130 



THE FROG 



upon as a depression of the skin, lined by pushed-in epiderm 

 cells. 



Epithelial cells having the power of manufacturing and 

 discharging a specific substance are called gland-cells, and 

 the process of manufacture is known as secretion. We 

 have already met with isolated gland-cells in the case of the 

 goblet-cells of the intestine (p. 109), which secrete mucus; 



FIG. 39. A, transverse section of frog's intestine ; B, small portion of the same 



highly magnified. 



b. v. blood-vessel ; c. in. circular layer of muscle-fibres ; ep. epithelium ; /. in 

 longitudinal layer of muscle-fibres ; pr. peritoneum ; m. muscular layer ; s. m. 

 submucosa. (After Howes.) 



but commonly, as in the present instance, gland-cells are 

 aggregated into a definite organ called a gland. 



The Intestine. A transverse section of the intestine 

 shows also a very definite and characteristic combination of 

 simple tissues. The mucous membrane, like the skin, is 

 composed of two layers, an epithelial layer (Fig. 39, ep\ 

 corresponding to the epiderm, and a connective-tissue layer 

 (s.m\ corresponding to the derm and called the submucosa. 

 The epithelium consists of a single layer of cells only (B, ep\ 

 all columnar, and with their long axes at right angles to the 



