138 INTRODUCTION TO CYTOLOGY 



Of the highest importance in this connection are the results of at- 

 tempts to maintain the cells and tissues of higher animals in the living 

 condition in artificial culture media outside the body. It has been shown 

 by the remarkable experiments of Carrel, Leo Loeb, Burrows, H. V. 

 Wilson and others that cells may be isolated from any of the highly 

 differentiated essential tissues of the body and kept actively growing and 

 multiplying in vitro for a length of time frequently far exceeding that to 

 which they would have lived in the body. They do not appear to grow 

 old: indeed it is not improbable that in such a constantly favorable 

 environment somatic cells are as " potentially immortal" as the germ 

 cells (see p. 403). In the words of Pearl (1921), "It is the differentiation 

 and specialization of function of the mutually dependent aggregate of 

 cells and tissues which constitutes the metazoan body which brings about 

 death, and not any inherent or inevitable mortal process in the indivi- 

 dual cells themselves." 



POLARITY 



Polarity is a feature which is exhibited in some form by the cells of 

 all higher organisms, and in at least many of the simpler ones, as shown by 

 Tobler (1902, 1904) for certain algae; indeed it is probable that it is 

 possessed in some -form and degree by all cells. Harper (1919) calls 

 attention to the fact that "in the presence of polarity and the various 

 symmetry relations we have a fundamental distinction between cell 

 organization and that of polyphase colloidal systems as they are com- 

 monly produced in vitro." 



This polarity has two aspects, the morphological and the physiological. 

 In the first place, the various constituents of the cell may be arranged 

 symmetrically about one or more ideal axes, so that the cell has more 

 or less distinctly differentiated anterior and posterior ends. This 

 structural aspect of polarity has been the one chiefly emphasized by 

 certain workers: van Beneden (1883), for instance, looked upon polarity 

 as "a primary morphological attribute of the cell," the axis passing 

 through the nucleus and the centrosome. Later writers, among them 

 Heidenhain (1894, 1895), made this conception of morphological polarity 

 the basis for interpretations of many of the phenomena of cell behavior. 

 (See Wilson 1900, pp. 55-56.) However, as Harper (1919) points out, 

 polarity "is apparently independent of the uni- or multinucleated condi- 

 tion of the cell, which shows that it is in some cases at least a more 

 generalized characteristic of the cell as a whole rather than a mere ex- 

 pression of the space relations of the nucleus and cytoplasm ..." Other 

 investigators (Hatschek 1888; Rabl 1889, 1892) early laid emphasis upon 

 the physiological expression of polarity. The cell shows a polar differ- 

 entiation in physiological labor: the processes in one portion of the cell 

 differ from those in another, this difference in the case of tissue cells 



