140 EMBRYOLOGY. 



Kingdom perceptible at an early stage of embryonic life, 

 quite as obvious as those found at maturity, and as the 

 phases of embryonic development furnish important indi- 

 cations for the natural classification of animals, we propose 

 to give the outlines of Embryology, so far as it may have 

 reference to Zoology. 



293. 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, (39.) These cells, however, are more or less diversi- 

 fied and modified, or even completely metamorphosed in the 

 full grown animal ; but, at the commencement of embry- 

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

 nearly the same form and consistence, originating within the 

 yolk, and constantly undergoing changes under the influence 

 of life. New cells are successively formed, while others 

 disappear, or are modified and so transformed as to become 

 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 

 is supplied by the transformation of blood. Similar changes 

 take place in the embryo, during its early life ; only, instead 

 of being limited to some part of the body, they pervade the 

 whole animal. 



295. The changes commence, in most animals, soon after 

 the eggs are laid, and are continued without interruption 

 until the development of the young is completed ; in others, 

 birds for example, they proceed only to a certain extent, and 

 are then suspended until incubation takes place. The yolk, 

 which at first consists of a mass of uniform appearance, grad- 

 ually assumes a diversified aspect. Some portions become 

 more opaque, and others more transparent ; the germinal 

 vesicle, which was in the midst of the yolk, rises to its upper 

 part where the germ is to be formed. These early changes 



