22 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



The Cell Types. All of the elementary tissues consist of cells and of 

 their altered equivalents. It will be as well therefore to indicate some of the 

 differences between the cells of the body. They are named in various ways, 

 according to their shape, origin, and functions. 



From their shape, cells are described as spherical or spheroidal, which is 

 the typical shape of the free cell; this may be altered to polyhedral when the 

 pressure on a mass of cells in all directions is nearly the same; of this the 

 primitive segmentation cells afford an example. The discoid form is seen 

 in blood corpuscles, and the scale-like form in superficial cells. Some cells 

 have a jagged outline and are then called prickle cells. Cells of cylindrical , 

 conical, or prismatic form occur in various places in the body. Such cells 

 may taper at one or both ends into fine processes, in the former case being 

 caudate, in the latter fusiform. They may be greatly elongated so as to 

 become fibers. Cells with hair-like processes, or cilia, projecting from their 



Remains of spindle. 



- Lighter substance 

 ' of nuc.eus. 



------ Hilus. 



FIG. 19. Final Stages of Karyokinesis. In the lower figure the changes are still more 

 advanced than in the upper. (Waldeyer.) 



free surfaces, are a special variety. The cilia vary greatly in size, and may 

 even exceed in length the cell itself. Finally, cells may be branched or stellate 

 with long outstanding processes. 



From theirfunction cells are called secreting, protective, sensitive, contractile, 

 and the like. 



From their origin cells are called epiblastic and mesoblastic and hypoblastic 

 (ectodermic, mesodermic, and endodermic). 



Modes of Cell Connection. Cells are connected together to form 

 tissues in various ways. 



They are connected by means of a cementing intercellular substance. 

 This is probably always present as a transparent, colorless, viscid, albu- 

 minous substance, even between the closely apposed cells of epithelium; 

 while in the case of cartilage it forms the main bulk of the tissue, and the cells 

 only appear as embedded in, not as cemented together by, the intercellular 

 substance. This intercellular substance may be either homogeneous or 

 fibrillated. In many cases, e.g., the cornea, it can be shown to contain a 

 number of irregular branched cavities, which communicate with each other ? 



