The Recapitulation Theory. 93 



the development of the pygostyle, but something must in turn 

 determine the development of these. Morse concluded from his 

 experiments that it is possible to suppress metamorphosis, but 

 that it cannot be induced in stages Jar removed from those in 

 which it would normally occur. A certain cycle of events determined 

 by heredity is necessary before any stimulating agent will cause 

 metamorphosis. 



Although we have spoken of recapitulatory characters having 

 been at one time terminal stages in the life-cycle, yet it appears that 

 they have not always been added terminally but have often been 

 intercalated at a subterminal or earlier stage. Thus in plants the 

 sporophyte ends inevitably with sporogenesis, so that the lengthening 

 of the sporophyte generation must always have taken place through 

 the addition of subterminal stages, involving the delay of 

 sporogenesis. 1 In animals, too, spermatogenesis or oogenesis is 

 usually delayed until the ontogeny is complete, although the 

 gi eater freedom of the germ cells in their relation to the soma 

 makes possible the occasional occurence of paedogenesis or reproduc- 

 tion by larvae. 



In concluding this chapter it may be pointed out that the 

 phenomena of recapitulation furnish another limitation of the cell 

 theory, recapitulatory characters being organismal, and embryonic 

 recapitulation apparently involving the inheritance of functional 

 modifications. 



1 This is a little different from Bower's well-known view of the sterilization 

 of potentially sporogenous tissue. 



