SIXTEENTH LECTURE. 



ON THE NATURE OF THE PROCESS OF 

 FERTILIZATION. 



JACQUES LOEB. 

 I. 



Leeuwenhook demonstrated in 1677 the existence of sper 

 matozoa. It was about one hundred and sixty years before 

 biologists convinced themselves that these spermatozoa were 

 no parasites. In 1835 K. E. von Baer was still of the opinion 

 that the spermatozoa had nothing to do with the process of fer- 

 tilization. The parasitic conception of spermatozoa was finally 

 done away with by Wagner's demonstration that only those 

 animals are capable of fertilizing eggs whose sperm contains 

 spermatozoa. Very soon afterwards histologists showed the 

 origin of spermatozoa from cells. 



Leeuwenhook was of the opinion that the spermatozoon repre- 

 sents the future embryo. On the other hand, it was not diffi- 

 cult to notice that the embryo in fishes, amphibians, and birds 

 develops from an egg furnished by the female. The question 

 arose as to what was the homologous element in the female of 

 mammals. In 1672 De Graaf discovered the follicle in the 

 ovary of mammals, and in 1827 von Baer discovered the mam- 

 malian egg cell. 



The next problem that was solved in this branch of science 

 was the relation of the sperm to the egg. Many scientists, 

 among them De Graaf, had assumed that no direct contact 

 between egg and sperm was necessary, that something volatile 

 in the sperm, the aura seminalis, was sufficient for the act of 

 fertilization. Contrary to this, Jacobi showed (1764) that fish 



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