Heredity — -Genetics 709 



(b) A period of multiplication of the primordial germ cells^ during 

 which they increase in number. 



(c) A period of rest, with no division or mitosis of the primordial germ 

 cells, but a gathering into one or two groups to form the primordia or 

 forerunners of the gonads (testes or ovaries) . 



(d) A second period of mitosis or multiplication which results in the 

 formation of large numbers of spermatogonia (in male animals) and 

 oogonia (in females). The stages in the embryologic development of 

 man and the frog are given in an earlier chapter. 



(e) Certain spermatogonia differentiate into nutritive or Sertoli cells, 

 while other spermatogonia remain undifferentiated; in the female certain 

 oogonia remain undifferentiated or germinal, while other oogonia dif- 

 ferentiate into nurse cells. 



(f) Then follows a period of growth without mitosis in which the 

 spermatogonia and oogonia grow to form, respectively, the primary 

 spermatocytes and primary oocytes. In this stage the homologous 

 chromosomes unite (appose) during the process of synapsis (union of 

 homologous chromosomes), and crossing-over of chromatin materials (in- 

 cluding genes) may take place. 



(g) Next, there is a period of maturation in which the number of 

 chromosomes is reduced to one-half the number ordinarily found in body 

 cells or the soma cells of that particular species. . Cells with a reduced 

 number of chromosomes are now called secondary spermatocytes and sec- 

 ondary oocytes. In the formation of the secondary oocyte there is an un- 

 equal division of the cell, producing one normal cell and a smaller, abor- 

 tive, nonfunctional cell called the polocyte or polar body. Such a special- 

 ized type of two consecutive mitoses is called meiosis (reduction division) 

 in which the sperm, or ovum, receives only a haploid (monoploid or 

 simplex) number of chromosomes instead of the diploid (double) number 

 usually present in cells. 



(h) In the next stage the secondary spermatocytes produce a num- 

 ber of spermatids, which are the forerunners of the future male sex cells 

 or sperm. The secondary oocyte again divides by unequal division in this 

 stage, producing a normal cell and a small polocyte. The polocyte of 

 the previous stage has divided equally into two abortive, nonfunctional 

 polocytes. We thus have three polocytes and one normal, functional egg 

 arising from each of the original primary oocytes. 



(i) In the next step there is a transformation of spermatids into male 



sperm. 



{']) The eog or ovum produced by the secondary oocyte can now be 

 fertilized by the sperm. In this process the hereditary factors or genes 



