104 SEX 



develop into females only, while others will 

 develop into males only. We are sure that 

 this is possible, because in the vine-pest 

 Phylloxera among Insects, and in Hydatina 

 among Rotifers, and in other cases, there are 

 large eggs which develop into females, and 

 small ones which develop into males. In 

 both these cases the ova develop without 

 fertilisation, so that there is no question as 

 to the possible influence of the sperm. 



Reuter has shown in one of the mites 

 (Pediculopsis), and Von Malsen has shown in 

 a primitive worm called Dinophilus, in both 

 of which fertilisation occurs as usual, that 

 there are large egg-cells which develop into 

 females, and small egg-cells which develop 

 into males. Size is never of much account 

 except as an index of something deeper, and 

 it is interesting to note that the Marchals 

 have proved that the asexually produced 

 spores of some of the mosses, though all 

 similar in appearance, are individually pre- 

 destined, indeed, fixedly predetermined, as 

 male-producing and female-producing units. 



We have already mentioned that " identical 

 twins," which are developed from one ovum, 

 are always of the same sex. In one of the 

 armadillos (Praopus or Tatusia hybrida) Von 

 Jhering found on two occasions eight embryos, 

 apparently from one ovum, and all male. 

 In some of the parasitic Hymenoptera, such 

 as Encyrtus, one ovum forms a group of 

 embryos, all of the same sex female if the 

 egg be fertilised, male if it be not fertilised. 

 These facts suggest that the sex of the off- 

 spring is already settled in the ovum, but 



