110 EMBRYOLOGY. 



To learn how this takes place is certainly sufficient to excite 

 the curiosity of every intelligent person. 



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

 bation for 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 ani- 

 mal ; while those which have been sat upon for a longer 

 period include an embryo chicken proportionally more de- 

 veloped. 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. 



292. The study of these changes constitutes that peculiar 

 branch of Physiology called EMBRYOLOGY ; and as there are 

 distinctions of the four great departments of the Animal 

 Kingdom perceptible at an early stage of embryonic 

 life, quite as positive as those found at maturity ; as also, 

 the phases of embryonic development indicate still other 

 grounds for natural classification, we propose to give the 

 outlines of Embryology, so far as it is concerned in zoologi- 

 cal arrangement. 



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

 bryonic development, we must bear in mind that the whole 

 animal body is composed of tissues, whose elements are 

 cells (39). These cells are much diversified in the full 

 grown animal ; but, at the commencement of embryonic 

 life, the whole embryo is composed of minute cells of nearly 

 the same form and consistence. These cells originate 

 within the yolk, and constantly undergo new changes under 

 the influence of life. New cells are formed, while others 

 disappear, or are modified so as to become blood, bones, 

 muscles, nerves, &c. 



294. We may form some idea of this singular process, by 

 noticing how, in the healing of a wound, new substance 



