CELL-DIVISION. 103 



The chromatin is the essential substance in the nucleus, and 

 in chemical composition is identical with nuclein, a material 

 which contains phosphorus in its molecule. The nucleolus is 

 composed of a substance known as paranuclein, or parachromatin, 

 and, during mitrosis, may possibly be partly converted into 

 chromatin, or a body from which chromatin may subsequently 

 be formed. 



In the cells of most plants below the mosses, and also in 

 certain cells during the reproductive cycle, in some of the 

 higher plants (Cycads) two peculiar structures are to be seen 

 lying close to the nucleus in the kinoplasm. These are the 

 centrosomes, and in lower plants and most animal cells, even 

 during vegetative divisions, they appear to possess an important 

 rdle. In the following description of mitosis the centrosomes 

 will be omitted, as in Higher Plants they are in all probability 

 absent, at least during ordinary vegetative divisions. 



2. The Details and Mechanism of Mitotic Nuclear Division, 

 or Karyokinesis* (see Plate I, Diagrams 76 to 80, and 

 Figs. 81a to 90 inclusive). 



The mitotic process is most conveniently divided into five 

 stages, the first four being termed phases, while the last 

 involves the formation of the cell-plate, or rudimentary par- 

 tition wall which divides the parent-cell into two. Thus it is 

 possible to distinguish between (a) Prophase, (/3) Metaphase, 

 (y) Anaphase, and (8) Telophase, in each of which certain 

 changes take place in the nuclear structures. It will be best to 

 study each of these phases separately and in order. 



(a) The Prophase, in which the nucleus prepares for division. 

 — At the beginning of mitosis certain conditions must be present 

 in a cell in order that the nucleus may be provided with 

 adequate powers to complete the process. These conditions are : 



(a) The presence of an adequate supply of soluble nitrOgenOUS 

 food and carbohydFates (elaljorated food-materials from the leaves). 



(6) The maintenance of an optimum temperature. 



(c) The presence of oxygen for the purposes of oxidation of waste 

 products arising during mitosis. 



(rf) Protoplasmic continuity between adjacent cells of a dividing 

 tissue. 



* See an article by the author in Knowledge and Scientific News, Feb., 

 1909, on "The Mechanism of Nuclear Division." Also one in same 

 magazine, Aug., Sep., 1909, on "Mitosis in Higher Plants." 



