ORIGIN OF THE GERM-CELLS 445 



given by the interesting experiments of Nussbaum ('97) on the rotifer 

 Hydatina, which is an especially favourable case since sex is here de- 

 termined at a very early period, before the egg is laid, the eggs being 

 of two sizes, of which the smaller give rise only to males, and the 

 larger only to females. The earlier experiments of Maupas ('91) on 

 this form seemed to show conclusively that the decisive factor was 

 temperature acting on the parent organism, since in a high tempera- 

 ture an excess of females produced male eggs, and in a low tempera- 

 ture the reverse result ensued. Nussbaum shows, however, that this 

 is not a direct effect of temperature, but an indirect one due to the 

 greater birth-rate and the greater activity of the animals under a 

 higher temperature, which result in a speedier exhaustion of food. 

 Direct experiment shows that, under equal temperature-conditions, 

 well-fed females produce a preponderance of female offspring, and 

 vice versa, precisely as in the Lepidoptera and Amphibia. These cases 

 show that sex may be determined by conditions of nutrition either 

 affecting the embryo itself (Lepidoptera, Amphibia) long after the e.gg 

 is laid, or by similar conditions affecting the parent-organism and 

 through it the ovarian G.gg. 



Nutrition is, however, not the only determining cause of sex, as is 

 shown by the long-known case of the honey-bee. Here sex is deter- 

 mined by fertilization, the males arising only from unfertilized eggs 

 by parthenogenesis, while the fertilized eggs give rise exclusively to 

 females, which develop into fertile forms (queens) or sterile forms 

 (workers), according to the nature of the food. This is a very excep- 

 tional case, yet here too it is the more highly fed larvae that produce 

 fertile females. It is interesting to compare with this case that of the 

 plant-lice or aphides. In these forms the summer broods, living 

 under favourable conditions of nutrition, produce only females the 

 eggs of which develop parthenogenetically. In the autumn, under 

 less favourable conditions, males as well as females are produced ; and 

 that this is due to the external conditions and not to a fixed cyclical 

 change of the organism is proved by the fact that in the favourable 

 environment of a greenhouse the production of females alone may 

 continue for years. ^ 



We are not yet able to state whether there is any one causal ele- 

 ment common to all known cases of sex-determination. The observa- 

 tions cited above, as well as a multitude of others that cannot here be 

 reviewed, render it certain, however, that sex as such is not inherited. 

 What is inherited is the capacity to develop into either male or 

 female, the actual result being determined by the combined effect of 

 conditions external to the primordial germ-cell. 



1 See Geddes, ^^jr, in Encyclop(rdia Britannica ; Geddes and Thompson, The Evo/uHon 

 of Sex, 1889; Brooks, The Law of Heredity, 1883; Watase ('92), The Phenomena of 

 Sex-differentiation. 



