94 



THE EVOLUTION OF SEX. 



which in all essentials is comparable to an amoeba. Even in the 

 simplest animals, however, the amoeboid phase 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 passivity to one in which passivity undoubtedly prepon- 

 derates, is associated with an increase of nutriment and reserve-pro- 

 ducts. The ovum feeds, becomes heavy with stored capital, becomes 

 less active, and more encysted in consequence. 



Fig. 25. — Ovum of a Threadworm (Ascaris), showing (a) the 

 chromatin elements of the nucleus, and (/') the appearance of the 

 surrounding yelk. — From Carnoy. 



III. Yelk. — 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 eggshell of the extinct giant-bird of Mada- 

 gascar (Epiornis) is big enough to hold the contents of one hundred 

 and fifty hens' eggs. Similarly the contrast between the eggs ot 

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

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



