HISTOLOGY OF THE FROG 87 



secrete a ferment or enzyme, the granules which are the 

 precursor of that ferment, are called zymogen granules. 

 When secretion takes place the granules are discharged from 

 the cells into the lumen of the gland, become converted into 

 ferment, and this is poured into the cavity of the gut. Amongst 

 the gland-cells of the frog's stomach mucus-secreting chalice 

 or goblet cells are found. In the pyloric end of the frog's 

 stomach the cubical epithelium is continued nearly down to 

 the blind end of the gland, its terminal portion only being 

 occupied by a few large gland-cells filling up its cavity. 



In the mammalian stomach the simple tubular glands are 

 slightly complicated by the branching of their extremities, so 

 that two or more tubes open into a single short neck or duct, 

 and in the cardiac end of the stomach the glands have two 

 kinds of cells in the walls of their deeper portions, chief cells 

 in the form of normal gland-cells, and parietal cells, which are 

 ovoid, filled with darkish granules, and lie between and behind 

 the chief cells, between them and their basement membrane. 

 The chief cells secrete the digestive fluid pepsin, the parietal 

 cells secrete the acid (hydrochloric) found in the gastric juice, 

 and hence are also known as oxyntic cells. 



Such glands as the pancreas, or the salivary glands of 

 mammals, are of the kind known as racemose. They may 

 easily be derived from the branched tubular condition of the 

 gastric glands, by supposing the branches of the latter to divide 

 and sub-divide, the proximal portions of the duct and branches 

 being lined by simple non-glandular cubical epithelium whilst 

 the distal end are dilated to form the so-called acini lined by 

 glandular cells. The whole structure is called racemose from 

 its resemblance to a bunch of grapes, the stalk and its divisions 

 being the ducts, the grapes the secreting portions or acini. 

 * jln the case of the liver the bile-ducts are lined with a 

 columnar epithelium which changes as the ducts break up 

 into finer and finer ramifications, the columnar cells passing 

 insensibly into the hepatic cells, whilst the cavities of the 

 ductules become no more than irregular spaces running 

 between the liver-cells, and are known as bile-capillaries. 

 The hepatic cells form the bulk of the liver ; they are large, 

 polygonal in shape, and possessed each of a single nucleus. 

 These liver-cells, especially if taken from an animal killed not 

 long after a meal, may be seen to contain numerous granules 



