History of ihe Theory of Heredity. 69 



difference. In some cases, as in Daphnia, all the par- 

 thenogenetic eggs hatch into females ; in other cases, 

 as in bees, they give rise to males alone; while in still 

 other cases, as in the gall-wasps, some of the unfertilized 

 eggs produce males and some females. 



In many cases the animals which are thus produced 

 are perfectly normal, and have nothing to distinguish 

 them from those born from impregnated eggs. They 

 have the ordinary structure of their species, and they 

 are perfectly capable of propagating their kind. In 

 some cases, as in the gallwasps, reproduction is pre- 

 ceded by the union of the sexes, and in other cases 

 the animals born from parthenogenetic eggs are them- 

 selves parthenogenetic. 



There is possibly one difference between ordinary and 

 parthenogenetic eggs, the presence of polar globules 

 in the one case and their absence in the other; and I shall 

 discuss this difference soon. 



Except in this particular, the history of the develop- 

 ment of the egg into the perfect animal is the same, 

 whether the egg is fertilized or not. Weismann, who 

 has studied the embryology of both parthenogenetic and 

 fertilized eggs in insects (" Beitrage zur Kenntniss der 

 ersten Entwicklungsvorgiinge im Insectenei "), shows 

 that all the minuter details in the process of building up 

 the embryo are the same, whether the egg is fertilized 

 or not. 



We must therefore believe that an ovum has in itself 

 the power to give rise to a new organism, and that al- 

 though it does not usually manifest this power, unless 

 the egg is fertilized, it may exhibt it under certain cir- ; 

 cumstances, as parthenogenesis. Of the character of } 

 the circumstances which lead to parthenogenesis we | 

 know nothing, except that such circumstances have 



