156 Heredity and Environment 



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 

 two maturation divisions (Fig. 51 C t D), and that the second of 

 these divisions is like an ordinary mitosis in that each chromo- 

 some splits into two and the halves move apart, such a division 

 being known as an equation division (Fig. 51 ), but it is pos- 

 sible that some chromosome pairs undergo an equation division 

 in the first maturation mitosis and a reduction division in the sec- 

 ond, while other chromosome pairs may reverse this order. 



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

 mosomes had been actually seen Weismann maintained on theoret- 

 ical 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 matura- 

 tion divisions; this hypothesis of Weismann's is now an estab- 

 lished fact. 



Mature Egg and Sperm. As the result of these two matura- 

 tion divisions four cells are formed from each cell (spermato- 

 cyte or oocyte) of the growth period. In the spermatogenesis 

 each of these four cells is transformed into a functional sperma- 

 tozoon (Figs. 41, 51 F) by the condensation of the nucleus into 

 the sperm head and the outgrowth of the centrosome and cyto- 

 plasm to form the tail. In the oogenesis only one of these four 

 cells becomes a functional egg while the other three are small 

 rudimentary eggs which are called polar bodies and which take 

 no further part in development (Figs. 41, 42 C-F). The fertili- 

 zation of the egg usually takes place coincidently with the for- 

 mation of. the polar bodies, and so we come back once more to 

 the stage from which we started, thus completing the- life cycle. 



