44 



AN INTRODUCTION TO ZOOLOGY 



interior droplets of a substance which is afterwards passed outside, 

 that is, they are constantly secreting, they constitute a glandular 

 epithelium. Since they are only in a single layer they also form a 

 simple epithelium. The secretion, a slimy fluid, is stored upon the 

 alveolus and passed to the surface through the duct, and by this 

 means the skin of the frog is kept moist. The glandular cells are 



h.i. 



d.l 



FIG. 14. A section of the skin of a frog, taken vertically to the surface, 

 highly magnified. From Borradaile. 



b.v., small blood-vessels ; cap., capillaries ; d.l., dense layer of connective tissue, consisting 

 of fibres which lie parallel to the surface ; der., dcrmis or corium ; ef>., epidermis ; gl'., gl"., gl'"., 

 glands of three kinds ; gl'. and gl". secrete a slimy mucus and pass it to the surface of the skin 

 by ducts which are not shown in the section ; gl"'. secretes a more watery secretion which 

 probably contains a substance of unpleasant taste ; all three kinds are simple glands of the 

 saccular type ; h.l., horny layer of the epidermis ; M.I., lowest row of the Malpighian layer of 

 the epidermis ; pig., pigment cells ; v.f., strands of vertical fibres in the connective tissue. 



continuous with those of the duct, but the latter are flattened and 

 squamous instead of being almost cubical like the former. The whole 

 is probably to be regarded as a specialised part of the epidermis 

 that has sunk below the general level, in order to carry out its function 

 more efficiently. Gland cells are found in many parts of an animal, 

 . and frequently close together, so as to form a glandular epithelium. 

 When this is brought together into a definite organ we speak of it 



