DEVELOPMENT OF THE YOUNG WITHIN THE ECG. 139 



SECTION H. 



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 AnimaJ 



