Reproduction and Life Cycles in Higher Animals 49 



ber happens to be 24 in man. The 2n or somatic number is 

 called the diploid (Greek, diploos, twofold, double; Latin, du- 

 plex) number whereas the n or gametic number is generally re- 

 ferred to as the haploid (Greek, haploos, single; Latin, simplex) 

 and occasionally as the monoploid (Greek, monos, alone, only) 

 number. 



The haploid number in the gametes is also called the reduced 

 number and is brought about by two successive mitoses which 

 differ from ordinary mitotic divisions in several important de- 

 tails. These peculiar mitoses do not occur in all parts of the 

 body but only in the ovary and testis. To differentiate them 

 from somatic cell divisions, these two divisions are called the 

 reduction divisions, meiosis, or, because they occur during the 

 maturation or differentiation of the germ cells, the maturation 

 divisions. There are always two meiotic divisions, and conse- 

 quently each cell that divides by meiosis produces four cells. 



In the male animal, a number of cells in the testis become 

 set apart and generally become larger than the others. They are 

 the primary spermatocytes and they undergo the first meiotic 

 division, by which each forms two secondary spermatocytes. 

 The second meiotic division immediately follows, with the result 

 that four spermatids have been produced from each original 

 primary spermatocyte. These spermatids do not divide further, 

 but usually change their shape by elongating and by developing 

 a tail. Each becomes a mature spermatozoon. 



In the ovary, when the eggs are about to form, certain cells 

 become very large. These primary oocytes divide to form two 

 cells but they are not alike. One is large and contains all the 

 stored food; the other is no more than a nucleus with some 

 cytoplasm around it. This small cell is the first polar body and 

 remains attached to the large cell which is the secondary oocyte. 

 The secondary oocyte and frequently the first polar body, also, 

 then undergo the second meiotic division. The secondary oocyte 

 farms a large functional egg and a small, nonfunctional sec- 

 ondary polar body, whereas the first polar body, if it divides, 

 forms two polar bodies. Thus each primary oocyte forms either 

 three or four cells, but only one of them is functional. 



In the higher animals, all the somatic cells are diploid. The 

 eggs and sperm of the animals possess the haploid number of 

 chromosomes and unite to produce new individuals which like 



