CYTOLOGY OF REPRODUCTION IN ANIMALS 



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one organ. Accompan\4ng the germ cells are certain accessory cells, such 

 as the nurse cells in the insect ovar,y and the Sertoli cells in the mammalian 

 testis. In some cases such special cells represent transformed germ cells, 

 while in others they are derived from other tissues. Up to this stage the 

 nuclear divisions in all the cells — somatic, germ, and accessory — are of 

 the equational type, every nucleus having a diploid chromosome com- 

 plement like that of the zygote nucleus from which they have all descended. 



Fig. 88. — Oogenesis and early embryogeny in a fly (Aliastor). 1, section through ovary. 

 At the top is a nurse chamber with several nuclei; with it is associated a young oocyte. 

 Earlier stages in the development of this condition are seen below. 2, oocyte nearly full 

 grown; note "pole plasm " at its lower end. 3, third embryonal mitosis following syngamy, 

 showing three of the four division figures (the small figure is a dividing polar body nucleus) . 

 The pole plasm is being provided with a nucleus. 4, pole plasm with its nucleus cut off as 

 the primordial germ cell; nuclei above continuing to divide without cytokinesis. 5, 

 primordial germ -cell has divided into eight oogonia (four shown); main portion of embryo 

 undergoing superficial cleavage into cells. {From R. Hegner: The Germ-cell Cycle in 

 Animals, The Macmillan Com-pany.) 



Spermatogenesis. — After the spermatogonia in the testis have ceased 

 multiplying, there is initiated a series of changes peculiar to this stage of 

 the life cycle. The spermatogonia commonly enlarge somewhat and 

 are then termed primary spermatocytes. Each of them then undergoes 

 two successive divisions which are mciotic in character : each spermatocyte 

 divides into two secondary spermatocytes, and these immediately divide 

 into four spermatids. The chromosomes in these divisions behave accord- 

 ing to the scheme described in the previous chapter, the nuclei in the 

 quartet of spermatids each having one genome, or monoploid complement, 

 in place of the diploid complement present in the spermatogonia and the 

 somatic tissues. 



In the two spermatocyte divisions the various cytoplasmic inclusions, 

 in particular the Golgi bodies and chondriosomes, are usually distributed 

 rather equally to the four spermatids. This results from a tendency' of 

 such inclusions to be grouped near the equator of the cell or about 



