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, 



