THEIR SIGNIFICANCE. IN HEREDITY. 345 



it would be a fact which harmonized with it, and negatived a 

 suggestion which, if confirmed, would have been fatal to the hypo- 

 thesis. Minot, Balfour, and van Beneden, from the point of view 

 afforded by their theories, were compelled to suppose that polar 

 bodies are wanting in parthenogenetic eggs ; and the facts which 

 were known at that time favoured such an opinion, for in spite 

 of many attempts, no one had ever succeeded in proving the 

 formation of these bodies by parthenogenetic eggs. 



During the summer of 1885 I first succeeded in ascertaining 

 that a single polar body is expelled from the parthenogenetic 

 summer-egg of one of the Daphnidae^ Polyphemus oculus 1 . Thus 

 my interpretation of the process in question received support, while 

 it seemed to me that Minot's interpretation of polar bodies had 

 been refuted ; for if these bodies are formed in the parthenogenetic 

 eggs of a single species, just as in eggs which require fertilization, 

 it follows that the expulsion of polar bodies cannot signify the 

 removal of the male element from the egg. 



The desire to throw light upon the significance of polar bodies 

 has been the only cause of my investigation. At the same time 

 I hoped by this means to gain further knowledge as to the nature 

 of parthenogenesis. 



In the third part of the essay on ' The Continuity of the Germ- 

 plasm' (see p. 225) I attempted to make clear the nature of 

 parthenogenesis, and I arrived at the conclusion that the difference 

 between an egg which is capable of developing without fertilization, 

 and another which requires fertilization, must lie in the quantity 

 of nucleoplasm present in the egg. I supposed that the nucleus of 

 the mature parthenogenetic egg contained nearly twice as much 

 germ-plasm as that contained in the sexual egg, just before the 

 occurrence of fertilization ; or, more correctly, I believed that the 

 quantity of nucleoplasm which remains in the egg, after the ex- 

 pulsion of the polar bodies, is the same in both eggs, but that the 

 parthenogenetic egg possesses the power of doubling this quantity 

 by growth, and thus produces from within itself the same quantity 

 of germ-plasm as that contained in the sexual egg after the 

 addition of the sperm-nucleus in fertilization. 



This was only an hypothesis, and the considerations which had led 



1 This observation was first published as a note at the end of the fourth Essay in 

 the present volume. See p. 249. 



