243 



Asexual 

 generation 



Sexual 

 generation 



M etaphyte 



zygote 2n) 



I 

 sporophyte (2w) 



I 

 spore formation and redu- 

 cing: division 



spores {\n) 



I 

 gametophyte (Iw), 



I ^ 



sexual organs {\ri)-^^ 



male female 



gamete (In) gamete (In) 



zygote (2w) 



M etaz oan 



zygote (2w) 



I 

 larva (2n) 



I 



"Apospory" (reducing division 



delayed till last available 



moment) 



adult form (2w) (gametozooid) 



I 

 sexual organs (2m) 



. I 

 reduction during oogenesis and 



spermatogenesis 

 spermatozoon (\n) egg (Iw) 



zygote (2w) 



In the above table n equals the number of chromosomes prior to the 

 duplication {In) at conjugation. 



nation of the why and wherefore of this affords an opportunity of 

 considering a very obvious objection to the view here adopted. 



It may be urged that, if the fact of the occurrence of an anti- 

 thetic alternation in animal development were admitted , the reason 

 for an aposporous formation of the gametozooid would not be at all 

 obvious. Spore-formation with the necessary reduction might none 

 the less occur at the proper juncture, and the gametozooid might, 

 directly or indirectly, arise from one of these spores; whilst the 

 remainder, whatever number of them there were, might be abortive. 

 As a matter of fact, as will be presently demonstrated, something of 

 this sort is the course of events in the conjugation of the Infusoria, 

 It does not hold for Metazoa for the following reasons: 1) Generally 

 speaking only one gametozooid makes its appearance on the larva, 

 there are no traces of abortive spores, and the formation of one true 

 spore alone appears to be out of the question ; for, in order that cells 

 with the reduced number of chromosomes, i. e. spores, should arise, 

 four of them at least must be formed. 2) That there is no spore- 

 formation is, of course, also proved by the circumstance that there is 

 no reduction before the origin of the gametozooid, and as previously 

 shown, the primary object of spore- formation is to effect a reduction. 



The modifications in development which this aposporous alter- 



16* 



