238 Fertilisation and Hybridization 



time. But gradually it was discovered that there are a 

 number of secondary details which may obscure the main 

 features, and we owe it chiefly to Strasburger that the 

 latter stand out clearly in the plant-kingdom. In the ani- 

 mal kingdom, however, there is still a series of cases 

 which do not follow this rule, and where the chromo- 

 somes of the pronuclei 'are not placed lengthwise side by 

 side at the moment of separation, but are connected at 

 one end. Hence the separation here takes the form of a 

 transverse division. Some insects and fresh-water crabs, 

 some molluscs and worms offer the best known instances, 

 but according to the most recent studies of de Sinety, Can- 

 non, and others, the assumption gains ground that here too 

 the microscopic pictures, on closer observation, disclose 

 a better fitting into the otherwise general scheme. It is 

 also possible that, after the longitudinal splitting, the 

 nuclear threads still remain connected for a while by their 

 ends, before they finally separate. 



The male and the female sexual cells usually originate 

 in separate organs, frequently on special individuals. This 

 goes to show that, at their origination from the body-cells, 

 the paternal pronuclei do not become sperms and the ma- 

 ternal ones egg-cells. On the contrary, the two pro- 

 nuclei of a mother-cell in the ovary can become egg-cells, 

 and the two pronuclei of a pollen mother-cell can both 

 give rise, by further splitting, to the formation of sper- 

 matozoids. Accordingly, one-half of the forming sperms 

 gets paternal or now grand-paternal pronuclei, and the 

 other half grand-maternal. The same is true of the 

 egg-cells, and this holds good in spite of the circum- 

 stance that, in consequence of the crowded condition of 

 the ovaries, the larger part of the female cells has regu- 



