CHAPTER III 

 THE MECHANISM OF EEPEODUCTION 



Theee is a division of labor in all the higher plants and 

 animals, the result of setting apart definite tissues for 

 producing germ cells. In addition, another important 

 matter is accomplished. The germ cells are insulated 

 from ordinary environmental changes, and are enabled to 

 go through a very exact routine of processes in prepara- 

 tion for the formation of the new organism — the zygote. 



In general the animal body or the sporophyte of the 

 higher plants can be considered as a double organization. 

 Various parts make up each of the cell units ; but of them 

 all the nucleus, and within the nucleus the chromosomes, 

 seem to be the most important. Each species has a char- 

 acteristic and constant number of these bodies, and it is 

 their distribution which parallels — and probably regu- 

 lates — ^the distribution of the hereditary differences 

 within a species. The double organization of the bodies 

 of the higher organisms is dependent upon the receipt of 

 one set of these chromosomes from each parent. And it 

 is the peculiar method by which these chromosomes are 

 apportioned to the gametes, together with experiments on 

 the actual distribution of characters in the generations 

 succeeding a cross, which have given us a fairly clear 

 idea of heredity as a mechanical process. 



In ordinary cell division during growth each chromo- 

 some divides longitudinally so that both daughter cells 

 apparently receive an exact half of the chromatin, al- 

 though possibly some sort of a special apportionment is 



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