44 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



its origin. The time required for complete division 

 varies from one to several hours. 



Amitosis is the direct cell division and occurs sel- 

 dom as a normal process. The nucleus merely con- 

 stricts, without the formation of chromatin loops or 

 filaments, thus producing two or more nuclei or 

 nuclear fragments. The cytoplasm may not take 

 part in this division, in which case polynucleated 

 cells are formed; however, polynucleated cells may 

 also arise from mitosis. Preceding the division the 

 nucleolus, if present, may subdivide, while the 

 centrosome does not seem to take any active part 

 whatever. In the human body certain leucocytes 

 have been described as dividing by amitosis, also 

 polynucleated pavement cells of the bladder. De- 

 generated cancer cells have been described as show- 

 ing amitotic division, but whether a cause or a con- 

 sequence of degeneration and disease can be argued 

 with equal force. On the other hand, certain em- 

 bryonic cells have been described as dividing by 

 amitosis, which later take up the regular cell di- 

 vision of amitosis, but these cases, if normal, must 

 be regarded as very exceptional. 



Laws of Cell Cleavage. (i) The cleavage plane is 

 always equatorial to the nucleus. 



(2) The position of the nucleus depends on (a) the 

 shape of the cell, and (6) on the distribution of food 

 material or secretions. If the cytoplasm is eccentric 

 in the cell, the nucleus is associated with it. 



(3) When each plane of division is parallel to the 

 preceding plane, a filament is produced. This is the 

 law in some plants, as spirogyra, nostoc, etc. 



