WHITE AND YELLOW YOLK OF OVA 469 
mediate stage in the formation and indeed of the de-formation 
(digestion), of yellow yolk; although they do strongly support 
that view. There seems to be an alternative, namely, that the 
figures under nos. 2 and 3 approach the composition of white 
yolk more and more, only because the amount of that sort of yolk 
originally present in the egg is not diminishing, or is diminishing 
but slowly, whereas the yellow yolk is here being digested very 
rapidly. For, it must be remembered that, although we are con- 
sidering a mature hen’s egg as our type of yellow yolk, it still 
contains white yolk in quantities not easy to estimate; though 
we are accustomed to think of this amount as small, probably 
between 5 and 15 per cent of the total. 
Parallel to the chemical data are the histological conclusions 
that it is always white yolk and never yellow yolk that is found 
applied to a surface into which yolk is being ingested. This is 
true for the germinal disc of pre-embryonic stages, and for the 
advancing entoderm and yolk-sac of the embryo (Balfour, Agas- 
siz). Virchow (91, p. 105) however, questions the correctness of 
this statement. It is certainly almost always true for the nucleus, 
or germinal vesicle of the primary oédcyte, a seemingly significant 
fact upon which I shall publish observations elsewhere. Our 
chemical data themselves show, however, that the alternative can- 
not be true unless there is several times as much white yolk in an 
egg as we have reason to believe exists there. In any event the 
certain and interesting fact remains that when the yolk complex of 
the hen’s egg is subjected to digestive and absorptive processes, the fat 
and phosphatids digest and disappear much more rapidly than does 
the protein. 
- ON THE MECHANISM OF YOLK FORMATION AND DE-FORMATION 
Having presented datato answer questions three and four of the 
introductory statement, we may now consider the first and second 
questions in the light of these results, and with the help of other 
facts. Precisely how and where does yolk originate? Why or 
how is it that there are two forms of yolk; or, what isthe relation 
between these? 
