138 THE INTEGUMENT 



Two types of alveolar jijlands are differentiated. A smaller 

 mucous secreting gland occurs generally in the connective tissue 

 below the epithelium. Another and larger type, called the poison 

 gland, extends more deeply into the corium and is surrounded by 

 smooth muscle cells. It is composed of cells filled with acidophilic 

 granules; the major portion of these cells disintegrate, liberating the 

 secretion into the lumen and leaving the nuclei scattered along 

 connective-tissue sheath. The function of these glands is protective, 

 their secretions are poisonous and presumably discourage many 

 animals from eating frogs. To the mucous glands are attributed 

 the function of keeping the skin moist. 



The skin of Xecturus has a stratified cuboidal epithelium forming 

 the epidermis and a network of chromatophores in the connective 

 tissue just below. Among the epithelial cells are scattered spherical 

 cells filled with acidophilic secretion. As in the frog, there are two 

 types of glands. One is a smaller mucous gland, while the other 

 is a larger serous type, in which the contents of the cells disin- 

 tegrate and fill the lumen with secretion, leaving the nuclei scattered 

 aroimd the periphery. 



INTEGUMENT OF REPTILES. 



With reptiles the land habitat is assmned, and a dry skin makes 

 its appearance. The epidermis of the reptile is relatively thin 

 and the superficial region in contact with the air is composed of 

 dead, cornified squamous cells. Folds of the cornified epidermis 

 alone form the scales of reptiles. Some reptiles shed their skin 

 periodically, the horny superficial epidermis (the corneum layer) 

 separates from the softer subjacent cell layers so that this dead 

 upper covering of the epidermis is shed in one piece. Glands are 

 restricted to regions about the mouth and anus. 



The corium in reptiles is relatively thick and is composed of dense 

 connective tissue, with numerous chromatophores located imme- 

 diately below the epidermal covering. When bony ])lates are 

 formed they are derived from the corium in a manner similar to 

 that described in intramembranous bone formation. In most 

 turtles, both scales and bony plates are formed. In turtles and 

 alligators, the tissue of each plate is not continuous with the l)()ny 

 tissue of adjacent ])lates, so that growth in size of the animal is 

 accompanied by continuous peripheral additions to the indi\i(hial 

 plates. When claws are present, they are formed by accunuilations 

 of the corneal layer of the epidermis. 



