THE EGG-CELL OR OVUM. 



95 



same is true for most mammals, except the very lowest. The differ- 

 ences in size, when very striking, are due not so much to any marked 

 disproportion in the essential parts of the ova, but to certain extrinsic 

 additions. The most important of these is the yelk, which serves as 

 nutritive capital for the embryo or young animal. Besides the yelk, 

 we have also to take into account the frequent pigment, so familiar in 

 frog-spawn, the albumen well seen in the white of birds' eggs, vari- 

 ous forms of protective and attaching viscid material, and, lastly, more 

 or less elaborate egg-envelopes or shells. The most important, how- 

 ever, is the yelk, and in regard to its origin and disposition a little 

 must be said. 



jL 



Fig. 26. — The relation between the disposition of the yelk and 

 the mode of segmentation. — ,4, diffuse yelk, for example, sponge. 

 B, polar; for example, frog. — C, central yelk; for example, cray- 

 fish. — D, predominant; for example, bird. — A ', total and equal 

 segmentation. — B', total and unequal segmentation. — C, periph- 

 eral segmentation. — D', partial segmentation. 



The egg has its nutritive capital increased in three different ways: 

 (a) Very generally it feeds on the nutritive elements in the general 

 lymph or vascular fluid of the body. (6) At the same time, or in another 

 case, it avails itself of the debris of surrounding cells. In many 

 instances — for example, in the minute ovary of hydra, or in the ovarian 

 tubes of insects — the ovum is but the surviving competitor among a 

 crowd of surrounding cells, which to start with were all potential ova. 



