478 OSCAR RIDDLE 
As regards yolk formation in insects, conditions are peculiar; 
the nurse cell seems here largely to carry out the work of yolk 
formation; while certainly the de-formation process normally is 
carried out by the egg only. 
All observers agree that the outermost layer of yolk in any 
egg or growing odcyte consists of finely granular yolk. If this 
were otherwise our general theory of yolk formation would be 
untenable. Sarasin (’83) was led to the odd idea that the zones of 
the Lacerta egg were developed outermost first, and the central 
ones last. I think our demonstration of the nature of this growth 
in the bird, and the considerations that have followed, will con- 
vince that Sarasin’s view is untenable. 
Yolk spherules have been seen to grow after the egg leaves the 
ovary by Agassiz (’59), Van der Stricht (’07) and others. This 
growth is quite surely due to the spherule taking up by osmosis 
water-particles from the albumen or other fluid encountered by 
the egg; such growth is not of the nature we have described, 
though neither of the above mentioned authors has made the dis- 
tinction. 
In regard to the conclusion of many cytologists that yolk arises 
from the egg-nucleus, and of still others that it arises from the 
follicular cell nuclei, or from these cells in toto, I may append the 
following to show that we can exclude all of these as inadequate 
in the case of the yellow yolk of the fowl’s egg. I have calculated 
that during the last day that a hen’s ovum remains in the ovary 
it may deposit more than 5000 cubic mm. of yolk! Evidently too 
much work for an egg-nucleus. Again, since the radius of such an 
egg is increased by 2.0 mm. per day, this means that if yolk for- 
mation be a function of the follicular cell, each such cell must 
here produce daily a column of yolk 2.0 mm. long and of the diam- 
eter of the cell; that is to say each such cell must form more than 
50 times its volume of yolk per day, or more than twice its volume 
per hour! Evidently too much vicarious labor for a cell. 
It appears then that an exclusive origin of yolk from the nu- 
cleus, or within the follicular cells is impossible in the birds. The 
quantities of yolk laid down daily are amounts compatible with 
substances undergoing physical translocation by osmosis, solution, 
