WHITE AND YELLOW YOLK OF OVA 483 
Yellow yolk, on the other hand, probably indicates, wherever 
it is found in ova, rapid growth under better nutritive conditions. 
9. The presence of alternating layers or zones of yolk (Schich- 
tung) in the ova of some animals thus receives an explanation. 
A period of poor nutrition corresponds to each of such zones of 
white yolk; a period of better nutrition to each layer of yellow 
yolk. 
10. The time of formation of a pair of such zones is known in 
the birds to be one day; in the turtle and myxinoid perhaps a 
year; in the skate possibly nearly a month; in the lizard this is 
quite unknown. 
11. This ‘Schichtung’ of the yolk, and other peculiarities of 
yolk distribution, have produced great similarity in the gross 
morphology of eggs of widely separated forms, e. g., selachian and 
bird; amphibian and marsupial. We can be confident that such 
similarities do not depend upon heredity in a strict sense, but 
upon the fact that these eggs have developed under like con- 
ditions. 
12. The gross chemical composition of white and yellow yolk, 
and of yellow yolk undergoing de-formation or digestion (a) by 
the embryo and (6) by the follicular cells, have been determined 
and comparisons made. | 
13. White yolk contains much more water, proteid, and extrac- 
tives, and much less fat and phosphatid than does yellow yolk. 
14. When yellow yolk is digested, in either of the two situa- 
tions named, its constituent parts are not digested, utilized, and 
absorbed at a uniform rate; but in such a way that the compo- 
sition of what remains approaches the gross normal composition 
of white yolk. In such digestion fat and phosphatid are broken 
down more rapidly than is protein. 
15. The immediate mechanism of yolk formation and of yolk- 
de-formation are the same. Chiefly involved are two factors— 
not previously applied here—which we recognize as (a) the rever- 
sible action of enzymes, and (b) the partition coefficients of the 
several constituents of yolk. 
16. The presence of the native lipochrome coloring matter 
—vitello-lutein—in the large spherules of yellow yolk only, is © 
