DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 141 



are accompanied, in some animals, by a rotation of the yolk 

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

 mollusks, especially in the snails. 



296. 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 sub- 

 divided into two more, and so on, till the whole yolk as- 

 sumes the appearance of a mulberry, each of the spheres of 

 which it is composed having in its interior a transparent 

 vesicle. This is the case in mammalia, most mollusks, 

 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. 



297. But whether complete or partial, this process leads 

 to the formation of a germ comprising the whole yolk, or 

 rising above it as a disk-shaped protuberance, composed of 

 little cells, which has been variously designated under the 

 names of germinative disk, proligerous disk, blastoderma, 

 germinal membrane. In this case, however, that portion of 

 the yolk which has undergone less obvious changes forms, 

 nevertheless, part of the growing germ. The disk again 

 gradually enlarges, until it embraces the whole, or nearly 

 the whole, of the yolk. 



298. At this early epoch, namely, a few days, and some- 

 times a few hours, 



after development 

 has begun, the 

 germ proper con- 

 sists of a single 

 layer composed Fig. 102. Fig. 103. 



* In the Birds and higher reptiles we find, in the mature egg, a peculiar 

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

 a similar process before it was laid. 



