THE REALISATION OF THE NUCLEAR FACTORS 



73 



parts died. In general, eggs which had divided into three devel- 

 oped better than those which had divided according to the 

 tetraster type. 



These facts give clear indications already as to the 

 importance of the chromosomes, because the difference be- 

 tween the three or four simultaneously formed cells lay in 

 their chromosome complements. But Boveri was able to do 

 more. He determined the size of the nuclei in the various parts 



Fig. 28. Diagram of the distribution of the chromosomes in a 

 tetraster division, taking place in a sea urchin egg after double 

 fertilisation, taking 4 as the haploid chromosome number, (a) the 

 12 chromosomes of the triploid egg become distributed at random 

 over the 4 spindles; (b) consequently, only one of the cells (bottom 

 left) receives a complete set of chromosomes, a, b, c and d. 



After Boveri. 



of germs developing from an intact egg, and in germs develop- 

 ing from isolated blastomeres. As we have seen, this is pro- 

 portional to the chromosome number. This method enabled 

 Boveri to establish that the success of development did not 

 depend only on the number of chromosomes present in the 

 nucleus. Sometimes parts with a larger nucleus were less 

 successful than other parts with a smaller nucleus, i.e. with a 

 smaller chromosome number. Boveri concluded that the result 

 of development is determined by the combination of chromo- 

 somes. He suggested that there were qualitative differences 

 between the chromosomes, and that only those parts of the egg 

 could develop normally which had at least one complete 

 (haploid) set of chromosomes in their nucleus (Fig. 28). 



