THE INTEGUMENT AND THE EXOSKELETON 55 



outlines of the cells of which it is composed. Examine with the microscope a 

 cross-section through the skin containing growing hairs, and study a hair follicle. 

 Each is a deep pit in the skin and is lined by epidermis. At the bottom of the 

 follicle the dermis forms a small bulb-shaped enlargement, the hair papilla. The 

 stratum germinativum over this papilla is seen proliferating a conical heap of cells 

 from which the hair arises. This cone extends up the follicle and after a short 

 distance a split is seen separating a central shaft, the root of the hair, from the 

 walls of the follicle. The transition from the ordinary epithelial cells of the pro- 

 liferating area to the horny cells of the root of the hair is readily observable. The 

 lining of the follicle is also somewhat cornified, and is known as the outer root 

 sheath. It is the white coat which clings to the roots of hairs when they are 

 pulled from their follicles. From this account it follows that hairs are epidermal 

 structures produced by the activity of the stratum germinativum. Draw a hair 

 follicle, showing its structure. Further details will be found in textbooks of 

 histology. 



2. Scales of mammals. A number of mammals possess horny scales like 

 those of reptiles. These may cover the body, as in the scaly anteater (Manis), fig- 

 ured in P and H, page 485, and N, page 375, but commonly occur on the tails only, 

 as in rats, beavers, and a number of other mammals. Such scaly parts are also 

 provided with scanty hairs. Examine a rat's tail and observe the scales and 

 hairs upon it. 



The armadillos are the only living mammals which possess, like the turtle, an 

 armor composed of both epidermal scutes and dermal plates. Obtain a dried 

 armadillo armor and examine the external surface. It is made up of an anterior 

 shield of small polygonal scales, a middle region composed of nine movable bands 

 with bare areas of skin between them, and a posterior shield similar to the anterior 

 shield. The outer surface of the armor consists of thin horny epidermal scales or 

 scutes, polygonal on the shields, triangular on the bands. The triangular scutes are 

 alternately reversed in position, so that in half of them the apex points anteriorly, 

 and in half posteriorly. The former bear hairs at their posterior margins. How 

 many hairs to each scute ? Draw some scutes and hairs to show their relation. 

 Turn the armor over and study the internal surface. It is composed of bony 

 plates of dermal origin, polygonal on the shields, rectangular on the bands. 

 With a knife point scrape off some of the epidermal scutes; note relative thickness 

 of scutes and plates, differences in the materials of which they are composed one 

 of horn, the other of bone and the impressions left on the plates by the scutes. 



Read W, pages 87-97, on the relation of scales to hair and to the friction ridges. 



I. SUMMARY 



1. The vertebrate skin consists of an outer epidermis, ectodermal in origin and composed of 

 epithelial cells, and an inner dermis, mesodermal in origin and composed of connective tissue. 



2. Exoskeletal structures are produced by the skin, either by the epidermis or the dermis 

 or both. 



