EFFECTS ON EGGS OF FRESH WATER PULMONATES 441 
themselves is shown by those cases in which by long or hard cen- 
trifuging the substances were very completely separated in stages 
just before the first cleavage. Such eggs also orient slowly, 
though the ^^ellow and gray zones are sharply separated from the 
clear one. The progressive changes in the qualities of the sub- 
stances at the two poles of the egg have already been mentioned, 
the gray substance passing from a finely granular to a coarsely 
granular condition, and the yellow substance from a coarselj' 
granular to a finely granular state. 
On the whole, then, several important physical changes take 
place in the substances of these eggs between the periods of matur- 
ation and cleavage; these changes affect the quantities and quali- 
ties of the different substances, as well as their relative weights 
and viscosities. 
3. As has been shown (p. 420) the germinal vesicle at the time 
of laying is slightly eccentric toward the future animal pole of the 
egg, but when the eggs are centrifuged in this stage they lie in 
all possible positions and consequently the axis, of stratification 
may bear any relation to the chief axis of the egg. Consequently 
this stratification may take place in the chief axis, at right angles 
to it or anywhere between these two extremes. Nevertheless 
in eggs centrifuged before the formation of the first maturation 
spindle, the polar bodies always lie on the clear zone or adjoining 
it on the borders of the yellow or gray zones. A study of such 
eggs at the time of maturation shows that the spindle lies in the 
clear zone and is apparently prevented from moving into the gray 
or yellow zones. In all cases then in which the axis of stratifi- 
cation does not coincide with the maturation axis the latter has 
been shifted so as to cause the polar bodies to lie on the clear 
zone. Such eggs almost invariably develop normally, as a glance 
at table 1, Exp. 1-4, will show, but in such cases the polar bodies 
do not always lie at the animal pole. It seems probable therefore 
that while the maturation axis has been altered by centrifuging 
the chief axis of the egg has remained unchanged. 
When eggs are centrifuged during the first maturation division, 
the polar bodies invariably form on the gray zone, though in some 
cases (Exp. 5) near the edge of this zone. Either the entire egg at 
