156 Heredity and Environment 



vidual chromosomes may have come from the father while others 

 have come from the mother. There is reason to believe that 

 homologous chromosomes show general resemblances but indi- 

 vidual differences, ,antf consequently when the members of each 

 pair separate and go into the sex cells these cells differ among 

 themselves because the individual chromosomes in different cells 

 are not the same in hereditary value (Fig. 58). 



Again this is comparable to the separation of the digits of the 

 two hands after these have been placed together in homologous 

 pairs, except that all the right digits must go with the right hand, 

 all the left ones with the left hand since they are permanently at- 

 tached to the hands. But the homologous chromosomes are free 

 and each chromosome of a pair may separate to the right or to 

 the left as the digits might do if they were severed from the 

 hands. Then each digit of a pair could separate either to the 

 right or to the left and it would rarely happen that all the right 

 hand ones would go in one direction and all the left hand ones 

 in the other, but since the members of each pair could separate in 

 two directions and since there are five pairs, the possible number 

 of different combinations after separation would be (2) 5 or 32, 

 and yet in each of these combinations the full set of digits from 

 the thumb to the little finger would be present. Finally the fer- 

 tilization of the egg may be likened to the game of "bean por- 

 ridge" in which different hands (gametes) each with its full set 

 of digits (chromosomes) are .struck together thus making new 

 c6mbinations of digits (chromosomes). 



In this way the number of chromosomes in the mature egg or 

 sperm comes to be one-half the number present in other kinds 

 of cells, and when the egg and sperm unite in fertilization the 

 whole number is again restored. The double set of chromosomes 

 is known as the diploid number, the single set as the haploid 

 number, and the maturation division in which this reduction from 

 the double to the single set takes place is the reduction division. 

 It is generally held that this reduction takes place in the first of the 



