WHITE AND YELLOW YOLK OF OVA 473 
portion of the reacting substances present is here, as elsewhere 
under these conditions, the factor that determines whether the 
amount of yolk shall from time to time increase, remain constant, 
or diminish. What then are the conditions in the fowl that would 
tend to modify the amount of these reacting substances in the egg? 
In answer to this we revert to the facts forecasted at the begin- 
ning of this paper in regard to our own earlier demonstration (’08) 
of a daily rhythm of better and poorer nutrition in birds; which 
rhythms coincide with periods of higher and lower blood-pressure. 
It was there made certain that very rapidly growing organs 
(feather-germs) were usually unable to pass over the period of 
the nightly (1:00-5:00 a.m.) reduction in blood-pressure without 
showing defects; which defects were proved to be due to insuffi- 
cient nutrition. 
Now we think there is no doubt that these facts lead to an 
answer to the above question. The egg (like the feather germs) 
doubtless derives fewer nutritive particles from the blood at this 
time than during the rest of the day. Possibly, even probably, 
the low blood-pressure induces at this time feeble but effective 
currents of fluid from these cells towards the blood and lymph; 
for it is probable that under low-blood-pressure the volume of 
the blood tends to increase at the expense of the fluid of the tissues. 
At any rate it seems certain that at this time the intake of the 
food substances from the blood is reduced, with the result that the 
equilibrium of the reaction is shifted. Thus the morphological 
picture becomes changed. Now growth will proceed more slowly. 
It is now that the granules must remain small, and poor in fat. 
It is now that some of the larger yolk spherules (yellow yolk) 
may possibly suffer reduction to smaller spherules (white yolk) ; 
(Hanriot) ; (4) presence of other bodies eg. lecithin (Hewlett); (5) reducing 
conditions, i. e., conditions favorable to the formation of fat from carbohydrate 
and protein by reduction. A further reason for only a mere mention of these 
factors here is that the data for the egg are at present too meagre. 
The factors which have to do with the formation and storage of the protein 
constituents of yolk, and of their union with lipoids and fats to form yolk, 
are less known than those factors which involve fat metabolism only; therefore 
the latter only are treated here. Macallum (’91) has some interesting state- 
ments on related subjects, and further points out that similar processes occur 
in the formation of yolk and in the production of pancreatic zymogen. 
