230 FOUNDATIONS OF BIOLOGY 



to elaborate the details of the chromosome cycle associated 

 with alternation of generations in plants attention must 

 be concentrated on the conditions as they exist in animals, 

 in which the somatic number of chromosomes is reduced one 

 half at the formation of the gametes. From the standpoint 

 of chromosome number, the sporophyte is comparable to 

 the animal soma and the gametophyte is represented by 

 merely a couple of cell generations during the formation of 

 the gametes in animals. (Fig. 124.) 



The MATURATION or 'ripening' of the germ cells of animals 

 involves two cell divisions by which each spermatogonium 

 gives rise to four sperm, and each oogonium to one functional 

 egg and three tiny, abortive eggs known as POLAR BODIES; 

 each and all with one half the number of chromosomes of the 

 somatic cells and of the germ cells up to this point in their 

 development. Consequently these two divisions, termed 

 MATURATION DIVISIONS, must be examined in some detail if 

 we are to appreciate the nicety of the process by which the 

 chromosome number is reduced one half without impairing 

 the chromatin heritage from cell to cell. We shall describe 

 first the origin of the sperm which, though it is fundamentally 

 the same as that of the egg, is somewhat simpler to under- 

 stand. 



3. Spermatogenesis 



A given SPERMATOGONIUM, with, let us say, eight chromo- 

 somes characteristic of the species, proceeds to increase in 

 size preparatory to the first maturation mitosis, and is desig- 

 nated a PRIMARY SPERMATOCYTE. At the close of the growth 

 period, when this cell is preparing to divide, the chromosomes 

 are arranged in pairs by a process termed SYNAPSIS. The 

 number of such pairs will obviously be half that of the chro- 

 mosome number. The synaptic pairs are then distributed in 

 the equator of the spindle exactly as the single chromosomes 



