xii CELL-DIVISION 563 



nuclei, as well as their attendant centrosomes, being 

 formed by the binary fission of those of the mother-nucleus. 

 But pari passu with the process of nuclear division, 

 fission of the cell-body is also going on. This takes 

 place by a simple process of constriction (F, G) in 

 much the same way as a lump of clay or dough would 

 divide if a loop of string were tied round its middle and 

 then tightened. 



In comparatively few cases the dividing nucleus instead 

 of going through the complicated processes just described 

 divides by simple constriction, but this seems to occur only 

 in the case of certain highly differentiated cells or of worn- 

 out cells. We have therefore to distinguish between direct 

 and indirect nuclear division. 1 



From what we have now learnt in this chapter as well 

 as in previous ones about the nucleus (compare e.g. p. 237) 

 and its complicated mode of division, we conclude that it 

 is of prime importance in regulating the life and deter- 

 mining the characters of the cell, and that the chromatin is 

 probably the substance on which these characters depend. 



In this connection the reader will not fail to note the 

 extreme complexity of structure revealed in cells and 

 their nuclei by the highest powers of the microscope. 

 When the constituent cells of the higher animals and 

 plants were discovered, during the early years of last 

 century, by Schleiden and Schwann, they were looked 

 upon as the ultima Thule of microscopic analysis. Now 

 the demonstration of the cells themselves is an easy 

 matter, the problem is to make out their ultimate con- 

 stitution. What would be the result if we could get 

 microscopes as superior to those of to-day as those of 

 to-day are to the primitive instruments of a hundred 

 years ago, it is impossible even to conjecture. 



1 To the latter very elaborate method the name mitosis or 

 karyokinesis is applied : direct division is then distinguished as 



amitosis. 



