REPRODUCTION AND LIFE CYCLES 



429 



mitotic cell division, and when ready for the final maturing stages 

 they first undergo a period of growth in which the cells increase some- 

 what in size. At this point one must look inside the cell to see what is 

 happening within the nucleus. Here the chromosomes are paired. 

 Each member coming from the male or from the female parent, re- 

 spectively, is identical as to shape and size with the exception in 



first 

 polar hsdy 



OOi 



SeconcC 

 bocCy 



primordiaX "penod. of 

 germ cells rnitoU'c 

 divisiorz 



drowth period 

 synapsis and. 

 tatrods formed. 



first msiotjo ^eixnd moXicnz^ 

 ciivision. meioticdiViSiai germ cells 



spermatogonia 



Secondary 



Diagram illustrating meiosis and the maturation of the germ cells. Explain 

 how a constant number of chromosomes is maintained for a given species. (After 

 Curtis and Guthrie.) 



certain cases of the pair of so-called sex chromosomes. The sperma- 

 togonium has now been transformed by this process into a primary 

 spermatocyte. When mitosis takes place each chromosome instead of 

 being split longitudinally as in the case of normal mitosis becomes 

 separated so that one entire member of each pair of homologous 

 chromosomes is passed to each daughter cell. This brings about an 

 actual reduction of the numbers of chromosomes present in each 

 daughter cell by one half. This division (meiosis) is spoken of as the 

 reduction division and the number of chromosomes as the haploid 

 number in contrast with the normal or diploid number found in nil 



