POLARITY OF THE CELL 



39 



general conception of cell-polarity has been developed in two differ- 

 ent directions, one of which starts from purely morphological con- 

 siderations, the other from physiological, and a parallelism between 

 them has not thus far been very clearly made out. 



On the y one hand, Van Beneden ('83) conceived cell-polarity as a 

 primary morphological attribute of the cell, the organic axis being 

 identified as a line drawn through the centre of the nucleus and the 

 centrosome (Fig. 17, A). With this view Rabl's theory ('85) of 

 nuclear polarity harmonizes, for the chromosome-loops converge tow- 

 ards the centrosome, and the nuclear axis coincides with the cell-axis. 

 Moreover, it identifies the polarity of the egg, which is so important 

 a factor in development, with that of the tissue-cells; for the egg- 



A 



VAN BENEDEN. 



B C 



RABL, HATSCHEK. 



Fig. 17. Diagrams of cell-polarity. 



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

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

 and Hatschek, B in a gland-cell, C in a ciliated cell. 



centrosome almost invariably appears at or near one pole of the 

 ovum. 



Heidenhain ('94, '95) has recently developed this conception of 

 polarity in a very elaborate manner, maintaining that all the struct- 

 ures of the cell have a definite relation to the primary axis, and that 

 this relation is determined by conditions of tension in the astral rays 

 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. In the present state of the 

 subject, Heidenhain's theories must be regarded as somewhat trans- 

 cendental, though they give many suggestions for further investigation. 



