THE WONDERS OP LIFE 



genetically in the cognate process of heterogenesis. This 

 it is which, especially in many of the articulates, causes 

 an immense increase of the species in a short time. 

 Among the insects we have the leaf -lice (aphides), and 

 among the Crustacea the water-fleas (daphnides), that 

 propagate in great numbers during warm weather by 

 unfertilized '"summer-ova." It is not until the autumn 

 that males appear and fertilize the large "winter-ova"; 

 in the following spring the first .parthenogenetic genera- 

 tion issues from the winter eggs. The two heterogenetic 

 generations are very different in the parasitic suctorial 

 worms (trematodes). From the fertilized ovvaa of the 

 hermaphrodite distoma we get simply constructed nurses 

 (psedogenetic larvae), inside which cercaria are generated 

 from unfertilized ova; these travel, and are afterwards 

 converted (inside another animal) into distoma once 

 more. 



I have given (General Morphology, chap, ii., p. 104) the 

 name of strophogenesis to the complicated process of 

 cell-reproduction, which we find in the ontogeny of most 

 of the higher histona, both phanerogams and coelomaria. 

 In these there is not a real alternation of generations, as 

 the multicellular tissue-forming organism develops direct- 

 ly from the iinpregnated ovum. But the process resem- 

 bles metagenesis in so far as the ontogenetic construction 

 consists itself in a repeated division of the cells. Many 

 generations of cells proceed by cleavage from the one 

 stem-cell (the impregnated ovum) before two of these 

 cells become sexually differehtiated, and form a genera- 

 tion of sexual cells. However, the essential difference 

 consists in the fact that all these generations of cells — in 

 the body of both the higher animals and the flowering 

 plants — remain joined together as parts of a single 

 bion (a unified physiological individual); but in the 

 alternation of generations each group produced is made 

 up of a number of bionta, which live as independent 



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