CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 347 



Organic polarity may be viewed from the standpoint of struc- 

 ture or of function ; neither of these aspects is complete in itself and 

 neither is at variance with the other but the two are complemen- 

 tary, la a series of important papers ('11-' 15) and in a recent 

 book ('16) Child has announced that the polarity of various 

 adult organisms and of certain eggs and embryos is shown 

 physiologically as a gradient in the rate of metabolism from one 

 pole to the other. But in spite of this important discovery 

 polarity can not be looked upon as a physiological process merely; 

 there must be a material, structural basis for such a gradient of 

 metabolism; furthermore it has not been demonstrated that 

 physiological differentiations are the causes of morphological 

 ones, for although functional changes are often more readily 

 visible than structural ones, there is every reason to think that 

 structure and function are inseparable in living organisms and 

 that neither is the cause of the other. In this paper attention 

 is devoted largely to the morphological aspects of polarity, but 

 it is not to be assumed therefore that the author considers the 

 physiological aspects as negligible. 



The polarity of the egg cell is the earliest recognizable and 

 most fundamental differentiation of morphogenesis; it is the 

 chief factor in determining the localization of developmental 

 processes, such as the segregation of different ooplasmic sub- 

 stances and of specific physiological activities, the orientation of 

 mitotic figures and cleavage planes, and finally the determina- 

 tion of the polarity and symmetry of the embryo and of the 

 adult. In short the polarity of the organism in the one-celled 

 stage of development is the chief condition and cause of the 

 polarity of all later stages. In many animals the polar differ- 

 entiation of the egg may be recognized even in the stages of its 

 development in the ovary and it is probable that in all cases 

 such polar differentiation exists at this time. 



When first recognizable this differentiation usually consists 

 in the eccentricity of the nucleus and centrosome and of the 

 greater part of the cytoplasm toward one pole of the egg and the 

 greater accumulation of yolk at the other pole, the former being 

 known as the animal pole and the latter as the vegetal. Dur- 



