HISTORY OF THE FERTILIZATION PROBLEM 13 



The modern period. The preceding period (1824-74) 

 was coincident, as we have seen, with the early history 

 of the cell theory, but the demonstration of the uni- 

 cellular character of the ovum and spermatozoon 

 had little effect upon the problems of fertilization. The 

 cell theory was still incomplete; the free formation of the 

 nuclei was still held by competent naturalists, and 

 nothing was known of the phenonema of karyokinesis. 

 The cytological investigations of the next ten years 

 (1874-84) were destined to lay the foundations of the 

 modern nuclear theory in its broad outlines. The 

 fertilization studies of tnis period were mainly morpho- 

 logical, and while it is correct to say that they were largely 

 dominated by the growing nuclear theory it is also strictly 

 true that they contributed in no small measure to its 

 upbuilding. Though the penetration of the sperma- 

 tozoon into the egg had long been suspected, it was first 

 clearly demonstrated at this time; the origin of the egg 

 nucleus by two successive divisions of the germinal vesicle 

 was discovered; the origin of the sperm nucleus from the 

 head of the spermatozoon, the sperm aster, the union of 

 the egg nucleus and the sperm nucleus, the relation of 

 these to the first cleavage spindle, the origin of the ferti- 

 lization membrane, the ill effects of polyspermy and the 

 theory of its prevention, and finally the doctrine of the 

 equivalence of the egg and sperm nuclei, and the bipar- 

 ental character of the nuclei of sexually produced organ- 

 isms, as first laid down by Van Beneden, were products 

 of the period also. No period of cytological research 

 seems to me of greater significance than this. 



There was almost a complete cessation of investi- 

 gation from 1855 to 1873, wne n the dawn of the modern 



