THE OVARIAN EGG. 283 
not be uninteresting to my readers. I will take 
the egg of the Turtle as an illustration, since 
that has been the subject of my own especial 
study; but, as I do not intend to carry my. re- 
marks beyond the period during which the his- 
tory of all vertebrate eggs is the same, they may 
be considered of more general application. 
It is well known that all organic structures, 
whether animal or vegetable, are composed of 
cells. These cells consist of an outside bag en- 
closing an inner sac, within which there is a dot. 
The outer bag is filled with a more or less trans- 
parent fluid, and the inner one generally with a 
more perfectly transparent fluid, while the dot 
has a dark appearance. In the language of our 
science, the outer envelope is called the Ecto- 
blast, the inner sac the Mesoblast, and the dot 
the Entoblast. Although they are peculiarly 
modified to suit the different organs; these cells 
never lose this peculiar structure; it may be 
traced even in the long drawn-out cells of the 
flesh, which are like mere threads, but yet have, 
at least while forming, their outer and inner sac 
and their dot. 
In the Turtle the ovary is made up of such 
cells, spherical at first, but becoming hexagonal 
under pressure, when they are more closely 
packed together. Between these ovarian cells 
the egg originates, and is at first a mere granule, 
