DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 279 



even completely metamorphosed, in the full-grown animal ; 

 but, at the commencement of embryonic life, the whole em- 

 bryo -is composed of minute cells of nearly the same form 

 and consistence, originating within the yolk, and constantly 

 undergoing new change^ under the influence of life. New cells 

 are successively formed, while others disappear, or are mo- 

 dified, and so transformed as to become blood, bones, muscles, 

 nerves, &c. 



451. We may form some idea of this singular process, 

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

 is supplied by the transformation of the blood. Similar 

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

 instead of being limited to one part of the body, they pervade 

 the whole animal. 



452. 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, 

 gradually assumes a diversified aspect. Some portions be- 

 come more opaque, and others more transparent; the germinal 

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

 upper part of it, where the germ is to be formed. These 

 early changes are accompanied, in some animals, by a rotation 

 of the yolk within the egg, as may be distinctly seen in the 

 eggs of some of the mollusca, especially the snails. 



453. At the same time the yolk undergoes a peculiar 

 process of segmentation. It is first divided into halves, 

 forming distinct spheres, which are again regularly subdivided 

 into two more, and so on, till the whole yolk assumes the ap- 

 pearance of a mulberry, each of the spheres, of which it i& 

 composed, having in its interior a transparent vesicle. This 

 is the case in mammalia, most mollusca, worms, c. In 

 many animals, however, as in the naked reptiles, and fishes,* 

 this segmentation is only partial, the divisions of the yolk not 

 extending across its whole mass. 



454. But whether complete or partial, this process leads 



* In the birds and the higher reptiles, we find in the mature egg a pecu- 

 liar organ called cicatricula, which may, nevertheless, have been formed 

 by a similar process before it was laid. 



