836 



BIOPHYSICAL PHENOMENA 



[PT. Ill 



Bellini, Viscosity 



the first place, Bellini has studied the viscosity of the yolk and the 

 white in the hen's egg during the early part of its development, using 

 Fano & Rossi's viscosimeter. The values he obtained are shown in 

 Fig. 207. Evidently if the eggs are not incubated the viscosity of 

 the white increases steadily but slowly, and that of the yolk de- 

 creases at much the same rate. If the eggs are incubated, the vis- 

 cosity of the white increases more rapidly, and that of the yolk 

 decreases far more so, descend- 

 ing from 30 units to less than 

 I by the 6th day of develop- 

 ment. This reciprocal inter- 

 change of viscosity between 

 the yolk and white evidently 

 goes on irrespective of the 

 embryo, though it is much 

 accelerated by the presence of 

 the latter, and Bellini rightly 

 concluded that it was the result 

 of a continuous dilution of the 

 yolk from the water of the 

 white. I shall return to this 

 point in the succeeding sec- 

 tion, when the whole question 

 of water metabolism in em- 

 bryos is being discussed, and 

 though, as we have already 

 seen, the osmotic pressure of 

 the yolk is much superior to 

 that of the white, Vladimirov 



Fig. 207. 



work has made it very unlikely that 

 this can be the sole cause of the flow of water. Again, the viscosity 

 of the white increases by only 4 units in the first 10 days, and 

 that of the yolk decreases by as much as 30 units, so Bellini en- 

 visaged other processes than the dilution of the yolk by the water 

 from the white as playing some considerable part in the decline of 

 yolk- viscosity. It is known that the yolk contains an abundance of 

 enzymes, which the incubation at 37° might allow to act, e.g. pro- 

 teases, lipases; diastases (see Plate XI). Experiments that Bellini did 

 with incubated infertile eggs showed that the great decline of yolk- 

 viscosity did not go so far as with, developing ones, but on the other 



