I ORGANIZING ASPECTS OF G AM ETO G ENESIS 315 



egg. The filling of the cytoplasm with yolk is completed by a special perinuclear 

 process of secondary vitellogenesis (Fig. 7). In arthropods, the earliest suggestion of 

 polarity seems to be the greater amount of RNA on the side of the still spherical 

 oocyte nearest the germinal vesicle (Mulnard, 1954). The antipole sometimes 

 shows a definite "yolk nucleus" (chilopods). This region grows intensely, while 

 yolk begins to be elaborated around the entire periphery and to accumulate 

 centripetally. The characteristic elongation of the egg occurs only secondarily, in 

 the hind part of the ovigerous tubes. The nuclear pole generally corresponds to the 

 dorsal side of the mother (Fig. 8). 



From these examples, it can be concluded that polarity is nothing transcen- 

 dental, and is not the expression of some intimate property which could be related 

 to a crystalline structure or to a uniform orientation of macromolecules. Polarity 

 is both inherited and acquired. It is inherited in the sense that it results either 

 from the structure of the ovary — a feature of the preceeding generation — or from 

 some attraction which associates the centriole, the Golgi vesicles, and the RNA 

 rich granules in a complex system. It is acquired in the sense that trophic, 

 extrinsic relations certainly play their role, as do also the physical conditions of 

 equilibrium among the substances elaborated in the cytoplasm. 



Also, polarity is the direct consequence of the formation of a giant cell in which 

 the nucleus and the centro-Golgian complex remain unique. It depends upon 

 the direction of the nutritive flow into the egg. It must be realized that the origin 

 of these nutritive materials may vary: they may be synthesized in the egg with 

 the help of the adjacent cells, which eventually form a follicle; or they may be 

 directly transformed into yolk by specialized vitelline cells; or they may be 

 elaborated at a distance, as the yolk nucleoproteins of l:)irds and fishes which are 

 synthesized in the liver' (Clavert, 1952; Clavert and Zahnd, 1956, 1957). 



C. Symmetrization during oogenesis 



In several instances it must be admitted that a bilateral symmetrized structure 

 is present in the unfertilized egg. There are at least four cases, as far as I can see, 

 in which the acquisition of symmetry can be followed in the growing oocytes. 

 The first case concerns the insect egg, where the correspondence between the 

 cephalo-caudal axis of mother and embryo has long been known (Hallez' rule). 

 Here the symmetry certainly depends upon the form of the ovarian cavity and 

 oviduct, and it is confirmed by the general existence of the posterior polar plasm 

 and sometimes by other details of cytoplasmic structure, recorded in Drosophila 

 and in Acanthoscelides (Fig. 8, c). In the viviparous teleost Lebistes, the eggs of 

 which are fertilized in the ovary, and develop in situ, a granular plasm, already 

 mentioned to explain the change of polarity, develops quite early in oogenesis 

 on one side of the nucleus, and endows the oocyte with a visible bilateral organi- 

 zation (Fig. 9). According to Fautrez and Vakaet (1954), this early established 



' See also the investigations, based on isotopic and serological methods, of Flickinger and 

 Rounds (1956). 



Literature p. 483 



