CYTOLOGICAL STUDIES OF CENTRIFUGED EGGS 649 
and the shrunken egg. At the same time the protoplasm with 
the remaining yolk shrinks into a small sphere. If we could 
account for a decrease of pressure over one pole in the region of the 
blastodisc, the movement observed may be accounted for, since 
the protoplasm, like any other fluid, w411 flow from a region of 
higher pressure to one of lower. The micropyle is the only direct 
communication with the outside. Through it water might escape, 
but that the formation of the normal blastodisc is not due to 
the presence of the micropyle above it is shown by the forma- 
tion of the blastodisc elsewhere when the eggs are shaken so that 
the egg is turned inside its membranes. 
The centrifuge increases the pressure on that pole that is turned 
outwards in the sense that the protoplasm being heavier than 
the yolk in the fish's egg is thrown more towards the outer side 
of the egg. In consequence the centrifugal force accomplishes in 
a minute that which it takes the normal egg an hour to bring 
about. This fact may throw some light on the nature of the 
normal phenomenon that has been so difficult to explain. The 
distribution of the protoplasm on the surface is due to the stretch- 
ing of the peripheral protoplasm by the yolk-sphere pressing it 
against the membrane. If the pressure were released the proto- 
plasm would flow towards that region where it is already thickest. 
But what releases the pressure? If substances are set free from 
the protoplasm so that it shrinks, the formation of the blasto- 
derm could be explained. Agassiz and Whitman describe the 
disappearance of watery spaces in the protoplasm. These may, 
it is suggested, produce the fluid that comes to lie between the 
egg and its membrane. 
If our analysis is correct for the normal process we see that the 
centrifuge brings about its results by the same method as that 
of the normal egg, and the physical phenomenon is the same. 
The centrifugal force by driving out the fluid from the vacuoles 
in the protoplasm hastens the formation of the blastodisc. 
