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 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, arid 

 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- 



