284 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 célls 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-struc- 
