342 



SENESCENCE AND REJUVENESCENCE 



which enter the cytoplasm of the egg and serve as food are begin- 

 ning to arise. The third figure (Fig. 189) shows a segment of the 

 egg at a still later stage with follicle and test cells in the peripheral 

 cytoplasm and yolk masses forming below them. Figs. 190, 191 



177 



FIGS. 176, 177. Embryo sacs of Helianthus (sunflower) and Rudbeckia (cone- 

 flower) at time of fertilization, showing egg, o; two male nuclei, sp l} sp 2 ', embryo sac 

 nucleus, en. From Nawaschin, 'oo. 



are two stages in the oogenesis of a fish egg, the first showing the 

 young egg at the beginning of yolk formation, the second, a later 

 stage in which the cytoplasm is loaded with numerous yolk spheres. 

 In various invertebrate groups the same individuals produce at 

 different times parthenogenic eggs, i.e., eggs which develop without 



