lOO THE EVOLUTION OF SEX. 



constantly shows a tendency to pass into greater quiescence, to 

 become in fact more or less encysted. So is it with ova, which 

 though at first often resembling various forms of amoeboid cells, 

 tend more or less quickly to pass into the encysted phase. 

 The protoplasm no longer flows out in irregular ever-changing 

 processes, but is gathered up into a sphere, rounded off, and 

 surrounded by a more or less definite envelope. This transition 

 from a state of relative equilibrium between activity and pas- 

 sivity, to one in which passivity undoubtedly preponderates, is 

 associated with an increase of nutriment and reserve-products. 

 The ovum feeds, becomes heavy with stored capital, becomes 

 less active, and more encysted in consequence. 



§ 3. Yolk. — The essential part of an egg-cell is always small, 

 though even in this there are great differences. The nucleus, 

 for instance, in the large eggs of amphibians, reptiles, and 

 birds, may be detected with the unaided eye ; while in other 

 cases, such as sponges, the entire ovum is very minute. Yet 

 every one knows that eggs vary enormously in size. The egg 

 of a skate is very much larger than the egg of a salmon ; and the 

 egg-shell of the extinct giant bird of Madagascar (^Tvpyornis) is 

 big enough to hold the contents of one hundred and fifty hens' 

 eggs. Similarly the contrast between the eggs of ostrich and 

 humming-bird is, as one would expect, extremely striking. 

 Yet the eggs of whales are "not larger than fern-seed," and the 

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

 differences 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 yolk, which serves as nutritive capital for the embryo or 

 young animal. Besides the yolk, 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, various forms of 

 protective and attaching viscid material, and, lastly, more or 

 less elaborate egg envelopes or shells. The most important, 

 however, is the yolk, and in regard to its origin and dis})Osition 

 a little must be said. 



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 body, {b.) At the same 

 time, or in another case, it avails itself of the debris of surround- 

 ing cells. In many instances, e.g.., in the minute ovary of 

 hydra, or in the ovarian tubes of insects, the ovum is but the 



