THE OVARIAN EGG. 285 
ture, differing from an ordinary cell only in 
having the inner sac, enclosing the dot, on the 
side, instead of in the centre. The outer mem- 
brane corresponds to the Hctoblast, or outer cell 
sac, the Purkinjean vesicle to the Mesoblast, or 
inner cell sac, while the dot in the centre an- 
swers to the Entoblast. When the Purkinjean 
vesicle has completed its. growth, it bursts and 
disappears ; but the mass contained in it remains 
in the same region, and retains the same char- 
acter, though no longer enclosed as before. 
At a later stage of the investigation, we see 
why the Purkinjean vesicle, or inner sac of the 
ege, is placed on the side, instead of being at 
the centre, as in the cell. It arises on that side 
along which the axis of the little Turtle is to lie, 
—the opposite side being that corresponding to 
the lower part of the body. Thus, the lighter, 
more delicate part of the substance of the egg 
is collected where the upper cavity of the ani- 
mal, enclosing the nervous system and brain, is 
to be, while the heavy oily part remains beneath, 
where the lower cavity, enclosing all the organs 
of mere material animal existence, is afterwards 
developed. In other words, when the egg is a 
mere mass of oil and albumen, not indicating as 
yet in any way the character of the future ani- 
mal, and discernible only by the microscope, the 
distinction is indicated between the brains and 
