344 



SENESCENCE AND REJUVENESCENCE 



18? 



egg (o) and the other three become nurse cells, which supply the egg 

 with nutrition. Three of these nurse cells thus contribute to the 

 formation of one parthenogenic egg. The zygogenic egg, however, 

 uses up not only three nutritive cells, but often several other cell 

 groups, including the young egg cells, i.e., a much larger amount of 

 nutritive material contributes to its formation than to that of the 

 parthenogenic egg. Fig. 193 shows the lower end of an ovarian 



tubule containing a zygogenic egg. 

 It is much larger than the par- 

 thenogenic egg and contains more 

 yolk. 



Among the insects, the plant 

 lice also produce both partheno- 

 genic and zygogenic eggs. In this 

 case the difference between the 

 two kinds of eggs is very marked, 

 the parthenogenic egg being much 

 the smaller and containing little 

 yolk (Fig. 194) as compared with 

 the zygogenic egg (Fig. 195). 

 Even the nurse cells, which here 

 form a sort of gland with which 

 the egg cell is connected by a pro- 

 toplasmic strand, are larger and 

 more highly developed in the latter 

 case. Similar differences have 

 been observed in other forms pro- 

 ducing the two kinds of eggs. If 

 the process of oogenesis is a pro- 

 cess of differentiation and senescence, we must conclude that in these 

 cases the parthenogenic egg does not proceed so far in development 

 as the zygogenic egg. Morphologically it is evidently less highly 

 differentiated and younger. 



Among the bees, however, where eggs which produce males, 

 i.e., the drones, apparently develop parthenogenically, while the 

 females, both workers and queens, develop from fertihzed eggs, no 

 characteristic morphological differences between the partheno- 



FlGS. 181, 182. — Primitive germ 

 cells and full-grown egg of Fasciola 

 (liver fluke), with stalk of attachment. 

 From Schubmann, '05. 



