ORIGIN OF THE INDIVIDUAL 233 



OOCYTE is very large, while the other is a tiny cell termed the 

 FIRST POLAR BODY. Both the large secondary oocyte and 

 first polar body now divide again; the former giving rise to 

 a large cell, the mature EGG, and a tiny SECOND POLAR BODY; 

 while the first polar body divides equally to form two polar 

 bodies. In this way arise the four cells, comparable to the 

 four sperm in spermatogenesis, each with half the somatic 

 number of chromosomes. But only one of these, the egg, 

 functions as a gamete. The three polar bodies, although 

 possessing a similar chromosome complex, are sacrificed in 

 providing one cell with its special cytoplasmic equipment. 

 The polar bodies get just enough cytoplasm to be regarded 

 as cells, and soon degenerate and disappear. 



Such is the outline of the essentials of spermatogenesis and 

 oogenesis in animals; processes which involve at one stage a 

 modification of ordinary mitosis to give each gamete half the 

 somatic number of chromosomes characteristic of the species. 

 It is clear that this is not merely a mass reduction of chromatin 

 material, but is a separating of definite chromatin entities, 

 the chromosomes, so that the gametes receive the reduced 



number. 



5. The Chromosome Cycle 



Throughout the animal kingdom, wherever sexual repro- 

 duction occurs, phenomena which can be interpreted as 

 nuclear reduction have been observed in the formation of 

 gametes. In some of the Protozoa this seems to be merely 

 an extrusion of a certain amount of chromatin, but since 

 whenever chromosomes can be observed and counted the 

 process has been found to follow in principle essentially the 

 same lines described above, we have every reason to believe 

 that it is never a haphazard mass reduction, and that the ripe 

 gametes emerge with a definite chromatin heritage, relatively 

 simple as this may be in the lowest forms. 



