310 GERMINAL ORGANIZATION INDUCTION PHENOMENA 



Fig. 4. Previtellogenesis in Xenopus oocytes, 

 Zenker fixation, (a) Young oocyte cut along 

 its axis of polarity (and possibly along its 

 plane of symmetry) and stained with tolui- 

 dine blue; the dark zone around the nucleus 

 is the RNA-rich cytoplasm; vitellogenesis 

 takes place at the periphery and dissociates 

 the cytoplasm as shown in (b) ; (b) Detail of 

 the animal pole showing, from top to bottom, 

 the unstained chorion (scarcely visible), the 

 granular layer of cortical cytoplasm (see Figs. 

 6, b and c) , a homogeneous subcortical layer 

 (see Fig. 6 a, under the cortical granules c), some pigment granules (darker spots) sticking to 

 this layer, clear columns of young platelets piling up from the periphery to the center, dark 

 radial strands of basophilic cytoplasm, dissociated by the vitellogenesis, and the granular 

 structure of the central cytoplasm; (c) somewhat more advanced oocyte cut along its polar 

 axis (and possibly along its plane of symmetry) and stained with hemalun-eosin; only 

 yolk platelets are stained, making conspicuous the centripetal progression ; some pigment 

 has appeared at the animal pole (upper part); numerous nucleoli are dispersed in the 



nucleus. (From Wittek, 1952). 



