48 POLARITY AND SYMMETRY; 



cause changes in polarity. We have mentioned above (p. 31) 

 that the structure of the egg can be profoundly modified by 

 centrifuging. As a rule, the protein yolk is heavier, and the 

 fatty yolk lighter than the clear cytoplasm. Therefore, they 

 will accumulate at opposite poles, with the cytoplasm occupying 

 the middle zone of the egg (Plate III). The direction of this 

 stratification need not coincide with the original polarity, but 

 may be at any angle to it. In this way, the normal yolk gradient 

 and the correlated gradients of metabolism are completely 

 destroyed. Nevertheless, the further development of such eggs 

 shows that in the majority of cases their polarity has remained 

 unchanged. The egg material, displaced by centrifuging, often 

 after a short time returns to its normal position with regard to 

 the original egg axis (Raven, 1938b; Raven and Bretschneider, 

 1942); polar bodies are given off at the original animal pole; 

 cleavage and development of the embryo are oriented with 

 regard to the original polarity, and are independent of the 

 direction in which the centrifugal force has been operating. 

 Evidently, the factors governing polarity have not been shifted 

 by the centrifuging; they must be inherent in a component 

 of the egg that is not moved by the centrifugal force. Presumably 

 this is the more solid outer layer of the egg, the cortex. 

 Polarity, then, depends on factors localised in the egg cortex; 

 the axial gradients arise only as a result of these factors. There 

 may be some exceptions to this rule. In amphibians, for example, 

 the yolk gradient seems to be of decisive importance for the 

 direction in which the embryo will later develop. If the eggs 

 of an amphibian are turned upside down, and fixed in this 

 position, the heavy yolk material will sink through the cyto- 

 plasm from the vegetative side toward the originally animal 

 side of the egg. According to Motomura (1935) and Pasteels 

 (1938-39), this inversion by gravity of the yolk gradient results 

 in an inversion of the polarity of the egg. 



Just as, in general, cortical factors are of importance for 

 polarity, so they are for symmetry. Pasteels has made a study 

 of amphibian eggs in which the internal structure was dis- 

 turbed by gravity or centrifugal force. Their further development 

 proved that the localisation of the primordia of the future 



