320 THE UNFERTILISED EGG AS A [pt. iii 



urgently than this, and the correlation of chemical constitution with 

 developmental type should offer a most attractive field for research. 



But it is not only correlations of this type that lie hidden under 

 the enigmatic character of analytical figures. The water-content of 

 the eggs may have a powerful effect on the sex-ratio, for King found 

 in 191 2 that reducing the water-content of fertilised frog's eggs con- 

 siderably lowered the proportion of males, while increasing it by 

 means of treatment with dilute acid considerably raised the pro- 

 portion. A discussion of these facts in relation to genetics as a whole 

 will be found in the review of Huxley. It is probable that the effect 

 which delayed fertilisation has upon the sex-ratio is to be explained 

 by difference in water-content of the eggs. Hertwig was the first to 

 observe this delayed fertilisation phenomenon in some work which 

 he published in 1905, and since then it has many times been observed 

 not only for amphibia but also for trout (Kuschakevitsch; Huxley; 

 Mrsic) . Riddle has suggested that the mammalian egg may be subject 

 to such influences as it passes from ovary to uterus. He quotes van 

 der Stricht's histological work on the bat's egg during this process, 

 and points out that the swelling of the yolk-granules would indicate 

 an absorption of water. The exact degree of hydration of the mam- 

 malian egg might thus conceivably have an effect on the mammalian 

 sex-ratio. 



Table 30 has several more important points which have not, so 

 far, been touched upon. It is interesting to follow in the figures of 

 Milroy the difference between the fish eggs which float at the surface 

 of the water during their development (pelagic ova), and those which 

 sink, or rather float, at lower and denser levels (demersal ova) — 

 the former have a water-content of about 90 per cent., the latter of 

 about 70 per cent. A knowledge of the chemical composition of fish 

 eggs throws a great deal of light upon their distribution in the sea, 

 and so indirectly upon ecological problems. Their fat-content, for 

 example, has been treated from this point of view by Polimanti, whose 

 work will be discussed in the section on the general metabolism 

 of the embryo; and the investigations of the specific gravity of fish 

 eggs, which are discussed in Section 5, have also an important 

 bearing upon these problems. Another point worth notice is the 

 approximately constant percentage of cholesterol in different eggs, 

 nearly always about 500 mgm. per cent, of the wet weight, a pro- 

 portion which, roughly speaking, holds for the egg of the hen as well. 



