WHITE AND YELLOW YOLK OF OVA 479 
etc., but not compatible with the probable rate of organic synthesis 
in the restricted regions of either the nucleus or follicular cell. 
It is of course necessary for all of the material entering into yolk- 
formation to pass through or between the follicular cells; but each 
particle of this material may have, by undergoing the synthesis in 
situ in the egg, twenty-four hours or longer to accomplish this; 
whereas we have seen that if it originated within the follicle each 
cell would there have to organize completely its own volume of 
yolk material and empty itself of this more or less solid material 
at least once in each twenty or thirty minutes of the day. 
Since such theories of yolk formation as have been proposed 
are now shown to be inadequate in a case where a test can be 
applied, and since it seems clear that the mechanism of yolk build- 
ing which we have here outlined and described is necessarily: 
present wherever and whenever yolk is formed, there is at present 
no valid reason for believing that any dissimilar method of yolk 
formation exists. 
In a certain sense, no general theory of yolk formation has as yet 
been stated. That is to say, no outline of the processes involved 
in yolk-building and of the conditions affecting these processes 
has been attempted, and our own effort leaves at least important 
chemical phases of the problem quite untouched. Previous 
efforts have been largely devoted to features of the histogenesis 
of yolk granules, and to the identification of some cell organ as 
the directive agent of yolk formation. Thus such cell structures 
as centrosome, nucleus, chromatin, nucleolus, mitochondria, 
yolk-nucleus, ete., have each been several times proposed as the 
seat or source of yolk. Whilst for some eggs, particularly those 
in which all of the yolk plainly could not have so circumscribed 
an origin, the seat or source of such yolk was centered upon a sim- 
ilar structure of the follicular cell; yolk particles have some- 
times been described as arising in such cell and later making their 
way through the follicle cell membrane, vitelline, or other egg 
membranes, into the periphery of the egg. But theory usually 
has extended only to the matter of the source of yolk, to the rela- 
tion between the white and yellow granules, or to the designation 
of one or another cell-organ as the directive agent of yolk forma- 
