66 ANIMAL BIOLOGY. [Part I. 



epithelium) as in the stomach and intestines, and in the tubules 

 and finer ducts of the kidneys, salivary glands, etc. ; sometimes 

 they form a tissue several layers deep (stratified epithelium) as in 

 the mouth, bladder, and epidermis. Sometimes the individual 

 cells are more or less spherical, as in the deeper layers of the 

 epithelium of the mouth, and in the peptic glands of the stomach; 

 sometimes they are flattened and squamous, as in the superficial 

 layer in the human mouth; sometimes polyhedral, as in the 

 external layer of the conjunctiva covering the eye ; sometimes 

 ciliated, as in the lining cells of the frog's mouth, and in 

 the trachea of mammalia ; sometimes columnar, as in the simple 

 epithelium of the intestine. Sometimes the cells become secreting 

 cells, as in the salivary glands or the pancreas ; sometimes horny 

 cells, as in the epidermis, in hair, and in nails; while in the 

 tubules of the testis they become seminal cells, giving rise to 

 spermatozoa. 



Some of these varieties we shall become further acquainted 



FIG. 24. BLOOD, EPITHELIUM, AND CONNECTIVE TISSUE. 



i. a. White blood-corpuscle, i. 6. Red corpuscle (frog), i. c. Red 

 corpuscle (rabbit), ii. Squamous epithelium, human mouth, iii. Ciliated 

 epithelium, mouth of frog. iv. Corneal epithelium, rabbit, v. Columnar 

 epithelium, frog's intestine, vi. Epithelioid cells of peritoneum of frog, 

 vii. White fibrous tissue, viii. Yellow elastic tissue, ix. Branched con- 

 nective tissue corpuscles, x. Corneal corpuscles, xi. Fat cell. xii. Pig- 

 ment cells : a. passive ; &. active. 



with in this chapter. At present we may content ourselves 

 with the following : 



(a.) Scrape gently the inner surface of your lip, and mount in 

 saliva. Flat nucleated squamous cells will be seen (Fig. 24, ii.). 

 They form the superficial layer of the stratified epithelium of 



