CHAPTER IX 

 CYTOLOGY OF REPRODUCTION IN ANIMALS 



One of the major cytological crises in the reproduction of organisms, 

 viz., raeiosis, has just been described in some detail. Another major 

 crisis, the fusion of nuclei in the union of gametes, has also received brief 

 mention. It now becomes necessary to relate these processes more closely 

 to life cycles by describing the various other events that precede, accom- 

 pany, and succeed them in the reproductive phases. This will be done 

 in three chapters. Again we should be reminded that such events 

 occur in numerous variations in different organisms and that our descrip- 

 tions are designed to include examples that are fairly representative of 

 what occurs in each class of organism considered, even though no single 

 species can be expected to show all the stages precisely as described. We 

 consider first the animals. 



The Germ Cells. — The term germ cells is applied in the case of animals 

 to those specialized cells whose ultimate descendants are to be female or 

 male gametes (eggs or spermatozoa) and often certain accessory cells, but 

 not somatic cells. When the specialization first becomes recognizable, 

 there may be but one 'primordial germ cell, or several, or a considerable 

 number of them. It is a striking fact that the differentiation of these cells 

 from the somatic cells occurs very early in the ontogeny of the organism. 

 They can be distinguished during larval stages, and in some animals it has 

 been determined that they are set apart from the somatic cells in one of 

 the earliest cleavage divisions of the fertilized egg. In extreme cases, 

 notably certain insects, the cytoplasm of the future germ cells can even be 

 distinguished in one end of the yet undivided egg (Fig. 88). In the case 

 illustrated one of the eight nuclei formed by the third embryonal mitosis 

 enters this specialized cytoplasm, which is then cut off as the primordial 

 germ cell from the larger somatic portion of the young embryo. 



The primordial germ cells, whatever their number and time of origin 

 from embryonic tissues, commonly pass through a period of multiplica- 

 tion. After their divisions cease, they migrate to the site of the future 

 ovaries or testes. These organs then develop and incorporate within 

 them the sperm cells, which undergo a new series of divisions. In the 

 ovary these multiplying cells are called oogonia; in the testis they are 

 called spermatogonia. Hermaphroditic animals may have separate 

 ovaries and testes, or both oogonia and spermatogonia may be present in 



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