DIGESTIVE TISSUES 



301 



but little differentiated from the epithelium lining the lumen of the proven- 

 triculus. From this axial primary tube many secondary tubes radiate 



FlG. 271. /I , transverse section of gastric gland in stomach of muskrat, Fiber zibethicus. 

 c.c., chief cells; a.c., acid cells; x, cleft between chief cells through which the secre- 

 tion of the acid cells passes into the lumen. B, parts of the walls of two adjoining 

 acini in the proventiculus of the pigeon, Columba. The epithelial cells are acid cells 

 exclusively. 



along its entire length and its fundus. The secondary tubes are structures 

 with spacious lumina. They empty into the axial tube which serves as an 

 excretory crypt or duct for the compound gland. The epithelium of 

 a glandular tube is supported by a membrana propria. It is composed 

 of large acid cells which are not generally crowded, hence are columnar. 

 In regions where a cell is crowded by its fellows it takes the triangular 

 form of mammalian acid cells (Fig. 271, A, ax.). 



Serous tissue. Serous tissue of a gland from the base of the bat's 

 tongue. This is a branching tubular gland. The cells when quite 

 active are greatly distended and columnar or even pyramidal in form. 

 They are rather small, measuring about twenty 

 microns in height and less in width. The cyto- 

 plasm contains numerous small excretion gran- 

 ules which are most numerous and largest at 

 the distal ends of the cell. The nuclei are oval 

 to rounded. They lie near the center of the 

 cytoplasm for the most part in the lower third 

 of the cell. Tubules bearing cells distended 

 with secretion particles have small lumina (Fig. 

 272). When the cells have given off their secre- 

 tion contents, they shrink and thus form a large lumen in the tubule. 



Hepatic tissues. The liver of Cryptobranchus must be considered a 



FIG. 272. Acinus of a serous 

 gland from the base of the 

 tongue in a bat. To show 

 the serous cells. X 700. 



