1 88 GENETICS 



development, it is possible to form a conception of some of 

 the main features in the action of genes in building up the 

 body. These are summarized in the following. 



The genes form a set of many diverse materials, differing 

 from each other chemically and physically, and having dif- 

 ferent chemical and physiological activities. They exist at 

 the beginning of the individual's development in the nucleus 

 of the fertilized egg, while surrounding them is a mass of 

 cytoplasm. 



The genes interact with the cytoplasm, changing it, pro- 

 ducing new materials that are incorporated into the cyto- 

 plasm. These varied cytoplasmic products of the different 

 genes interact with each other, producing again new types 

 of material. 



Some of the gross features of the interaction of genes 

 and cytoplasm were described in Chapters II and III (see 

 figure 15, page 42). As there shown, the chromosomes 

 (groups of genes) visibly take up material from the cyto- 

 plasm, alter it, and give it off again into the cytoplasm. 



Though the genes thus interact with the cytoplasm, mod- 

 ifying it, they are not themselves used up in this process so 

 as to lose their essential character. They supply material 

 which combines with cytoplasmic material. But there is left 

 an unchanged part of each gene, and this by division pro- 

 duces additional genes of the same type and action. 



As the cells divide, each chromosome divides, and each 

 of its contained genes divides. One of the two halves of 

 each gene goes after division to each of two cells produced. 

 Thus each new cell produced contains the full set of genes, 

 all those that were present in the fertilized egg. 



In most organisms it appears probable that this continues 

 throughout at least all the early stages of development, if 

 not to the end of development, so that all the cells contain 

 all of the genes. For in the young embryo, after many hun- 

 dred cells have been produced, it can be shown that any 



