Cells and Their Activities 273 



MEIOSIS 



If the offspring of any animal is to have the same chromosome num- 

 ber as the parents, there must be some method by which the chromo- 

 somal number of the parents is halved. If this were not so, the union 

 of a sperm and &gg would result in doubling of the chromosome num- 

 ber. It would not take many such doublings to result in cells being 

 overloaded with chromosomes. 



Fortunately this contingency is avoided by a remarkable modifica- 

 tion of cell division known as meiosis or gametogenesis. In this type 

 of division, the resulting cells have only half the initial number of 

 chromosomes. This is brought about by two cell divisions and but one 

 chromosomal division. In the male this is known as spermatogenesis, 

 in the female as oogenesis. 



An understanding of this process is absolutely essential to a com- 

 prehension of genetics, for it is the cytological basis of heredity. 



The actual process of gametogenesis begins in the very early em- 

 bryo when the primordial germ cell is set aside from those cells des- 

 tined to become somatic or body cells. Before meiosis begins, this 

 primordial germ cell divides many times by mitosis to produce a large 

 number of germ cells. Each of these resulting cells has a diploid 

 number of chromosomes. Meiosis does not begin until sexual maturity 

 is attained. It continues throughout the reproductive life of the in- 

 dividual. 



Spermatogenesis. — Those cells which result from the proliferation 

 of the primordial germ cell in the male are known as spermatogonia. 

 Since these are produced by mitotic divisions, each spermatogonium has 

 a diploid (2N) number of chromosomes, with one of each homologous 

 pair derived from the maternal parent and the other from the paternal 

 parent. 



When maturity is attained by the organism, certain of the sperma- 

 togonia enlarge to become primary spermatocytes, each of which is des- 

 ignated as spermatocyte I. It is within this cell that the true maturation 

 or meiotic divisions begin. The actual divisions of the primary sperma- 

 tocyte resemble mitosis, and the various phases can be recognized with a 

 spindle being formed in the typical manner. During the prophase, the 

 homologous chromosomes of each pair are in close contact, even twist- 

 ing about one another, in a condition known as synapsis. At this point 

 it can be seen that each maternal chromosome and each paternal chromo- 



