278 EMBEYOLOGY. 



SECTION II. 



DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 



447. 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 composed, 

 as we have seen (fig. 287), of the albumen and the yolk, with 

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

 a certain time, a living animal, composed apparently of ele- 

 ments entirely different from those of the egg. Endowed with 

 organs perfectly adapted to the exercise of all the functions of 

 animal and vegetative life, having a pulsating heart, a digestive 

 apparatus ; organs of sense for the reception of outward im- 

 pressions, and having, moreover, the faculty of performing 

 voluntary motions, and of experiencing pleasure and pain. 

 These phenomena are certainly sufficient to excite the curi- 

 osity of every intelligent person. 



448. 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 incubation 

 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 deve- 

 loped. Modern researches have taught us that these gradual 

 changes, although complicated, and at first sight so mysterious, 

 follow laws which are uniformly the same in each department 

 of the animal kingdom. 



449. The study of these changes constitutes that branch of 

 Physiology called EMBRYOLOGY ; as there are differences in the 

 fourgreat departments of the animal kingdom perceptible at an 

 early stage of embryonic life, quite as obvious as those found 

 at maturity ; and, as the phases of embryonic development af- 

 ford important indications for the natural classification of 

 animals, we propose to give the outlines of Embryology, so 

 far as it may have reference to zoology 



450. In order to understand the successive steps of em- 

 bryonic development, we must bear in mind that the whole 

 animal body is formed of tissues, the elements of which are 

 cells. These cells are more or less diversified and modified, or 



