856 GENERAL METABOLISM [pt. iii 



this subject is that of Aggazzotti, but earlier workers made some 

 observations of the kind. Thus in 1863 Davy reported that he had 

 found in many kinds of birds' eggs that the albumen was always 

 alkaline and the yolk acid. In 1884 Tarchanov, in the course of his 

 work on " Tataeiweiss " already referred to (p. 272), measured the 

 titratable acidity of the egg-whites of various eggs, obtaining the 

 following results : .„ ,. . 



Alkalinity expressed as grams 

 potassium hydroxide in 

 per cent, of dry weight 

 Nidicolous Raven (fresh) 4-9 



„ (dev. 2 days) 1-4 



,, (still more) o-8 



Pigeon (fresh) 4-7 



,, (i week) 2-8 



Nidifugous Hen (fresh) 7-1 



,, (i week) 4-7 



,, (1^ weeks) 2-7 



,, (2 weeks) 2-3 



Thus the titratable acidity was greater in the case of the whites of 

 nidicolous birds than in those of nidifugous ones, and in all cases 

 it increased as development proceeded. This has often since been 

 confirmed. A solitary figure of Reiss' is available for the yolk and 

 the white of an elasmobranch egg, pH 5-6-6'0 in the case o^ Scyllium 

 canicula}. 



Aggazzotti's careful work on the hen's tgg, published in 191 3, 

 involved many measurements, both of pH. and titratable acidity, 

 which are incorporated in Figs. 211 and 212. The measurements of 

 />H were all made electrometrically. Taking first the graph which 

 shows the />H, it can at once be seen that Davy had been quite right. 

 The yolks of the eggs investigated by Aggazzotti had an average pYl 

 before incubation of 4-5, and the egg-whites one of 8-3. If the eggs 

 were not incubated, this hydrogen ion concentration remained un- 

 altered, as is shown on the graph by the dotted lines, and no change 

 took place over an even longer period. If incubation occurred, 

 however, there were marked changes in the fertile egg. The j&H of the 

 yolk rises steadily, attaining /?H 6 about the loth day of development 

 and neutrality by about the i6th, while that of the white equally 

 regularly falls, reaching neutrality on the loth and pYi 6 on the 

 15th day of incubation. There is thus a cross-over point when 50 per 

 cent, of development has been completed, and after that the white is 



^ Nothing is known about the pH of reptile eggs but their egg-white is non-coagulable, 

 like that of nidicolous birds (Deraniyagala) . 



