456 _ OSCAR RIDDLE 
withstanding this great amount of study and description, the 
literature fails to give satisfactory answer to any of the following 
questions: (1) Precisely how and where does yolk originate? 
(2) Why, or how is it that there are two kinds of yolk, (a) smaller 
spherules (often with enclosures) of white yolk, and (b) larger 
spherules of finely granular (often pigmented) yellow yolk? and, 
what is the relation between these? (3) What is the meaning of 
the stratified condition of the yolk of some eggs, eggs in which 
layers of white yolk alternate with layers of yellow yolk? (4) 
What are the chief chemical differences between these two kinds 
of yolk? 
Thinking that we are now able positively to answer questions 
3 and 4, and that these solutions bring some light upon the first 
and second questions, we submit the following data and considera- 
tious. These are presented with a minimum of refereace to the 
enormous literature; otherwise this communication must have 
been increased to several times its actual size. 
In carrying out this work, and now in the presentation of it, 
the author would say that he has not forgotten that ‘yolk’ is 
‘non-living substance’ and therefore from a certain standpoint 
has but a minor interest to biologists. But, standpoints change. 
Until Johannes Miiller declared, and Van Beneden clinched the 
point, that the yolk of eggs is not living matter, and that it con- 
trasts absolutely with the other part of the egg—the protoplasm— 
yolk had an all-absorbing interest to naturalists as a substance 
per se. In the years that followed, yolk has been studied largely 
with a view to cataloguing its diverse occurrences, forms, origins, 
distribution, tingibility, etc.; its interest to most students has 
flagged; though its often overweening bulk in the most studied 
of all cells has frequently won for it unwilling and tedious descrip- 
tion. Perhaps one day we shall have a new standpoint. At any 
rate, we are only now beginning to realize that, though yolk is 
non-living substance, it is nevertheless organized substance and a 
very refined product of the vital laboratory; that it is a product 
laid down in the meshes of protoplasmic elements; and that the 
very act of its laying down is a signal of important metabolic 
states and capabilities of these living elements. More of the im- 
