462 OSCAR RIDDLE 
THE COINCIDENCE OF THE AMOUNT OF YOLK DEPOSITED IN 
A DAY, WITH THE AMOUNT OF YOLK CONTAINED IN A 
STRATUM OF WHITE AND YELLOW YOLK 
A comparison of the two sections of table 1 shows quite convinc- 
ingly, I think, that the figures, which in the one column indi- 
cate the amount of a day’s growth, are of the same order of magni- 
tude as those which in the other column indicate the thickness of 
a stratum of yolk. This fact, and another one, namely, that we 
know that there exists in birds a daily nutrition rhythm capable of 
producing daily growth-marks in their rapidly growing feathers, 
convince us that a layer of white yolk and another of yellow yolk ws 
laid down during each twenty-four hours. 
The well-developed appearance of the yellow yolk, its large 
yolk-spherules andits much greater thickness than that of the white 
layer, all indicate, moreover, that this layer, like the broad fun- 
damental bar of the feather, is grown under the best nutritive 
conditions; while the narrow layer of white yolk with its small 
spherules gives indication that it, like the fault-bar of the feather, 
is grown under poor nutritive conditions. 
Since I have shown that the poor nutritive conditions which pro- 
duce the fault-bar occur in the later hours of the night—1 :00-5:00 
a.M.—I consider it as practically certain that the white yolk of the 
ovum is produced at the same time, and that the yellow yolk is pro- 
duced during all other hours of the day. 
The layer of white yolk of the hen’s egg is then a growth-mark 
left at the ever-changing boundary of the ovum; it represents 
the results of yolk formation. under sub-optimal conditions. It 
is indeed incomplete, unfinished yolk, as is apparently indicated by 
the histological data already known, and by the chemical evidence 
which I shall present in another section. 
YOLK STRATIFICATION IN OTHER ANIMALS AS SEEN IN THE 
LIGHT OF ITS CAUSATION IN BIRDS 
With the story of the white and yellow yolk of the bird in mind 
it becomes most instructive to reéxamine many of the peculiar 
types of yolk distribution which from time to time have been re- 
