264 LECTUEES ON BIOLOGY 



finally a union and fertilization of the sexes takes place. But 

 while formerly the egg-cells passed their embryonic development 

 in the roomy brood-chamber of the mother, situated between the 

 chitinous armour and the dorsal surface of the body, and come 

 into life as developed crustaceans, the fertilized ova enter the 

 brood-chamber only for the purpose of being furnished with 

 a solid chitin-shell, the ephippium. After that they are left to 

 their fate : the death of the mother, which soon after intervenes, 

 would in most cases render further care impossible. 



What is the meaning of this maleless reproduction which, 

 apart from the daphnids, we find in numerous other animals ? 

 How is it possible for eggs to develop without fertilization ? 

 The answer to the latter question must be postponed till later, 

 but I will here recall what we have already heard on a previous 

 occasion. No matter how well the germ-cells may be adapted 

 for fertilization, it happens not infrequently that ova, and in one 

 case even male sex-cells, proceed to parthenogenetic development. 

 The value of parthenogenesis lies principally in the fact that it 

 increases the fertility of the species and produces in a short time 

 an enormous number of individuals. We meet, therefore, with it 

 chiefly in animals whose conditions of life are favourable only 

 for a brief time, and which are afterwards threatened with 

 destruction from numerous causes. However important ferti- 

 lization and the resulting mixing of qualities may be for the 

 development and adaptation of organisms, parthenogenesis un- 

 doubtedly confers in such cases a greater advantage. It becomes, 

 however, never so much the exclusive method of reproduction 

 that sexual union is entirely suppressed. 



A simple calculation will show the degree to which partheno- 

 genesis is able to increase reproduction. In the first genera- 

 tion, when all ova develop into female individuals,- the number 

 of descendants is doubled, but with each further generation of 

 females the number increases rapidly, because the number of 

 the ova increases in geometrical progression. We need only 

 think of our illustration of the chess-board to understand the 

 extent of this extraordinary multiplication. 



The strict dependence of their reproduction upon external con- 

 ditions of life in the water-fleas has been proved by most careful 

 experiments made by Weismann, and recently by Issakowitsch. 



