346 



SENESCENCE AND REJUVENESCENCE 



FIG. 186. Part of an ovarian 

 tubule of Dytiscus (beetle), show- 

 ing eggs alternating with groups of 

 nutritive cells : the dark regions of 

 the eggs are dense aggregations of 

 granules derived from the nutritive 

 cells. From Korschelt, '91. 



the physiological condition of the 

 bee's egg is so near the boundary 

 line between parthenogenesis and 

 zygogenesis that slight differences 

 suffice to determine it one way or 

 the other. 



Many other interesting cases of 

 oogenesis might be added to the few 

 described here, but the fact that the 

 formation of the female gamete in 

 organisms is a process of growth and 

 morphological differentiation requires 

 no further evidence. 



The gametes then in both plants 

 and animals are to all appearances 

 the final stages of a period of growth 

 and differentiation. Except in a few 

 of the unicellular organisms where 

 body and gamete are the same cell, 

 the gametes are highly specialized 

 cells, different from any other cells 

 of the body and bearing not the 

 slightest resemblance to embryonic 

 or undifferentiated cells. Of course 

 it is possible to assume with Weis- 

 mann and others that, in addition 

 to the oogenic and spermatogenic 

 protoplasm which is responsible for 

 the differentiation, the cells each con- 

 tain "undifferentiated germ plasm," 

 but we can find neither morpho- 

 logical nor physico-chemical support 

 for such an assumption. Not only 

 is such germ plasm not visible, but 

 from a physico-chemical point of 

 view it is difficult to conceive how it 

 could continue to exist through the 

 course of differentiation of the 



