THE MECHANISM OF DEVELOPMENT 



which it were well to consider before proceeding further with 

 the search after the " ultimate unit of living matter," and 

 therefore of growth, of differentiation, and of variability. 



SECTION III THE MECHANISM OF CELL DIVISION 

 (MITOSIS) 



Growth in the sense of increase of size is the direct result of 

 cell division. Large bodies do not have larger cells than small 

 ones, but they have more of them. Growth is, therefore, in 

 proportion to cell division, the mechanism of which is exceed- 

 ingly suggestive of the methods by which lines of descent are 

 preserved and the proper development assured. 



When the protoplasm of an ordinary growing cell, plant or 

 animal, has absorbed material until it has reached a certain maxi- 

 mum size, it then prepares for division. This is not a lump 

 division in which the new cells each get one half of the bulk of 

 the parent cell, but it is qualitative as well as quantitative, and 

 is based on an exceedingly orderly procedure, which insures not 

 only that each daughter cell shall receive its share of the mass 

 but also that this share shall be identical in quality with that 

 inherited by the sister cell of the same division. 



Those portions of the cell contents most intimately concerned 

 in the process of division, and therefore of chief interest here, 

 may be briefly described as follows : 



Floating in the general protoplasmic mass (the cytoplasm) is 

 a small body (the nucleus) of greater density than its surround- 

 ing matter and the evident seat and initial point of all construc- 

 tive processes. 



Scattered through the mass of the nucleus and generally, but 

 not always, in the form of minute granules is the so-called 

 " chromatin matter," named from its intense reaction to staining 

 agents. 



These granules of which the chromatin matter is (apparently) 1 

 composed are the " chromatin granules " of some authors, the 



1 The word " apparently " is inserted because the granular character of chro- 

 matin matter has not in every case been made out, and because its granular 

 character is less pronounced at some times than at others. 



