274 BIOLOGICAL LECTURES. 



eggs can only be fertilized by bringing the sperm into direct 

 contact with the eggs, and Spallanzani showed the same for 

 the frog. He succeeded in producing artificial fertilization of 

 mammals by introducing sperm into the vagina. But even 

 Spallanzani did not realize that the spermatozoa were the 

 essential element in the sperm. In 1824 Prevost and Dumas 

 proved this by filtering the sperm, and demonstrated that the 

 sperm whose spermatozoa had been retained by the filter lost 

 its power of impregnating the egg. These observations estab- 

 lished the fact that the spermatozoon has to come in contact 

 with the egg in order to bring about fertilization. 



The next step was the observation made by Barry in 1843 

 that the spermatozoon actually enters into the egg. This 

 observation was confirmed ten years later by a number of 

 authors, Meissner, Newport, Bischof, etc. It is rather remark- 

 able that it was one hundred and sixty years after the discovery 

 of the spermatozoon and the follicle before the fact was recog- 

 nized that the spermatozoon has to enter the egg in order 

 to bring about fertilization. Had the biologists during these 

 one hundred and sixty years lost their interest in the investiga- 

 tion of this problem ? This was certainly not the case, but they 

 spent their energy not in fruitful research, but in speculations 

 and controversies which were admired by their contemporaries 

 and made their authors famous, but which were a mere waste of 

 time. History has since repeated itself in other fields of biology. 

 The outcome of the .facts gathered concerning the process of fer- 

 tilization was four apparently different theories of fertilization, 

 which, however, have much in common. 



The first theory of fertilization is a morphological one. 

 According to this theory, it is the morphological structure 

 of the spermatozoon which is responsible for the process of 

 fertilization. 



The second theory is a chemical one. According to this 

 theory it is not a definite morphological or structural element 

 of the spermatozoon, but a chemical constituent, that causes the 

 development of the egg. Against this second view Miescher 

 has raised the objection that his investigations showed the 

 same compounds in the egg and the spermatozoa. I do not 



