DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 139 



SECTION II. 



DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 



290. The formation and development of the young ani- 

 mal 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 albumen and yolk, 

 with a minute vesicle in its interior, there is produced, at the 

 end of a certain time, a living animal, composed apparently 

 of elements entirely different from those of the egg, en- 

 dowed with organs perfectly adapted to the exercise of all 

 the functions of animal and vegetative life, having a pul- 

 sating heart, a digestive apparatus, organs of sense for the 

 reception of outward impressions, and having, moreover, the 

 faculty of performing voluntary motions, and of experi- 

 encing pain and pleasure. These phenomena are certainly 

 sufficient to excite the curiosity of every intelligent person. 



291. By opening eggs which have been subjected to incu- 

 bation during different periods of time, we may easily satisfy 

 ourselves that these changes are effected gradually. We 

 thus find that those which have undergone but a short incu- 

 bation exhibit only faint indications of the future animal ; 

 while those upon which the hen has been sitting for a 

 longer period include an embryo chicken proportionally 

 more developed. Modern researches have taught us that 

 these gradual changes, although complicated, and at first 

 sight so mysterious, follow a constant law in each great 

 division of the Animal Kingdom. 



292. The study of these changes constitutes that peculiar 

 branch of Physiology called EMBRYOLOGY. As there are 

 differences in the four great departments of the Animal 



