WHITE AND YELLOW YOLK OF OVA 465 
period, is significant in that here may be found the means of a 
nutritional depression which produces a layer of white yolk in 
all of the remaining eggs of the ovary. If this be the true explana- 
tion one can readily understand the lack of stratification inthe 
eggs of the related Petromyzon (fig. 2) since this form spawns but 
once in a lifetime. 
In the skate the main growth period of the odcyte is probably 
completed in less than a year. The nine orten pairs of strata 
figured by Rickert (fig. 6) are probably produced at the rate of 
about one per month. Whether this refers merely to the number 
of times the animal has fed during this time, or otherwise, nothing 
seems to be known. 
The amphibian egg has a short growth period, and derives its 
growth material too from substances stored in the body, and is 
thus independent of external food supply. Doubtless these facts 
—together with its usually moderate size—will account for the 
actual configuration of its yolk. 
The eggs of two reptiles—turtle and lizard—show very evident, 
but dissimilar, yolk strata. What the time, or the nature of the 
nutritive fluctuations are, that may produce these strata in La- 
certa, I can make no suggestion. 
In the egg of the tortoise Munson (’04) seems not to have 
identified (fig. 1) the so-called inner and outer cytocoel as layers 
of white yolk. A study of my own preparations, however, con- 
vinees me that such is their nature and the term cytocoel there- 
fore 1s unnecessary. The turtle’s egg has then alternate layers of 
white and yellow yolk somewhat comparable to those of the bird. 
I have found indications of four pairs of such zones in some eggs; 
or rather, by comparing the strata of different eggs from the 
same animal I have found such indications. But I'am not now 
sure that four such pairs exist, nor that only four exist. Cer- 
tainly several very thin strata can sometimes be found within 
2 mm. of the periphery of some ova. 
One wonders much whether the well-marked innermost layers 
of the turtle’s egg can be the indications of years of growth. Agas- 
siz (757) showed that these ova undergo their greatest growth in 
four interrupted stages extending over four years. Our predic- 
