ORGANIZING ASPECTS OF GAM ETO GENESIS 



3" 



Fig. 5. Melanogenesis at the animal pole of Xenopus eggs fixed in a picro-formol mixture. 

 (a) PAS reaction; (b) Hale reaction. Both reactions selectively stain the granules located 

 at the animal pole. In (a) the cords of peripheral vitellogenesis are visible (clear). Courtesy 



of Wartenberg, 1956. 



to mention only some of the recent data. These observations fall in line with 

 others of general cytology, which show that the nucleus is an organ of active 

 elaborations. 



It is during their time of growth that eggs acquire the properties of organization 

 which endow them with a morphogenetic pattern. In all groups, they become 

 polarized ; in several, they acquire some differential property on one side of the 

 axis, and eventually attain a bilaterally symmetrical organization (symmetri- 

 zation). 



Polarization of the egg cell can no longer be considered as a primary structural 

 property inherent to the whole cytoplasm. It has been recognized that, in ovaries 

 with a tubular structure, the eggs preserve the polarity which they had while 

 arranged in an epithelium. In the case of the snail Limnea, the young egg cells 

 detach themselves from the wall, wander in the lumen by amoeboid movement, 

 and settle down again: their region of adherence becomes the antipole (tradition- 

 ally called vegetal or vegetative pole) and, most probably, the polarity axis is 

 established in that way (Bretschneider and Raven, 195 1). In the Portuguese 



Literature p. 483 



