10 INTRODUCTION PROTOPLASM THE CELL 



shape. If expansion during the growth of such cells is limited 

 or resisted by surrounding tissues, pressure will be applied to 

 the cell in many directions, and it consequently assumes a poly- 

 hedral shape. If, on the other hand, the pressure is excessive in 

 two opposing directions, the cell gradually becomes flattened or 

 squamous. If, again, the pressure on its four sides should exceed 



FIG. 10. VARIOUS FORMS OF CELLS. 



a, squamous epithelium from the tongue; 6, a columnar cell from the small intestine; 

 c, a polyhedral or spheroidal cell from the liver ; d, a smooth muscle cell from the mus- 

 cular coat of the stomach, x 550. 



that applied to the poles of the cell, it would necessarily assume 

 an elongated, prismatic, or columnar shape. 



Reproduction. Two forms of cell division may be observed 

 within the human body : direct division or fission, and indirect 

 division, karyokinesis or mitosis. 



Direct Division (amitosis, fragmentation, fission). This method 

 of cell division is the least common. It occurs in some of the 

 epithelial cells of the urinary bladder, in certain cells of red bone 

 marrow, and perhaps occasionally in leucocytes and in glandular 

 epithelium. 



The process, according to Nemiloff,* commences by an elonga- 

 tion, followed by constriction, and finally fission of the nucleolus. 

 These changes are accompanied by a similar elongation of the 

 nucleus, followed by constriction and cleavage along such a median 

 plane that a daughter nucleolus is included within each daughter 

 nucleus. The daughter nuclei then travel toward opposite poles 

 of the cell, and constriction and cleavage of the cell protoplasm 

 complete the process. 



* Anat. Anz., 1903. 



