ALTERNATION OF GENERATIONS. 



191 



I J!.. I 9. 



A sexless fern-plant forms special reproductive cells (spores), which develop 

 parthenogenetically into a sexual prothallus, from the fertilized egg-cell of which 

 the fern-plant arises. 



Fig. 70. 



The difference between these two alternations has been as often pointed out 

 as it has been ignored. The former is called true alternation of generations (or 

 metagenesis) ; the latter is called by zoologists, in reference to flukes for 

 instance, heterogamy. Comparisons between the alternations in plants and 

 animals have seldom recognized the distinction. 



Let it be recognized, however, and we can readily proceed to more 

 complicated cases where the two are combined. Returning to the liver-fluke 

 and others like it, we find that the sporocyst sometimes multiplies in a genuinely 

 asexual fashion — without the intervention of precocious ova, special repro- 

 ductive cells, germs, or spores, call them what you will — by direct division 

 or budding. For such cases the formula must be modified as follows : — 



Fig. 



7 1 - 



The complication is not serious. It is simply that, before the multiplication 

 by special cells sets in, there may be more than one (A', A") entirely asexual 

 (and not merely sexless) generation. 



V. Alternation of Juvenile Parthenogenetic Reproduction with the Adult 

 Sexual Process. — We have already noted the curious precocity of some 

 midge larvae, which reproduce while still young. Cells within the body, 

 apparently precocious ova, develop parthenogenetically into larvae, which prey 

 upon the mother larva, eventually kill her and leave her, only themselves to 

 become in turn similar victims of precocity. This mav continue for a series ot 

 generations, with continuous decrease in the size of the reproductive cells, till 

 finally true sexuality and adult life is attained. The reproductive cells here are 



