REPRODUCTION. 57 



same time permits the entrance of oxygen and moisture, and 

 conducts heat, all being essential for the development of the 

 germ within this large food-mass. The shell serves, evidently, 

 chiefly for protection, since the eggs of serpents (snakes, turtles, 

 etc.) are provided only with a very tough membranous cover- 

 ing, this answering every purpose in eggs buried in sand or 

 otherwise protected as theirs usually are. As the hen's egg is 

 that most readily studied and most familiar, it may be well to 

 describe it in somewhat further detail, as illustrated in the 

 above figure, from the examination of which it will be ap- 

 parent that the yelk itself is made up of a white and yellow 

 portion distributed in alternating zones, and composed of cells 

 of different microscopical appearances. The clear albumen is 

 structureless. 



The relative distribution, and the nature of the accessory or 

 non-essential parts of the hen's egg, will be understood -yfhen it 

 is remembered that, after leaving its seat of origin, which will 

 be presently described, the ovum passes along a tube (oviduct) 

 by a movement imparted to it by the muscular walls of the 

 latter, similar to that of the gullet during the swallowing of 

 food ; that this tube is provided with glands which secrete in 

 turn the albumen, the membrane (outer), the lime salts of the 

 shell, etc. The twisted appearance of the rope-like structures 

 (chalazoe) at each end is owing to the spiral rotatory movement 

 the egg has undergone in its descent. 



The air-chamber at the larger end is not present froni the 

 first, but results from evaporation of the fiuids of the albumen 

 and the entrance of atmospheric air after the egg has been laid 

 some time. 



THE ORIGIN AND DEVELOPMENT OF THE OVUM. 



Between that protrusion of cells which gives rise to the bud 

 which develops directly into the new individual, and that which 

 forms the ovary within which the ovum as a modified cell arises, 

 there is not in Hydra much difference at flrist to be observed. 



In the mammal, however, the ovary is a more complex struct- 

 ure, though, relatively to many organs, still simple. It consists, 

 n the main, of connective tissue supplied with vessels and nerves 

 inclosing modifications of that tissue {Graafian follicles) within 

 which the ovum is matured. The ovum and the follicles arise 

 from an inversion of epithelial cells, on a portion of the body 



