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MANUAL OF ELEMENTARY ZOOLOGY 



St. I. 



in a stout rod-shaped or conical body. These compose 

 the layer of "rods and cones" which lines the retina. 

 Glandular epithelium is a kind of simple epithelium in 

 which the cells have become specialised for the manu- 

 facture or secretion of chemical substances. It may occur 

 as single cells scattered among those of ordinary columnar 

 epithelium. This is seen, for instance, in the intestine of 

 the frog, where some of the cells 

 store at their outer ends granules 

 of a substance which, when they 

 discharge it, forms the mucus which 

 gives the lining of the alimentary 

 canal and other spaces the name 

 of mucous membrane. After the 

 discharge of this substance there 

 is left a cup- shaped hollow in 

 the cell, on which account it is 

 called a goblet cell. The hollow 

 is presently filled again by the 

 activity of the protoplasm of the 

 cell. Isolated gland-cells in an 

 epithelium are sometimes known 

 as unicellular glands. Collections 

 of gland-cells form multicellular 

 glands. The simplest kind of 

 these is found in the mucous 

 membrane of the stomach. The 

 Fig. 49. -One of the glands epithelium here dips down into 

 of the frog's stomach. the underlying connective tissue 

 d., Duct;/., the secreting part of as holl ° w tubular processes like 

 the fingers of a glove. The mouths 

 of these tubes are lined with 

 ordinary columnar epithelium 

 which deeper in the tube is succeeded by somewhat lower 

 cells. This region is the duct of the gland. At the end of 

 the tube the cells are large and more nearly cubical and 

 contain in their protoplasm granules of a substance which, 

 when it is discharged, forms the enzyme of the juice 

 secreted by the gland. 1 The granules do not leave a hollow 



1 The granules themselves consist not of the enzyme but of a 

 precursor called the zymogen. 



the gland, known as the 

 fundus or alveolus ; si./., epi- 

 thelium lining the stomach. 



