54 THE DOCTRINE OF IMMORTALITY 



for the remarkable alterations which produce from a sexual 

 cell, as the case may be, an ovum or a spermatozoon. In 

 the fifth lecture we shall return to the consideration of the 

 visible alterations during this transformation. 



Let us now assume that we have eggs and spermatozoa, 

 and occupy ourselves with their further history. Science has 

 acquired correct notions of ' these elements very gradually. 

 A hundred years have not yet passed since the publication of 

 the discovery of the eggs of mammals by Carl Ernst von Baer. 

 Eighty years ago one considered the spermatozoa as parasites, 

 although they had been known since 1628. The investiga- 

 tions of Koelliker first demonstrated the true significance of 

 spermatoza. That the semen acted to fertilize ova has been 

 long known, but so long as one did not know the male and 

 female sexual elements of the higher animals one could have 

 no clear conception of reproduction. During the period of 

 ignorance all sorts of wonderful theories arose, which, how- 

 ever, had no value because precisely that which they should 

 explain was, in its essentials, unknown. We must express a 

 warning against theories of this sort, because even to-day we 

 are much inclined to make up for lacking knowledge by 

 theories. It was not until the seventies of the previous 

 century that it became possible to understand the role of 

 sexual elements in reproduction through the epoch-making 

 investigations of the gifted Oskar Hertwig. Hertwig was at 

 that time Privatdozent in Jena, and I rejoice that it is 

 permitted me to express here the admiration which all biolo- 

 gists bestowed on his discovery. Hertwig showed that 

 fertilization consists essentially in the union of one spermato- 

 zoon with one ovum. Since the ovum is very large in pro- 

 portion to the male element we are accustomed to describe 

 this union as the penetration of the spermatozoon into the 



