CHAPTER VI 



The realisation of the nuclear factors 



We have seen in Chapter II how in fertilisation the nuclei of 

 egg and sperm unite to form a single nucleus, the zygote nucleus 

 or synkaryon. This contains the chromosomes of both nuclei, 

 i.e. it has a double set of chromosomes. Genetical and cyto- 

 logical research, which lies outside the scope of this book, has 

 shown that we must regard the material carriers of heritable 

 properties, the so-called genes, as localised in the chromosomes. 

 Therefore, the nucleus of the fertilised egg contains two sets 

 of genes, one paternal, and one maternal; these transmit the 

 heritable characters of both parents to the new individual. 

 These characters are thus present, in an undeveloped form, in 

 the fertilised egg, and must be realised in the course of the 

 embryo's development. We shall now discuss this process. 



During cleavage, the synkaryon divides first into two, then 

 into four, and so on, until a great number of cleavage nuclei has 

 been formed, one in each blastomere. 



We have seen above (p. 35) that, according to Weismann's 

 theory, there would be a qualitative difference between the 

 products of these nuclear divisions. The hereditary factors 

 (Weismann's "determinants") would thereby be distributed 

 over the various cleavage nuclei so that, in the end, each cell 

 would contain certain of these factors only. We have also dis- 

 cussed the experiments which refute this theory; there are in 

 fact no qualitative differences among the products of the 

 nuclear divisions during cleavage. At least in the earlier stages 

 of development, the cleavage nuclei are identical with each 

 other, and with the zygote nucleus. Each of them contains all 

 the genes, and there are good grounds for the assumption that 

 even in the course of further development only genotypically 



