40 GENERAL SKETCH OF THE CELL 



Hatschek ('88) 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 jn 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. 17, B, C.) This is 

 typically shown in epithelium, which, as Kolliker and Hackel 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, towards the source 

 of food, while differentiated products of the 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. 



But, although this conception of polarity has an entirely different 

 point of departure from Van Beneden's, it leads, in some cases at 

 least, to the same result ; for the cell-axis, as thus determined, may 

 coincide with the morphological axis as determined by the position 

 of the centrosome. This is the case, for example, with both the 

 spermatozoon and the ovum ; for the morphological axis in both is 

 also the physiological axis about which the cytoplasmic differentiations 

 are grouped. Moreover, the observations of Heidenhain, Lebrun, and 

 Kostanecki indicate that the same is true in epithelium ; for, accord- 

 ing to these authors, the centrosome is always situated on that side 

 of the nucleus turned towards the free end of the cell. How far this 

 law holds good remains to be seen, and, until the facts have been 

 further investigated, it is impossible to frame a consistent hypothesis 

 of cell-polarity. The facts observed in epithelial cells, are, however, 

 of great significance ; for the position of the centrosome, and hence 

 the direction of the axis, is here obviously related to the cell-environ- 

 ment, and it is difficult to avoid the conclusion that the latter must 

 be the determining condition to which the intracellular relations con- 

 form. When applied to the germ-cells, this conclusion becomes of 

 high interest ; for the polarity of the Q,g^ is one of the primary con- 

 ditions of development, and we have here, as I believe, a clue to its 

 origin.^ 



' Cf. pp. 288, 320. 



