THE REALISATION OF THE NUCLEAR FACTORS 67 



number. The latter he obtained by shaking normal eggs a few 

 minutes after fertilisation; this caused irregularities in the 

 first nuclear division. Boveri found that the size of nuclei and 

 cells depended on the number of chromosomes they contained. 

 When the chromosome number was doubled, both the size of 

 the nucleus (according to Boveri its surface area), and the 

 volume of the cell were doubled too. However, the total number 

 of cells of which the embryo will eventually consist, is inversely 

 proportional to the number of chromosomes. This is due to 

 the fact that in embryos with higher chromosome numbers 

 (larger nuclei) the cell divisions are slower, and fewer in 

 number. Therefore, the final size of the embryo is more or less 

 constant and independent of the chromosome number. 



The validity of these so-called "rules of Boveri" has repeat- 

 edly been confirmed in other animals, though it was not the 

 surface area but the volume of the nucleus which was, as a 

 rule, found to be proportional to the chromosome number. 

 Evidence was also obtained that the regulation of the body 

 size can go even farther. In amphibian larvae with chromosome 

 numbers varying from haploid to pentaploid, and accordingly 

 with cells of very different sizes, it was found that not only the 

 total body size, but also the thickness of the Vx^alls of the organs 

 was approximately constant. This implies that not only the 

 number of cells varies with the chromosome number, but their 

 shape as well (Fankhauser, 1945). However, this regulation 

 of the body size is not without exceptions. In plants and in 

 several invertebrate animals, polyploids often are larger than 

 normal diploid specimens, at least in the first few generations 

 after the production of polyploidy. 



During cleavage the egg cytoplasm is distributed over a great 

 number of cells. In as far as chemodifferentiation has already 

 taken place, there will be differences in physical and chemical 

 properties of the cytoplasm among the various blastomeres. 

 Therefore, the genotypically identical cleavage nuclei will be- 

 come located in non-identical parts of the egg cytoplasm. The 

 interactions of these nuclei with the cytoplasm surrounding 

 them will result in differences among the reactions that are 

 set going in the various parts of the egg. The nuclear factors 



