PARTHENOGENESIS 185 



more and more easily and change over to the sexual 

 phase of the cycle. 



What has just been said about the successive broods 

 might be said equally of the first-born offspring of the 

 successive generations, as Papanicolau's table shows 

 (Fig. 94). Later born offspring respond more readily 

 than do those that are historically nearer to the fer- 

 tilized egg. 



It seems to me that these results become a little less 

 obscure if we suppose some substance is produced during 

 fertilization, that is carried by successive broods and 

 successive descendants in an ever decreasing amount. 

 As it becomes used up, the change is indicated by the 

 color change in the egg. When it disappears, the sexual 

 phase comes on. Its disappearance may be hastened 

 by cold or by starvation. 



A third type, Hydatina senta (Fig. 95), an almost 

 microscopic worm-like animal belonging to the rotifers, 

 reproduces by parthenogenesis. 



The resting egg always gives rise to a parthenogenetic 

 female, which also reproduces by parthenogenesis. 

 Whitney has obtained 500 generations produced in this 

 way. But from time to time another kind of individual 

 appears. She is externally like the parthenogenetic 

 female, but has entirely different capacities. Her 

 eggs may be fertilized, and if they are they become 

 resting eggs inclosed in a hard case. The sperm enters 

 when the eggs are immature and still in the ovary of 

 the mother. The presence of a spermatozoon in an egg 

 determines that the egg goes on to enlarge and to pro- 

 duce its thick coat. But if perchance no males are 

 there to fertilize the eggs, this same female produces a 



