IJd 



THE FRO(; 



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. ^Ve 

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



Fkj. 39. — A, traiis\erse section of frog's inlcsliiic ; 1!, small portion of the same 

 highly niagnifieij. 

 b.v. bIood-\essel ; c. lu. circular layer of muscle; t-/, epithelium; /. ///. longi- 

 tudinal layer of muscle-fibres ; fir. peritoneum ; ///. muscular layer ; j". 7«. sub- 

 mucosa. (After Howes.) 



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

 but commonl)', as in the present instance, gland-cells are 

 aggregated into a definite organ called ^ gland. 



The Intestine.— A trans\erse section of the intestine 

 shows also a very definite and characteristic combination of 

 simple tissues. 'I'hc mucous membrane, like the skin, is 

 composed of two layers, an epithelial la)er (Fig. 39, </), 

 corres[)0nding to the epiderm, and a connective tissue layer 



