CELLS. 35 



zndfy, a small soft semi-solid mass of matter, with no 

 definite boundary- wall, but with, most often, a small 

 granular substance in the centre, called, as in the first 

 case, a nucleus. In both cases, the nucleus may contain 

 a nucleolus. Fat-cells (fig. n) are examples of the first 

 kind of cells ; white blood-corpuscles (fig. 26) of the 

 second. 



The cell-wall, when there is one, never presents any 

 appearance of structure : it appears sometimes, as in blood- 

 cells, to be a protein substance ; sometimes a horny matter, 

 as in thick and dried cuticle. In almost all cases (the dry 

 cells of horny tissue, perhaps, alone excepted) the cell- 

 wall is made transparent by acetic acid, which also pene- 

 trates into the interior and distends it, so that it can hardly 

 be discerned. But in such cases the cell-wall is usually 

 not dissolved ; it may be brought into view again by 

 merely neutralizing the acid with soda or potash. 



The simplest shape of cells, and that which is probably 

 the normal shape of the primary cell, is oval or spheroidal, 

 as in cartilage-cells and lymph-corpuscles ; but in many in- 

 stances they are flattened and discoid, as in the red blood- 

 corpuscles (fig. 26) or scale-like, as in the epidermis and 

 tesselated epithelium (fig. 2). By mutual pressure they 

 may become many-sided, as are most of the pigment cells 

 of the choroidal pigmentum nigrum (fig. 12), and those 

 in close-textured adipose tissue ; they may assume a conical 

 or cylindriform or prismatic shape, as in the varieties of 

 cylinder- epithelium (fig. 4) ; or be caudate, as in certain 

 bodies in the spleen ; they may send out exceedingly fine 

 processes in the form of vibratile cilia (fig. 6), or larger 

 processes, with which they become stellate, or variously 

 caudate, as in some of the ramified pigment-cells of the 

 choroid coat of the eye (fig. 13). 



The contents of all living cells, including the nucleus, are 

 formed in a greater or less degree of protoplasm, less as 

 the cell grows older. But, besides, cells contain matters 



