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984. THE OVARIAN EGG. 
so minute, that, when placed under a very high 
magnifying power, it is but just visible. This 
is the incipient egg, and at this stage it differs 
from the surrounding cells only in being some 
what darker, like a drop of oil, and opaque, 
instead of transparent and clear like the sur- 
rounding cells. Under the microscope it is found 
to be composed of two substances only: namely, 
oil and albumen. It increases gradually, and 
when it has reached a size at which it requires 
to have its diameter magnified one thousand 
times in order to be distinctly visible, the outside 
assumes the aspect of a membrane thicker than 
the interior and forming a coating around it. 
This is owing, not to an addition from outside, 
but to a change in the consistency of the sub- 
stance at the surface, which becomes more closely 
united, more compact, than the loose mass in the 
centre. Presently we perceive a bright, lumi- 
nous, transparent spot on the upper side of the 
egg, near the wall or outer membrane. This is 
produced by a concentration of the albumen, 
which now separates from the oil and collects at 
the upper side of the egg, forming this light spot, 
called by naturalists the Purkinjean vesicle, after 
its discoverer, Purkinje. When this albuminous 
spot becomes somewhat larger, there arises a 
little dot in the centre, —the germinal dot, as it 
is called. And now we have a perfect cell-struo 
