CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 351 



said, a Sdsible mosaic work.' All of these orientations of de- 

 velopment find their earUest visible expression in the polar dif- 

 ferentiation of the egg; the problem of the causes of these 

 orientations is perhaps the greatest problem of embryogeny. 



Causes of cell polarity 



Any satisfactory explanation of the causes of polarity and 

 symmetry of cells must be able to explain the following 

 phenomena : 



a. The typical locaUzation of substances in cells, such as 

 yolk at the vegetal pole and of cytoplasm and nucleus at the 

 animal pole, together \\'ith the typical orientation of spindles, 

 centrospheres and other cell constituents during and after 

 mitosis. 



6. The return of all cell substances to their typical positions 

 after they have been displaced if sufficient time and opportunity 

 for this return are given, and if the injury' to the cell is not too 

 severe. 



It is therefore evident that the cause of polarity in cells is one 

 of the most fundamental problems in the study of the structure 

 and functions of cells and in the processes of differentiation and 

 regulation. The localization of formative substances in eggs 

 determines the localization of the parts of the developing embr>'o, 

 and the return of these substances to their normal positions 

 when once they have been displaced is a remarkable case of 

 adaptation or regulation in which the organization concerned is 

 merely the polarity of a single cell. Because of the apparent 

 simplicity of this problem of the polarity of the cell, the hope is 

 raised that a thorough analysis of it may throw light on the 

 problems of differentiation and regulation in general. 



It is conceivable that the causes of this polarity may be due 

 (1) to electric charges on the colloidal particles of protoplasm, or 

 more particularly on centrospheres and cell membrane, or (2) 

 to external or internal surface tension phenomena, or (3) to a 

 framework of viscid protoplasm which is so elastic or contrac- 

 tile that it recovers its nonnal form after distortion. The 



