CELLS AND EPITHELIAL MEMBRANES. 15 



The mechanism of the process of generation by 

 cleavage is as follows : 



The primitive nucleus developes itself into two 

 secondary nuclei, or, two nuclei may exist, from* its 

 formation, in the primitive cell ; then the cell wall 

 contracts like an hour-glass enclosing a nucleus in 

 either end, and finally a separation takes place at the 

 contracted portion, the result of this metamorphosis 

 being two perfect new cells ; (Ex. cells of cartilage, 

 cells of epithelium of intestine). 



SECT. II. EPITHELIAL MEMBRANES. The term Epi- 

 thelium is applied to a class of membranes formed 

 exclusively of cells, and ordinarily very thin and deli- 

 cate. They invest all of the free surfaces which the 

 body presents ; thus the external integument is every- 

 where clothed with an expansion of epithelium, or 

 epidermis, and the same is true of all mucous, serous, 

 and synovial membranes, and of the membranes lining 

 the cavities of the secreting glands, blood-vessels, and 

 lymphatics. In view of the different shapes of the 

 cells of which epithelial membranes are composed, 

 they are divided into three groups, or classes: 1st, 

 polygonal or scaly epithelium ; 2d, cylindrical or co- 

 nical ; and 3d, ciliated epithelium. 



Epithelial cells are in some instances spread out so 

 as to form a simple lamina ; in others they are found 

 in several layers, one superimposed upon another. 

 The first mode of arrangement constitutes simple epi- 

 thelium / the second is known as stratified epithelium. 



Heretofore the study of cells and epithelial mem- 

 branes has been too much neglected ; and yet there 

 are, in truth, no histological elements of more import- 



