114 The Mechanistic Conception of Life 



himself that under such conditions the eggs remained unferti- 

 Hzed although the "aura seminalis" was not prevented from 

 acting upon the eggs. This ended the reign of the ''aura 

 seminalis.'' 



It was reserved to two experimenters, Prevost and Dumas 

 (the latter the famous chemist) to prove that the spermatozoa 

 are the essential element in the sperm. They made the simple 

 experiment of filtering the sperm and demonstrated that the 

 sperm whose spermatozoa had been retained by the filter had 

 lost its power of fertilizing the eggs. But even this did not 

 convince many of the descriptive biologists and nine years later 

 K. E. von Baer still expressed the opinion that spermatozoa 

 had nothing to do with fertilization. In 1843 the entrance of 

 the spermatozoon into the egg was directly observed by Barry 

 and this fact has since been verified by an endless number of 

 investigators for the egg of all kinds of animals. It is probably 

 no exaggeration to say that with the general recognition of the 

 experimental method in biology it would probably have taken 

 about as many years as it took centuries to establish the simple 

 fact that the spermatozoon is the essential element in sperm. 



The mere observation of the fact that the spermatozoon 

 must enter the egg in order to bring about fertilization did not 

 lead to any understanding of the mechanism of the activation of 

 the egg. Nevertheless four theories or rather suggestions were 

 offered. 



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 fertiliza- 

 tion. 



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 

 raised the objection that his investigations showed the same 



