THE GAMETES IX PLANTS AXD AXIMALS 



343 



fertilization, and zygogenic eggs which require fertilization before 

 development. It is a fact of great interest that in such cases the 

 parthenogenic eggs usually dilTer morphologically from the zygo- 

 genic eggs. In Sida crystallina, one of the cladoceran Crustacea, for 

 example, the parthenogenic eggs are smaller and contain less yolk 



Figs. 178-180. — Three 

 stages in the differentiation 

 of the egg, 0, of Hydra. 

 From Downing, '09. 



than the zygogenic eggs. Fig. 192 shows an ovarian tubule of this 

 species containing various stages of parthenogenic oogenesis. The 

 primitive cells {pc), formed at the upper end of the tubule, after 

 the period of division is over arrange themselves in groups of four 

 {gi g) of which the third from the upper end develops into an 



