HISTORY OF THE FERTILIZATION PROBLEM 9 



pressing them." It is perfectly clear that Spallanzani 

 himself never held that the spermatozoa themselves 

 were the fertilizing agents, but, on the contrary, he 

 contests this idea strongly as leading to spermatist 

 delusions. 



After Spallanzani there was no real advance in the 

 theory of fertilization until the publication of Prevost 

 and Dumas' New Theory of Reproduction in 1824. They 

 observed that young animals incapable of breeding, 

 old animals beyond the breeding stage, the infertile 

 mule, and birds outside of the breeding season possess 

 no spermatozoa, and they conclude that these facts 

 "sufficiently prove the importance of the animalcules, 

 and show that there exists an intimate relationship be- 

 tween their presence in the reproductive organs and 

 the fertilizing power of the animal." In a long series 

 of experiments they investigated the conditions of 

 fertilization in frogs: all conditions that destroy the 

 animalcules destroy also the fertilizing power of sperm 

 suspensions ; the filtrate of a sperm suspension devoid of 

 spermatozoa will not fertilize; the redissolved residue 

 of a suspension evaporated to dryness will not fertilize, 

 etc. ; the number of eggs fertilized is always less than the 

 number of "animalcules" employed. They came to 

 the conclusion, therefore, that "the prolific principle 

 resides in the spermatic animalcules." 



In their subsequent publications they concluded 

 that it is "infinitely probable that the number of animal- 

 cules employed in fertilization corresponds to that of 

 the embryos developed .... so that the action of 

 these animalcules which we regard as the male repro- 

 ductive elements is individual, not collective." 



