SUMMARY AND CONCLUSION 327 



may be actually derived from the centrosome (p. 321); but Lillie finds 

 in some cases that in the same mitosis the entosphere is formed by a 

 different process, arising by a differentiation of the cytoplasm around 

 the central granule. The former case, therefore, may be interpreted 

 to mean simply that the centrosome may give rise to other cytoplasmic 

 elements (as has already been shown in the formation of the sperma- 

 tozoon, p. 172), the material of which may then contribute either 

 directly or indirectly to the building of the aster ; and the facts do 

 not come into collision with the view that the astral rays are in gen- 

 eral formed from the cytoplasmic substance. 



G. SUMMARY AND CONCLUSION 



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

 clusion 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. Per- 

 manent organs, such as the nucleus or plastids, are constant areas in 

 the same basis, which never are formed de novo, but arise by the divi- 

 sion of preexisting areas of the same kind. These two extremes are, 

 however, connected by various intermediate 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. There is now considerable evidence that the 

 centrosome itself may in some cases have the character of a perma- 

 nent organ, in others may disappear and re-form like the asters. 



The facts point toward the conclusion, which has been especially 

 urged by De Vries and Wiesner, that the power of division, not only 

 of the cell-organs, but also of the cell as a whole, may have its root in 

 a like power on the part of more elementary masses or units of which 

 the structural basis is itself built, the degree of permanence in the cell- 

 organs depending on the degree of cohesion manifested by these elemen- 

 tary bodies. If such bodies exist, they must, however, in their primary 

 form, lie beyond the present limits of the microscope, the visible struc- 

 tures arising by their enlargement or aggregation. The cell, therefore, 

 cannot be regarded as a colony of "granules " or other gross morpho- 

 logical elements. The phenomena of cell-division show, however, that 

 the dividing substance tends to differentiate itself into several orders 

 of visible morphological aggregates, as is most clearly shown in the 

 nuclear substance. Here the highest term is the plurivalent chromo- 

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



