50 JOURNAL OF THE [April, 



selves in mantle form around this mass in a kind of pocket, des- 

 tined later to become the outer hair sheath. This proliferation 

 of Malpighian cells now assumes a bottle-shaped form, and a dif- 

 ferentiation of the constituent cells into central and peripheral 

 portions takes place (Fig. 4, e, F, K). The central part consists 

 of elongated cells and grows rapidly outward to form the hair 

 shaft ; the peripheral layer now becomes a sheath. The base of 

 the shaft will now be observed to have assumed a knob-like form; 

 an infolding of its base occurs to contain the nutrient blood ves- 

 sel (Fig. 4, <?, /, K, B, P). Sebaceous glands for oiling the hair 

 are also produced by proliferations of the Malpighian cells (Fig. 

 4, e,f, G). When a shedding of hair takes place a new papilla 

 may be formed at the base of the old one. 



Hair plays a very important part in the life history of the Mam- 

 malia. It is their most distinguishing characteristic, and here, as 

 much as in any animal type, we can see the great protective value 

 of integumentary derivations, the very nature of the hair mass 

 retaining so large a quantity of the heat of the body. 



When pigment is present in Mammals, it occurs in the cells of 

 the Malpighian layer, showing a marked difference from lower 

 forms where it is found principally in the derma. 



Epidermic structures are worthy of note in this group. They 

 include many and varied forms : the baleen plates of whales, the 

 spines of the hedgehog and porcupine, the nasal horns of the 

 rhinoceros, the claws of cats and dogs, the bristles of the hog, and 

 many others all belong in this category. 



The principal glands are sweat and sebaceous glands. These 

 make up two general classes, the former the simpler, the latter 

 more highly developed histologically. The mammary glands, 

 which characterize this group, can be also looked upon as modi- 

 fied sebaceous glands. 



IV. Conclusion. 



The five great types of back-boned animals having been ex- 

 amified, it remains for us to consider briefly a few of the relations 

 of the integumentary structures to general organic evolution and 

 to each other. 



In the first place, as we have seen, they are divided into two 

 classes : those originating in the derma, in which the epidermis 



