SECT. 6] OF THE EMBRYO 879 



make it seem as if its water-content remains unaltered after that 

 time, whereas the results of other workers show a descent to about 

 the same value as in the unincubated egg. It is probable that there 

 is a descent, for the dense and viscous condition of the yolk at hatching 

 is familiar^. The course of affairs, then, maybe summed up by saying 

 that for the first 10 days a flow of water passes from white to yolk. 

 After that time the water-content of the white remains constant, but, 

 owing to its now very small size, becomes quite unimportant in 

 absolute reckoning, while that of the yolk declines again to its original 

 figure. Carini, without giving any figures, stated that the yolk- 

 volume increased at the expense of that of the white, which is in 

 agreement with the rest of our knowledge about it. He also found 

 that the digestibility of egg-white by pepsin decreased as incubation 

 proceeded, and he put this down to its decreasing water-content. 



What is the mechanism of this passage of water into the yolk? 

 Greenlee supposed that the greater concentration of osmotically 

 active substances in the yolk would amply account for it, and that 

 the transference of water was entirely due to osmotic causes. 

 He found that the rate of loss of water by infertile whites 

 followed an extremely regular course, rising with the temperature, 

 and falling with the time. He constructed curves for this, and was 

 able to express the whole process by an equation on the basis of 

 which the moisture content of the yolk and white of an infertile egg 

 at any given time after laying could be predicted for a given tempera- 

 ture and a given initial state. But the evidence in favour of the 

 water-current of infertile eggs being purely osmotic in nature is 

 not satisfactory (see p. 816). 



In the developing egg, however, matters are certainly more com- 

 plicated. Vladimirov criticised the older viewpoint, showing by 

 means of a simple calculation that the observed osmotic pressures 

 would not account for the movements of the water. He suggested 



^ The question of the " bound " water in the egg is one of much interest. Bound 

 water may be defined as that part of the water in which added solutes will not dissolve. 

 Zawadzki showed that the free water in the yolk is probably identical with the inter- 

 micellar liquid (the ultrafiltrate) of Bialascewicz (seep. 361). By adding known amounts 

 of sucrose and urea Hill found that 97 % of the water in the egg-white was free, and 

 85 % of that in the yolk. One egg-yolk would thus contain (besides 7-95 gms. solid) 

 6-05 gms. of free and 1-05 gms. of bound water ; one egg-white would contain (besides 

 6-39 gms. solid) 22-9 gms. of free and 0-71 gms. of boimd water. Nothing is known of 

 the variations which may occur in these factors during the development of the embryo, 

 or of the bound water in the embryo itself. 



