270 Cell as Basis oj Organic Activity 



CELL DIVISION 



The growth of an organism involves both an increase in size of 

 individual cells and an increase in numbers of these cells. In multi- 

 cellular forms, this increase in cell number is brought about by cell 

 division. While it might appear simpler for a cell merely to pull in 

 two, this is not the way it actually happens. Rather an indirect method 

 known as mitosis occurs. In this, the nuclear membrane first breaks 

 down and the chromosomes divide; this is followed by division of the 

 cytoplasm. The importance of this indirect method is evident when 

 it is recalled that the chromosomes carry the hereditary material which 

 must be distributed to each daughter cell. 



Cell division is a continuous process once initiated ; however, for 

 convenience it is arbitrarily divided into several phases. 



EARUr PROPHASE 



LATE PROPHASE 

 latsral via* 



LATE PROPHASE 

 polar vi«« 



METAPHASE ANAPHASE TELOPHASE 



Fig. 81. — Mitosis in thie roundworm Ascaris (somewhat diagrammatic). 



Mitosis: the Interphase. — The undividing cell as described above 

 is considered to be in the resting stage or interphase. The term inter- 

 phase is a more descriptive one inasmuch as the living cell is never 

 truly resting, rather it is always undergoing metabolic activity. During 

 this phase, the chromatin is scattered throughout the nucleus on the 

 chromonemata, the centrioles lie close together, and the nuclear mem- 

 brane is intact. 



Early Prophase. — The first indication that the cell is about to 

 divide occurs when the chromonemata and its chromatin divide longi- 



