342 



f 

 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 



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

 flower) at time of fertilization, showing egg, o; two male nuclei, spz, spi\ 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 



