TttE Fkoci 



cHaP. 



epiderm. The whole gland with its duct 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 



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

 highly magnified. 

 h. V. blood-vessel ; c. m. circular layer of muscle ; ep. epithelium ; I. ;«. longi- 

 tudinal layer of muscle-fibres ; pr. peritoneum ; Jn. muscular layer ; j. m, sub- 

 mucosa. (After Howes.) 



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

 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 



