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. 



More recently Grosvenor and Smith have found for 

 Moina that if females are reared alone (at 25°-30° C.) 

 no sexual broods appear, while parallel cultures of 

 females crowded together give 30 or higher per cents 

 of males. Agar has carried isolated females through 46 

 parthenogenetic generations and has found that mothers 

 from late brood also give only parthenogenetic offspring 

 in a suitable environment. 



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

 microscopic wormlike animal belonging to the rotifers, 

 reproduces by parthenogenesis. 



The resting egg always gives rise to a parthenogenetic 

 female, i.e. she 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 



