236 SOME PROBLEMS OF CELL-ORGANIZATION 



G. SUMMARY AND CONCLUSION 



A minute analysis of the various parts of the cell leads to the 

 conclusion that all cell-organs, whether temporary or "permanent," 

 are local differentiations of a common structural basis. Temporary 

 organs, such as cilia or pseudopodia, are formed out of this basis, 

 persist for a time, and finally merge their identity in the common 

 basis again. Permanent organs, such as the nucleus or centrosome, 

 are constant areas in the same basis, which never are formed dc novo y 

 but arise by the division of pre-existing areas of the same kind. 

 These two extremes are, however, connected by various interme- 

 diate gradations, examples of which are the contractile vacuoles of 

 Protozoa, which belong to the category of temporary organs, yet in 

 many cases are handed on from one cell to another by fission, 

 and the attraction-spheres and asters, which may either persist from 

 cell to cell or disappear and re-form about the centrosome. 



The facts point strongly to the conclusion, which has been espe- 

 cially urged by De Vries and Wiesner, that in many if not in all 

 cases the division of cell-organs is in the last analysis brought about 

 by the division of more elementary masses of which they are made 

 up ; and furthermore that the degree of permanence depends on the 

 degree of cohesion manifested by these masses. The clearest evi- 

 dence in this direction is afforded by the chromatic substance of the 

 nucleus, the division of which does not take place as a single mass- 

 division, but through the fission of more elementary discrete bodies 

 of which it consists or into which it is resolved before division. 

 Several orders of such bodies are visible in the dividing nucleus, 

 forming a series of which the highest term is the plurivalent chro- 

 mosome, the lowest the smallest visible dividing basichromatin-grains, 

 while the intermediate terms are formed by the successive aggrega- 

 tion of these to form the chromomeres of which the dividing chromo- 

 somes consist. Whether any or all of these bodies are " individuals " 

 is a question of words. The facts point, however, to the conclusion 

 that at the bottom of the series there must be masses that cannot be 

 further split up without loss of their characteristic properties, and 

 which form the elementary morphological units of the nucleus. 



There is reason to believe that the linin-network is likewise com- 

 posed of minute bodies, the oxychromatin-granules, which are closely 

 similar in appearance to the smallest chromatin-grains, and differ 

 from them only in chemical nature as shown by the difference of 

 staining-power. Whether the oxychromatin-granules have also the 

 power of growth and division is unknown ; but if, as Van Beneden 

 and Heidenhain maintain, the basichromatin- and oxychromatin-gran- 



