EMBRYOGENY 



117 



of all animals but also a large amount of detail and generalization which 

 is outside all embryogenies. 



142. Types of Egg Cells. — A part of the process of maturation in the 

 egg cell consists in the accumulation of yolk, but the amount of yolk 

 thus stored and its distribution in the cell are not the same in all egg 

 cells. 



All egg cells show polarity. This is indicated by the polar bodies 

 being formed at or near the upper pole and by the nucleus, which is 

 always more or less excentric, being located nearer this pole. The 

 opposite pole is called the lower pole. 



In some cases the yolk is not very great in amount and is scattered 

 throughout the cytoplasm. Such egg cells are termed homolecithal 

 (Fig. 45 A and C). Most of the lower invertebrates and almost all of 

 the mammals possess this type of egg cell, though in mammals the 

 condition is not primary but secondary and is due to adaptation to the 

 peculiar mode of development. In some homolecithal egg cells there 



Fig. 46. — Diagrams of teloleeithal and centrolefithal egg cells, and discoidal and super- 

 ficial cleavage. A, telolecithal egg cell, in which the protoplasm is all at the upper pole. 

 B, the discoidal cleavage which occurs in a telolecithal egg cell. C, centrolecithal egg cell, 

 with a superficial layer of protoplasm, protoplasm around the nucleus, and strands of pro- 

 toplasm connecting the two. D, the superficial cleavage of a centrolecithal egg cell; an 

 early stage, when the nucleus has divided into several nuclei and each, with a portion of 

 protoplasm about it, is migrating toward the periphery. E, later stage in superficial 

 cleavage showing the nuclei and cytoplasm at the periphery, forming a superficial layer of 

 cells, and the yolk at the center. The protoplasm is stippled, the yolk indicated by the out- 

 lines of globules. 



is somewhat more protoplasm toward the upper than toward the lower 

 pole and somewhat more yolk toward the lower than toward the upper 

 pole. 



In the egg cells of almost all vertebrates but the mammals, however, 

 the yolk, which is present in very large amount, is massed toward the 



