DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 109 



less plays but a secondary part in the history of their devel- 

 opment. It is not formed in the ovary, like the yolk, but is 

 secreted by the oviduct, and deposited around the yolk 

 during the passage of the egg through that canal. It con- 

 sists of several layers, one of which, the chalaza (c), is 

 twisted. On this account, the eggs of those animals in 

 which the oviduct is wanting are generally without the albu- 

 men. Like the yolk, the albumen is surrounded by a single 

 or double membrane, the shell membrane, which, in birds 

 and some reptiles and mollusks, is again protected by a cal- 

 careous covering, forming a true shell (s). In most cases, 

 however, this envelop continues membranous, particularly in 

 the eggs of the mollusks, most crustaceans and fishes, sala- 

 manders, frogs, &c. Sometimes it is horny, as in the sharks 

 and skates. 



SECTION II. 



DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 



290. The formation and development of the young animal 

 within the egg is a most mysterious phenomenon. From a 

 hen's egg, for example, surrounded by a shell and com- 

 posed, as we have seen (Fig. 101), of the albumen and the 

 yolk, with a little vesicle in the middle, there is produced, at 

 the end of a certain time, a living animal, composed in part 

 of totally different elements ; endowed with organs perfectly 

 adapted to the exercise of all the functions of animal and 

 vegetative life, having a pulsating heart, intestines fitted for 

 digestion, organs of sense for the reception of outward im- 

 pressions, and having, moreover, the faculty of performing 

 voluntary motions, and of experiencing pain and pleasure. 



10 



