14 HEREDITY AND EUGENICS 



The organism begins its development from the 

 union of the nuclei of egg and sperm. But when 

 this union happens it is not a mere interminghng 

 of two fluid substances, for the chromosomes, which 

 are highly viscous in the condensed condition, 

 maintain their separate identity ; and in the subse- 

 quent nuclear divisions they frequently arrange them- 

 selves in pairs, each pair consisting of one chromo- 

 some of paternal origin (from the male germ cell) 

 and one of maternal origin (from the female germ 

 cell). In man}^ animals and plants the various pairs 

 are distinguishable from each other in size or shape. 

 The chromosomes may therefore be said to possess 

 individualit}' and to show genetic continuit}^ from 

 generation to generation. 



When the germ cells of an organism undergo 

 maturation as the organism develops, the chromo- 

 some number in them is reduced to one-half. The 

 essential feature of this complicated process is the 

 separation of the pairs of chromosomes which are 

 characteristic of the somatic nuclei, so that the nuclei 

 of the eggs and sperms receive one member of each 

 pair and hence have half as many chromosomes as 

 the somatic cells.* Half the germ cells will thus 

 receive one member of each pair, and half the other. 

 This maturation process has been studied in great 

 detail in hundreds of plants and animals, as well as 

 in man (see p. 20). In the separation of pairs in the 

 reduction divisions there is free assortment of the 

 chromosomes. There are many reasons for believing 

 that the chromosomes are the basis of Mendelian 

 inheritance, and that the segregation of characters, 

 which IMendel's experiments indicated took place 

 in the formation of the germ cells, really depends 

 on the separation of the chromosome-pairs in the 

 reduction divisions. 



* Further complications of this process need not concern us here. 



