WHITE AND YELLOW YOLK OF OVA 463 
ported and figured by embryologists and cytologists; for now 
we can feel fairly sure that wherever we meet alternate layers of 
white and yellow yolk, such layers indicate just so many alterna- 
tions of better and poorer nutritive conditions during the time 
these layers were being formed. The better and poorer nutritive 
conditions doubtless applying to the organism as a whole.’ 
A zonal arrangement of yolk similar to that of the bird has been 
reported in at least four other groups, viz., turtles, lizards, skates, 
and myxinoids. Some yolk patterns are known which are not 
distinctly zonal but intermediate to it and the type of yolk 
arrangement which is usual in small eggs; these help to bring 
all yolk distribution under a single principle or set of principles. 
In order to avoid much tedious description in the text, and also 
to present more clearly and accurately this part of the subject, 
I have prepared plate 3, which is to a large extent a reproduction 
of figures which are not new. To what is shown in the plate, and 
in the explanation which accompanies it, I here add the following: 
In all ripe ova, as in all the growth stages during which yolk 
is being deposited in the ovum, a layer of yolk composed of very 
‘ small spherules (white yolk) is to be found at the extreme pe- 
riphery of the egg. If larger yolk spherules (yellow yolk) also 
occur, they occupy more central portions of the egg. There is, 
moreover, scarcely an exception to the rule that the germinal 
vesicle or egg-pronucleus is immediately surrounded by similar 
small spherules and not by large ones. 
It seems also to be very generally true that in those ova in which 
considerable yolk is developed, and in which the germinal vesicle 
makes its way from the center to the periphery of the egg (or 
remains near one side of the cell) it leaves in its wake a cylinder of 
white yolk to which in some cases has been given the name of 
Pander’s nucleus. 
All of these features are shown in eggs of such widely separated 
forms as the skate (fig. 6) the amphibian (fig. 5), the lizard (fig. 
3 On the other hand, some eggs, e.g., those of the salmon, may undergo their 
chief growth at the expense of the somatic tissues and while no food whatever is 
being ingested. The conditions here, however, are essentially constant and there- 
fore produce no stratification of the yolk. 
