132 



COMPARATIVE ANATOMY 



among the mesodermal epithelia. The endothelium of blood vessels is 

 one of the thinnest of layers. In studying sections of a tissue it is often 

 difficult to ascertain whether or not a certain space in the tissue is lined by 

 endothelium. The peritoneal epithelium of a frog tadpole is of utmost 

 thinness. 



In such organs as the digestive tube and urinary bladder the Hning 

 epithelium is stretched as the wall of the organ becomes distended and the 

 form of the individual epithelial cell varies accordingly. A cell which is 

 tall-columnar when the organ is contracted may be cuboidal or even flat 

 when the wall is stretched. 



■^nt'ttfflaa&Muw 



'^*fl**"ini-""iiiMttfoi.i. w 



n 







Fig. 93. — Columnar ciliated epithelium from human trachea. Most of the cells 

 are slender, with axes more or less curved, and extend from the basement membrane 

 to the free surface of the epithelium. Occasional short cells, basal cells, lie at or near 

 the basement membrane and do not extend to the free surface. Several swollen mucous 

 cells ("goblet" cells) are shown. (From Bremer, Text-book of Histology.) 



Stratified Epithelium. On Amphioxus, a slender marine animal only 

 four or five centimeters long, an epidermis one cell thick affords adequate 

 protection. On an elephant it would not. In such large arthropods as 

 crabs and lobsters the epidermal epithehum is simple but it attains con- 

 siderable thickness by virtue of the fact that its columnar cells are exceed- 

 ingly tall. These cells secrete the characteristic "shell" consisting of 

 organic chitinous material hardened by deposit of calcium salts. As the 

 animal grows, the shell is periodically cast and, until the new shell becomes 

 hardened, the animal is in a somewhat precarious " soft-shelled " state. 

 While this arrangement serves the crab passably well, it would have obvious 

 disadvantages for, again, the elephant. Certain surfaces of large heavy 

 animals are exposed to excessive mechanical friction and impact. The 

 inevitable loss of material at the surface is best compensated for by a 



