482 OSCAR RIDDLE 
chromatin, mitochondria, or centrosome, etc., of the egg, may 
later be shown to have special causal significance with regard to 
such changes in amount of food-intake, or with the production 
of reducing centers, which we now recognize as basic and unknown 
features of the cunditions which primarily initiate yolk formation. 
If so, then such cell-structure will have been shown to bear indi- 
rect causal relation to a result which it was formerly credited 
with ‘causing’ directly; the test of this hes with the future. But, 
some at least, of the more direct and immediate features of yolk- 
building are quite certainly those which have been described in 
these pages. 
SUMMARY 
1. A method of measuring the rate of growth of large, rapidly 
growing ova has been found. It consists in feeding, at known 
intervals, the fat stain Sudan ITI to animals developing such ova. 
2. Ova of the common fowl smaller than 6 mm. in diameter 
grow extremely slowly as compared with ova of larger size. 
3. The time interval between the beginning of rapid growth of 
the 6 mm. egg, and the breaking of the egg from the ovarian folli- 
cle (ovulation) is normally between five and eight days. In most 
cases it is either six or seven days. 
4. The radii of ova which are larger than 6 mm. usually 
increase nearly 2 mm. during each twenty-four hours. 
5. The thickness of a layer of white yolk together with an 
adjacent layer of yellow yolk is nearly 2 mm. 
6. A pair of such yolk layers is therefore produced during 
each twenty-four hours. 
7. We conclude that the layer of white yolk in the hen’s egg 
is laid down during poorer nutritive conditions obtaining in the 
later hours of the night (1-5 a.m.) and that the yellow yolk is 
deposited during the better growth conditions of the rest of the 
day. ; 
8. Reasons are found for believing that white yolk wherever 
found is but a stage in the formation, or the de-formation, of yellow 
yolk. That it remains as the final form of yolk, only where it is 
slowly grown or is halted by sub-optimal growth conditions. 
