464 OSCAR RIDDLE 
8), the birds and at least in some mammals (fig. 3). These are 
the forms, too, which—with the exception of the mammal—in 
addition show a stratification of the main body of the yolk. 
Two other forms are known, the turtle and the cyclostome 
(Bdellostoma) in which the stratification and other features 
occur, as in theabove mentioned eggs, except that no Pander’s nu- 
cleus has been found. 
How may we explain at one and the same time the essential 
similarity of the yolk distribution in eggs of widely separated 
forms, and the often essential dissimilarity of its distribution in 
the eggs of closely related species? There seems now no doubt 
that all can be accounted for when one knows two things: first, 
the length of the growth period; and, second the chief fluctuations 
in the nutrition of theanimal during the growth period of the eggs. 
Most ova have no stratification, then, because the yolk is 
grown in a short season—the animal not being subjected to such 
severe alternations as winter and summer, while the process is 
going on; or, because the eggs remain very small and develop 
little yolk; or, again, because some ova have the extraordinary 
capacity of growing at the expense of somatic tissues. In such 
cases fluctuations in the nutrition of the animal are of little 
moment to the egg; the latter being able to feed well at the ex- 
pense of the organism as long as it continues to live. 
When stratification is present, however, I believe this to be a 
positive declaration that nutritive fluctuations did occur in the 
organism, and the number of the strata tobe a reliable index to 
the number of such fluctuations. The presence of yolk stratifica- 
tion in the eggs of an animal then is an invitation to the natura- 
list and physiologist to look for important nutritional variations 
in that animal. 
Thus far definite causal and time relations between such stratifi- 
cation and nutritional fluctuation has been determined only for 
the bird. What this time period is in Bdellostoma we can now 
only conjecture; but the fact that in a mature specimen eggs of 
a wide range of size exist possibly argues that these eggs are 
several years in forming. The further fact, that the animals lose 
much blood and become much weakened at each yearly spawning 
