THE CELLULAR BASIS OF HEREDITY 127 



other, which are unlike any other cell divisions. These are known as 

 the first and second maturation divisions and they are the last divisions 

 which take place in the formation of the egg and sperm. In one or the 

 other of these two maturation divisions the pairs of chromosomes sepa- 

 rate along the line of junction, one member of each pair going to one 

 pole of the spindle and the other to the other pole, so that in each of the 

 daughter cells thus formed only a single set of chromosomes is present 

 (Fig. 34 C and D) ; but since the position of the pairs of chromosomes 

 in the spindle is a matter of chance it rarely happens that all the 

 paternal chromosomes go to one pole and all the maternal ones to the 

 other; thus each of the sex cells comes to contain a complete set of 

 chromosomes though particular individual chromosomes may have come 

 from the father while others have come from the mother. There is 

 reason to believe that homologous chromosomes show general resem- 

 blances but individual differences, and 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 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 divi- 

 sion 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 two maturation divisions (Fig. 34, 0, D), and 

 that the second of these divisions is like an ordinary mitosis in that 

 each chromosome splits into two and the halves move apart, such a divi- 

 sion being known as an equation division (Fig. 34 E), but it is possible 

 that some chromosome pairs undergo an equation division in the first 

 maturation mitosis and a reduction division in the second, while other 

 chromosome pairs may reverse this order. 



It is an interesting fact that long before the reduction of chromo- 

 somes had been actually seen Weismann maintained on theoretical 

 grounds that such a reduction must occur, otherwise the number of 

 chromosomes would double in every generation, and he held that this 

 reduction must take place in one of the maturation divisions; this 

 hypothesis of Weismann's is now an established fact. 



As the result of these two maturation divisions four cells are formed 

 from each cell (spermatocyte or oocyte) of the growth period. In the 

 spermatogenesis each of these four cells is transformed into a functional 

 spermatozoon (Fig. 34 E), by the condensation of the nucleus into the 

 sperm head and the outgrowth of the centrosome and cytoplasm to form 

 the tail. In the oogenesis only one of these four cells becomes a func- 



