236 Fertilisation and Hybridisation 



And since the double nuclei of the body originated during 

 fertilization, it is evident that the conjugating cells must 

 have single nuclei, and therefore that the separation of the 

 pronuclei must take place at the origination of these cells. 



This fact is now so generally established, for animals 

 as well as plants, that it may be regarded as one of the 

 strongest foundations of the whole theory of fertilization. 

 Wherever it is possible to count the chromosomes, we find 

 in the somatic cells twice as many as in the sexual cells. 

 The former contain double nuclei, the latter single nuclei, 

 or pronuclei. 



The sexual cells in animals originate directly from the 

 somatic cells, but in plants there is more or less prepara- 

 tion. Correspondingly, the two pronuclei separate in ani- 

 mals at the formation of the egg- and sperm-cells, but in 

 the case of plants before that. In the seed-bearing plants 

 it is the period of the origination of the mother-cells of the 

 pollen and of the embryo-sacs. Therefore all cell-genera- 

 tions which appear after this moment, and up to the final 

 production of the egg-cells in the embryo-sac, and of the 

 sperm-cells in the pollen-grains and their tubes, possess 

 only pronuclei. Such cells are called sexual, and the 

 period of their formation the sexual generation. In ferns 

 the entire life-period of the prothallium lies between the 

 origination of the sexual cells and the appearance of the 

 egg- and sperm-cells. This small plantlet, though built up 

 of hundreds of cells possesses, therefore, as Strasburger 

 has demonstrated, only pronuclei. The alternation of the 

 sexual prothallia and the asexual fern-plant is called the 

 alternation of generations ; the two generations are hence 

 distinguished from each other fundamentally by their 

 nuclei, which in the leafy plants are always double nuclei, 

 and in the prothallia always pronuclei. This difference 



