314 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



i^S^iBPwfr' ^(p^p 





t^J 



>^'. -ff 



.'j>. ' - 



Fig. 8. Oogenesis in a bean-weevil, (a) young oocyte (Serra, Unna-Brachet) with much 

 RNA at the animal pole and the beginning of peripheral vitellogenesis in other parts; 

 (b) more advanced follicle (fixed in Bouin-Allen; transverse section; stained with iron- 

 hematox.) showing the peripheral yolk granules (dark) ; (c) longitudinal section of an 

 oocyte in full vitellogenesis (ethanol, Unna-Brachet) with its central remnant of RNA, ex- 

 tending to the posterior pole. From Mulnard, 1954. 



been established that the first sign of polarity is the "yolk nucleus" and that the 

 location of this structure has no constant relation to the axis of the ovary. But the 

 "yolk nucleus" elaborates only lipids, and stimulates the growth of the antipolar 

 cytoplasm, while vitellogenesis first takes place under the cortex and progresses 

 centripetally^ (Fig. 4). In this early period of vitellogenesis granules are produced 

 which crowd the cortex of the pole, and which are detected by several tests used 

 for mucopolysaccharides (Fig. 5) ; they seem to be prepigment granules (Voss and 

 Von Wartenberg, 1955; Von Wartenberg, 1956). Simultaneously, an elaborate 

 structure appears in the cortex. Studies with electron microscope (Kemp, 1956) 

 show that the aspect of the zona striata known in the chorion is due to microvilli of 

 the cytoplasm interdigitating with the finger-like processes of the follicular cells^ 

 (Fig. 6). This increase of contact evidently favors assimilation and growth of the 



' This is also true in fish eggs (cf. Konopacka, 1937; Yamamoto, 1956-7). 

 2 Similar villi also exist in invertebrate oocytes (Rebhun, 1958). 



