228 Fertilization and Hybridization 



and we thus find, in the material bearers of the hereditary 

 characters, the duahty of which Goethe sang in his 

 "Spriiche in Reimen," and which the spHttings of hybrids 

 put so clearly before our eyes. Van Beneden chose the 

 name pronuclei for the male and the female nuclei that are 

 thus united, and speaks of a pronucleus male and a pronu- 

 cleus femelle. This designation has been retained since 

 that time, and recommends itself especially for the reason 

 that the union of the two nuclei is usually simply called the 

 nucleus of the cell ; and this latter designation will prob- 

 ably not be changed, although the double nature of the 

 nucleus is recognized. Therefore the pronuclei are the 

 entities that concern us ; the nuclei are really double nuclei. 



If the border line between the two pronuclei remained 

 as distinct through life as before the first cleavage and at 

 the time of it, van Beneden's view would hardly meet with 

 any difficulty. But this is not so. Gradually the line of 

 demarcation becomes blurred, and in most cases nothing 

 more is to be seen of it in later life. But the richness of 

 forms in nature is fortunately so great that the general 

 phenomena in different organisms appear to us with an 

 extremely varied distinctness. And thus it is also here. 

 In one species the border line of the pronunclei is lost 

 sooner, in others later. It is only a case of finding the 

 best illustrations, that is, of selecting a species in which 

 the paternal and the maternal inheritances remain longest 

 visibly separate. 



The discovery of such instances is the great merit of 

 Riickert and Hacker. In the one-eyed water-flea of our 

 fresh waters, the well-known Cyclops vulgaris, and its 

 nearest allies, they found a group of animals in which the 

 pronuclei remained distinctly separate for a long time. 

 Sometimes during several consecutive cell-divisions, some- 



