56 



GENERAL SKETCH OE THE CELL 



focussed at the centrosome. On this basis he endeavours to explain 

 the position and movements of the nucleus, the succession of division- 

 planes, and many related phenomena.^ 



Hatschek {^^^) and Rabl ('89, 92), on the other hand, have ad- 

 vanced a quite different hypothesis based on physiological considera- 

 tions. By "cell-polarity" these authors mean, not a predetermined 

 morphological arrangement of parts in the cell, but a polar differen- 

 tiation of the cell-substance arising secondarily through adaptation of 

 the cell to its environment in the tissues, and having no necessary 

 relation to the polarity of Van Beneden (Fig. 22, B, C). This is 



A 



Van Beneden. 



B C 



Rabl, Hatschek. 



Fig. 22. — Diagrams of cell-polarity. 



A. Morphological polarity of Van Beneden. Axis passing through nucleus and centrosome. 

 Chromatin-threads converging toward the centrosome. B.C. Physiological polarity of Rabl and 

 Hatschek, Zj' in a gland-cell, C'in a ciliated cell. 



typically shown in epithelium, which, as Kolliker and Haeckel long 

 since pointed out, is to be regarded, both ontogenetically and phy- 

 logenetically, as the most primitive form of tissue. The free and 

 basal ends of the cells here differ widely in relation to the food- 

 supply, and show a corresponding structural differentiation. In such 

 cells the nucleus usually lies nearer the basal end, toward the source 

 of food, while the differentiated products of cell-activity are formed 

 either at the free end (cuticular structures, cilia, pigment, zymogen- 

 granules), or at the basal end (muscle-fibres, nerve-fibres). In the 

 non-epithelial tissues the polarity may be lost, though traces of it 

 are often shown as a survival of the epithelial arrangement of the 

 embryonic stages. 



1 Cf. p. 105. 



