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. Thexouter mem- 

 brane corresponds to the .Ectoblast, or x 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 

 egg, 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 



