PROCESS OF REPRODUCTION IN ORGANISMS. 25 



and rejuvenescence in the egg by bringing about increased 

 metabolism. Isolated pieces of Planaria which are "too small," 

 when kept at a certain temperature, to regulate into new wholes 

 can be made to undergo such regulation by raising the tempera- 

 ture a few degrees and by various other methods. I believe that 

 such cases are not fundamentally different from cases of artificial 

 parthenogenesis. 

 5 . A Uernation of Generations in Plants and Metagenesis in A nimals. 



In many of the lower plants in which sexual reproduction is known 

 maturation is not immediately followed by fertilization, but the 

 mature cells are spores which pass through a developmental cycle 

 and this results in the formation of the gametophy te generation : 

 this finally produces the gametes, \vhich after fertilization give 

 rise to the sporophyte. In the higher plants the gametophyte 

 generation is much reduced and never becomes an independent, 

 free living organism. The sporophyte produces two kinds of 

 spores, the microspore or pollen grain and the macrospore or 

 embryo sac. These structures in their further development 

 represent all that remains of the gametophyte and they give rise 

 to the gametes. In many plants then there are two reproductive 

 cycles instead of one, although in the higher plants one of these 

 cycles is much reduced. 



The production of spores by the sporophyte is apparently 

 associated with the senescence of this generation or of parts of it 

 and as Klebs and others have shown can often be induced, at 

 least after the earlier stages of the vegetative period, by condi- 

 tions which decrease metabolism. But the spore resembles the 

 parthenogenetic egg in that it reacts to the stimulus of isolation 

 either with or without a period of quiescence by initiating a 

 developmental cycle. It differs from the parthenogenetic egg, 

 however, in that this developmental cycle is different from that 

 in the course of which it arose. The spore then is not so highly 

 differentiated or so old that it is unable to react to the stimulus 

 of isolation by a regulatory process, but this process gives rise 

 to an organism of different character from the preceding genera- 

 tion. In short, a second cycle of dedifferentiation and rejuve- 

 nescence occurs between maturation and the formation of the 

 gametes. The dedifferentiation in this case, however, does not 



