CHAPTER LXXXII 



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HEREDITY AND PROTOPLASM 



478. What is inherited? It is common to speak of the 

 inheritance of characters as though something passed on from 

 parents to offspring. But a moment's thought will show that 

 nothing is transmitted in the ordinary literal sense. 



What is really meant by saying that a plant or animal has 

 inherited certain characters from his parents is that there 

 is something in the fertilised egg that makes possible the 

 development of those characters, and ^vhatever is in the egg 

 must have come from the gametes, and so, presumably, from 

 the parents. 



479. Nuclear division. A study of cells shows that the 

 nucleus contains a tangle of substance which behaves in a very 

 definite way before cell division takes place. This substance is 

 called chromatin because it readily absorbs various anilin dyes 

 and thus appears highly colored, in contrast to other portions 

 of the cell, when looked at under the microscope after treat- 

 ment with the dyes (Fig. 244). Just before the cell divides, 

 the chromatin breaks up into separate bits of chromatin called 

 chromosomes, which means " color-bodies." Each chromosome 

 splits lengthwise into two pieces, and one of these pieces 

 goes into one of the two new cells (see Fig. 244). In this way 

 each cell as it divides distributes one half of its chromosomes 

 to each of the two daughter cells. Thus it comes about that 

 each cell has exactly the same number of chromosomes as the 

 other cells in the body. 



480. Formation of germ cells and zygotes. This process of 

 nuclear division goes on as the developing organism continues 

 to grow by the production of new cells. In the formation of 



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