6 EPITHELIAL TISSUES. 



hcemoglohin — a substance which plays such an important part in 

 respiration. Besides blood and lymph corpuscles we find other 

 isolated cells in the body, the ova and spermatozoa, which 

 become detached as single cells from the epithelial walls of the 

 male and female organs, the testes and ovaries. The form, 

 especially of the spermatozoa, varies greatly : in most cases the 

 spermatozoa have a long thread-like tail attached to the nucleated 

 cell. 



(6) Epithelicd tissues consist of groups of cells, which in 

 simple layers line the exterior and interior of the body surface! 

 The internal lining is known as " endothelium." There are 

 four chief types of epithelium, each distinguished by the form 

 of the cells — namely, (1) cylindrical, (2) ciliated, (3) pavement, 

 and (4) glandular epithelium. 



The lower cells of these cell-masses retain their natural form ; 

 but the upper ones may become hardened. Thick stratified 

 layers of such cells occasionally become fused, and produce 

 horns, nails, claws, hoofs, &c. Sometimes the outer walls of 

 the epithelial cells are thickened, forming a " cuticle." These 

 cuticular membranes are perforated by small pores aiid also by 

 larger passages : in these cuticular pores are placed the hairs 

 and feathers. The cuticular secretions may form a hard shell 

 or case for the organism, an exoskeleton, as seen in the 

 Crustacea and Hexapoda. 



Glandular epithelium is that epithelium in which some cells 

 secrete not a solid but a liquid substance. In the most rudi- 

 mentary cases the gland is formed by a single epithelial cell, 

 the secretion passing out by either a special opening or through 

 the superficial membrane. Several of these cells may arrange 

 themselves around a central space and pour their secretion into 

 it ; the gland then forms a blind invaginated sac opening to the 

 exterior or interior by the neck of the whole glandular mass. 

 From this simple gland a compound gland is built up by re- 

 peated regular or irregular outgrowths. The terminal portion 

 of each gland is converted into a dvict in most glands, for the 



