6 EPITHELIAL TISSUES. 



in some fluid medium. The corpuscles of the blood, chyle, and 

 lymph are excellent examples of free cells. In the invertebrate 

 blood, which is normally colourless, will be found numbers of 

 pale amoeboid bodies. In vertebrate blood these amoeboid cor- 

 puscles are augmented with red blood corpuscles (fig. 1), round 

 cell-discs which contain the colouring matter of blood — namely, 

 hcemoglobin — a substance which, as we shall see later, 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 



t..(»^ f/ #1 & ^ 



Fig. 1. — Blood Corpuscles of Vertebrata. 

 a, Of man ; b, of goose ; c, of crocodile ; d, of frog ; e, of skate. (Nicholson.) 



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) Epithelial 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-m!asses 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 and also by 

 larger passages : in these cuticular pores are placed the hairs 

 and feathers. The cuticular secretions may form a hard shell 



