130 



THE FROG 



CHAP. 



is to be looked 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 ; but commonly, as in the present in- 



53 



FIG. 39. A, transverse section of Frog's ileum ( x 25) ; B, small portion of the 



same highly magnified. (:< about 125.) 



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

 longitudinal layer of muscle-fibres ; m. m. muscularis mucosa? (sec p. 133 ) ; 

 pr. peritoneum ; m. muscular layer; s. m. submucosa. (After Howes.) 



stance, 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, its deeper part being called the submucosa (see 



