3l6 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



plane of syinmetry is preserved during the whole development^ In the growing 

 oocytes of amphibians, a bilateral symmetry is often, but not always visible, 

 owing to the distribution of yolk, pigment and RNA (Fig. 7, a). In this case, it seems 

 to be caused, at the origin of the oocvte, bv some excentricitv of the "volk nucleus". 



Fig. 9. First steps of growth in the oocyte of the guppy fish, Lebistes retic. H is the hilum of 

 the ovary given as a point of reference. In a, the nucleus is already excentric, and a primary 

 polarity is estabHshed (arrow). While this continues (b), a plasm which is proteic in nature 

 (circles) appears near the primary "animal" region. In (c), growth has proceeded by 

 synthesis of cytoplasm around the nucleus (small radiating arrows) while the germinal 

 vesicle has migrated to a new "animal" region, and the definitive polarity (long arrow) has 



appeared. From Vakaet, 1955. 



Finally, in several mammalian eggs, symmetry is acquired along with polarity. 

 Although a preferential direction of growth is provided by the so-called "yolk 

 nucleus" complex — a still more inappropriate designation, since practically no 

 yolk will be formed — some deviation from this direction seems to be caused by the 

 location of the one or two primary follicular cells. As a result, the primary follicle 

 is distinctly more active on one side of the polarity axis (Fig. 10). In this way, 

 the growing oocyte appears to acquire a denser, more RNA-rich cytoplasm on 

 one side, while the other side is more areolar, and will, in certain species (e.g. 

 guinea pig, mole) become loaded with lipids. 



The remarks made in connection with polarity could be repeated here. This 

 bi-axial organization is not the expression of any intimate structure of the cyto- 

 plasm. It is only a consequence of the anatomical or histological features of the 

 mother organism. But, once impressed upon the egg, it is likely to have important 

 consequences. At this point, let us consider the degree of stability of this primary 

 egg pattern, and where it might be most firmly established in the egg cytoplasm. 

 All the results of centrifugation agree on the stability of the cortical structure of 

 the unfertilized egg. This ectoplasmic layer (or cortical layer, or cortex), which 



^ Attention has, however, been drawn (Clavert and Filogamo, 1958) to the fact that the 

 orientation of the polarity axis mentioned above (p. 313) is not valid for full grown oocytes, 

 and that the location of the blastodisc itself can be modified by inverting for a day or so 

 the normal position of the mother fish. So, this problem is not quite settled. 



