144 The Phenomena of Morphogenesis 



appears to be a property not of the individual cells but of the aggregate 

 that they form. 



Three Aspects of Polarity. Polarity is evidently a complex phenomenon 

 which is intimately related to the whole process of development. It may 

 be broken down, for purposes of more detailed examination, into several 

 different aspects or elements which may possibly involve different physio- 

 logical or developmental processes. 



First, one may recognize the oriented behavior of living substance, as 

 distinct from axiation or bipolarity. This is evident in the differential 

 growth of cells and tissues, where one dimension increases more rapidly 

 than the others; in the controlled plane of cell division, in which the 

 cytoplasm, as evident especially in vacuolate cells, sets up a pattern 

 oriented in a definite direction; and in coenocytes and plasmodia where 

 growth, movement, or direction of nuclear spindles is similarly oriented. 

 The fundamental basis of this behavior is not known. There may be in- 

 volved the orientation of micelles or other submicroscopic units, the 

 paracrystalline properties of cytoplasm, the orientation of molecules at 

 cell surfaces or interfaces, or the nature of the fine structure of the cell 

 wall. It is reasonable to suggest that some sort of cytoplasmic anisotropy 

 is concerned in this oriented behavior. Here is evidently a major problem 

 for the student of the ultimate structure of protoplasm, a problem inti- 

 mately related to the whole question of directed growth and thus of or- 

 ganic form. Whatever the basis of oriented behavior may be, in some 

 cases it can evidently be changed in direction readily by environmental 

 factors, but in others, when once established, it becomes firmly fixed. 



A second aspect or element of polarity is axiation. The oriented be- 

 havior of living material most commonly, though not invariably, is ex- 

 pressed in cellular systems which develop symmetrically in relation to an 

 axis or plane of symmetry parallel to the direction of orientation. Most 

 cells and most multicellular structures possess an axis. Such structures as 

 the cells and filaments of unattached filamentous algae may show no evi- 

 dent difference between the two ends of the axis, either in cell or filament, 

 but they are clearly axiate. The problem of the symmetrical growth of a 

 living system about this axis, so characteristic of almost all organic de- 

 velopment, is an essential part of the general problem of pattern. Experi- 

 mental attack upon this phenomenon of symmetry is promising, for its 

 character can often be changed by modifying the environment. 



The third aspect of polarity is polar difference, the appearance of dis- 

 similarity between the two ends of the axis. This is regarded by many as 

 the essential characteristic of all polarity and is present in the great ma- 

 jority of organic axes. In not a few cases, as we have seen, cytoplasm may 

 show oriented behavior, or an axis of symmetry may develop, without any 

 demonstrable evidence of difference between the two ends of the system. 



