ZOOLOGICAL SCIENCE 189 



how the seminal fluid of the male was related to the genera- 

 tive process. After the spermatozoa and the microscopic ova 

 of many animals were discovered in the latter part of the 

 seventeenth century, the role of each was long in dispute 

 the spermatists of the eighteenth century maintaining that 

 the embryo arose from the sperm, the ovists that it came 

 from the ovum. During this period a considerable amount 

 of experimentation was carried on in the attempts to deter- 

 mine, by filtration and similar methods, whether the sper- 

 matozoa or the fluid portion of the semen constituted the 

 fertilizing agent (Spallanzani, 1785). 



During the first half of the nineteenth century it came to 

 be acknowledged that the spermatozoon and not the fluid of 

 the semen, was the activating agent. But the morphological 

 facts of fertilization remained obscure, until, in 1875, Oscar 

 Hertwig described correctly the cellular phenomena of 

 fertilization in the egg of the sea-urchin. It was shown that 

 fertilization consisted in the entrance of a single spermat- 

 ozoon into the egg, and the union of egg-nucleus with 

 sperm-nucleus to form the nucleus of the one-cell stage 

 from which the many-celled organism originated by cell- 

 division. Virchow's doctrine, omnis cellula e cellula (1856), 

 was fully confirmed; and the nature of the continuity 

 between generations was explained in terms of the cell- 

 doctrine. 1 



Following 1875 there ensued a period of morphological 

 study, during which the exact nature of cell division and 

 the structure of the nucleus of egg and sperm-cells was ascer- 

 tained. It became apparent that fertilization involved two 

 distinct phenomena, which should be investigated independ- 

 ently, despite their intimate association. On the one hand 

 were the phenomena related to the genetic problem of how 

 egg and sperm constituted the physical basis for continuity 



1 Lillie, F. R., "The History of the Fertilization Problem," Science, Jan. 14, 

 1916, contains an authoritative summary of the history and recent status of 

 knowledge concerning this fundamental process of reproduction. 



