REPRODUCTION AND GROWTH 



177 



far the greater number of these cells differentiate into tissue- 

 units — muscle-cells, bone-cells, nerve-cells, etc. — and they 

 undergo tectonic arrangement into a body, or soma. But a 

 relatively small number of the original cells remain in an un- 

 differentiated condition and they are usually localized in some 

 particular part of the body as germ-cells. 



At about the time of reproductive activity the germ-cells 

 reproduce by mitotic divisions. Consider such a fish as a plaice. 

 In its body there are the gonads (ovaries in the female and testes 

 in the male). Consider the ovaries (i in Fig. 24). These organs 

 are sacs and their walls are lined by germinal epithelium. The 



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Ovarves 



Fig. 24. 



1, Transverse section through the body of a fish ; 2-5, stages in the proliferation of a small 

 part of the germinal epithelium in the ovary. 



cells of this epithelium are mostly germ-cells. Consider a small 

 part of the germinal epithelium (2 in Fig. 24) : 



Consider one germ-cell : it divides mitotically, 2, 3, and one 

 of the daughter- cells is shed out into the cavity of the ovary, 4 ; 

 the remaining cell still occupies the epithelium and it divides 

 again, 5 ; one daughter-cell from this division passes out into the 

 ovarian cavity and the other remains in the epithelium and so on. 

 By and by the ovarian sacs become filled up with germ-cells 

 (unripened ova). Presently these cells mature, imbibe water, 

 swell out and become fully developed ova. 



This is called a process of proliferation of the germinal epi- 

 thelium. In innumerable ways the process is modified among 

 animals. Ova or embryos are spawned, gestated, born, etc., 

 but the essential thing is that the germ-cells proliferate. 



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