WHITE AND YELLOW. YOLK OF OVA 467 
development of yellow yolk. This same conception had been 
urged on histological grounds by several workers, though opposed 
by others. The chemistry of the two substances was then ap- 
pealed to for further evidence of a sort which it alone could 
give. 
An examination of the rather abundant literature on the chem- 
istry of yolk showed that it contained none of the data which our 
problem required. Analyses of yellow yolk have indeed been made 
by Prout, by Gobley and by Parke; but it was believed that the 
extraction methods of their time did not effect a complete separa- 
tion of the fat from the other constituents of the yolk. These 
determinations have therefore been made anew. That such was 
really necessary may be indicated by the fact that Parke (’67) 
extracted only 66.7 per cent of fat and phosphatids, whereas my 
analyses always yielded more than 70 per cent of these constitu- : 
ents. It was also imperative of course that results of analyses 
which were to be compared should be obtained by identical 
methods. Apparently no analysis of white yolk had been made, 
so that this had to be done. 
Since, moreover, the metabolism of yellow yolk includes not 
only its formation but also its de-formation into absorbable con- 
stituents, it was considered necessary to take account of yolk in 
a late stage of such modification. Such yolk is met with in two 
rather different situations: Normally, the whole yolk of the egg 
(yellow yolk) is subjected during the incubation period to the 
digestive, 7.e., disintegrative action of the embryonic tissues— 
entoderm and yolk sac. Under such modifying action does yellow 
yolk become more like white yolk, or does it become less like it? 
A similar digestive action occasionally overtakes an ovum in 
situ, 7.e., while still in the ovary and surrounded by follicular cells. 
These are the so-called ‘resorbed ova.’ How does the yolk of such 
an ovum in an advanced stage of resorption compare with the 
yellow yolk which it was before the beginning of resorption? Has 
it become more like, or less like white yolk? 
The complete results of my analyses with a consideration of 
their points of chemical interest, and an account of the preparation 
of materials, and of methods used, will be published elsewhere. 
