peter on the total acidity of the yolk 

 and white, I made an estimate of the extent to which the respiratory 

 quotient values would be affected if carbon dioxide were retained 

 and neutralised instead of being excreted. The results, shown in 

 Fig. 144 by special points, showed that the values might come higher 

 by as much as 0-5 respiratory quotient units when so corrected, but 

 not more. The effect of the alkali reserve may therefore be regarded 

 as quite small. The line joining the series of points in Fig. 144 repre- 

 sents the respiratory quotient calculated from various chemical 

 analyses; this will be referred to in detail later. On the whole, the 

 evidence points to a combustion of fat after the 8th day (Bohr & 

 Hasselbalch's average then was o-68) and to more complicated events 

 before that time. 



4- 1 2. Heat-production of Avian Embryos 



Bohr & Hasselbalch's observations on the heat-production of the 

 egg are shown graphically in Fig. 145. There are several remarkable 

 things about this curve. Firstly, during the first few days of develop- 

 ment they observed an absorption of heat, not an output. They 

 were convinced that this process could not be accounted for as 

 a meaningless effect due to technique, but that it was a real 

 physiological phenomenon, and they associated it with the produc- 

 tion of oxygen which Hasselbalch had previously shown to go on 

 before the 3rd day. They pictured the existence of some endothermic 

 synthetic process which gave off oxygen as a by-product, and even 

 suggested that this might go on throughout development obscured 

 by the mass of the usual respiratory exchange. No satisfactory 

 explanation has so far been advanced for the initial heat absorption 

 shown in Bohr & Hasselbalch's work, and, as no calorimetry of the 

 egg has since been published, it has remained unconfirmed, although 

 Barott, I understand, has unpublished experiments indicating that it 

 is an artifact. Its possible theoretical importance has already been 

 indicated in connection with the work of Rapkine on the echinoderm 

 egg (see p. 648). The second striking thing about the figure is the 

 fact that the observed values for heat-production agree so well with 

 the values calculated from the oxygen taken in and the carbon dioxide 



