CYTOLOGICAL STUDIES OF CEXTRIFUGED EGGS 637 
absorbed water — i.e., not on ovarian eggs, — and consequently cen- 
trifiiging, especially if slow, may do the same thing. There is a 
consequence of some importance involved in this conclusion. If 
the contents revolve as a whole it retains {i.e., it carries with it) 
the former polar or molecular structures. Consequently the polar 
bodies may be given off even after rotation through 180,° and the 
cleavage may begin at the light pole, and produce there small 
cells. The failure of the ectosarcal layer to take part in the 
movements of the interior may lead one to a false conclusion, 
if a comparison between the frog and other forms is made, unless 
this factor is taken into account. In other words it may appear 
that the polar bodies of the frog's egg may be given ofT at any 
point of the surface, if the immobile ectosarcal layer be taken as 
evidence of polar relation, while in reality the rotation, en masse, 
of the contents beneath the surface ma}' be responsible for the 
results. Other eggs than the frog's may be expected to give 
essentially the same results. 
3. CENTRIFUGING THE EGG OF HYDATINA 
Whitney has shown for the eggs of Hydatina senta that the 
typical three zones appear after centrifuging,and that normal 
embryos result irrespective of what part of the egg contains these 
materials. Since the egg is not fertilized, and since the polar 
bodies are given off at the side, this egg offers exceptional oppor- 
tunities for the study of the relation between the visible sub- 
stances in the egg and development. The results corroborate in 
ever}' way those obtained for the sea urchin, Cumingia, frog and 
other forms. 
The question has been raised whether the effects of centrifug- 
ing may not be different at different periods in the maturation of 
the egg — i.e. J differences may result according to whether the sep- 
aration takes place before or after the breaking down of the ger- 
minal vesicle. That differences may exist is abundantly shown 
from the evidence here adduced concerning what the centrifuge 
may do in regard to the nucleus and spindle, to take but two 
examples. These questions, however, do not touch the real point 
