WHITE AND YELLOW YOLK OF OVA 475 
2. The role of the partition coefficient of the elements of yolk 
Of less importance than the reversible action of enzymes, but 
following upon it, is the distribution between the yolk® and the 
blood of the soluble substances concerned in these reactions accord- 
ing to their relative solubilities in these two solvents. There can 
be no doubt that this distribution, or partition coefficient is a 
factor in determining the amount of soluble substance which comes 
from the blood, lymph, follicular cells, or vitelline membrane, to 
the periphery of the yolk, and vice versa. Such is a physical 
necessity. The constitutents of fat for example, quite certainly 
enter the egg in soluble form and must there be subject to the laws 
of solubility. 
The chief thing incumbent upon us in this connection is to 
point out how this partition coefficient may act selectively in - 
modifying the amount of the reacting substances in the egg; 
t.e., how this principle may contrive at one time to increase, and 
at another to decrease the quantity of yolk contained in the egg. 
Remembering that it is the amount of reacting substances present 
that decides whether yolk formation or yolk de-formation may 
occur, the answer can perhaps be more easily given in refer- 
ence to fig. 1. Let this figure now represent the periphery of the 
ovum of a turtle, in contact with the lymph and blood streams. 
During the summer, when the constituents of fat are probably 
most abundant in the blood, some of these must, on account of 
their solubility, pass into the egg and there later be built into 
yolk; their former places being continually taken by new particles 
from the constant supply of the blood stream. Under these con- 
ditions yolk spherules grow, as is represented by section A. In 
winter, conditions become asin C. The turtle cannot now digest 
food (Riddle, ’09). Its heart-beat and other activities, however, 
require food for their continuance, and the blood becomes depleted 
of food. The reversible action of-its yolk enzymes is not like- 
wise suppressed, but these now as before set free soluble yolk 
5 The word ‘yolk’ ishere made tostand for the whole body of the egg cell. Perhaps 
egg-protoplasm, follicular cell, vitelline membrane, and yolk, should all be men- 
tioned instead of ‘yolk’ alone. 
