INTKOUMKNT. -27 



of the hair-sacs, the glands of the prepuce, the perineal glands 

 present in many Mammals, the glandular dorsal grooves of the 

 neck of the Chamois, the Meibomian glands of the eyelids, and 

 many others. 



Epidermic structures play a very important part in Mammals : such 

 are claws, nails, hairs, bristles, and spines (Hedgehog, Porcupine) ; the so-called 

 whale-bone (baleen) of the Mystaceti, the nasal horns of the Rhinoceros, the 

 scales of Manis, and the palatal plates of Sirenia, belong to the same category. 



MAMMARY GLANDS. The mammary glands, which stand in 

 such a close relation to reproduction, are entirely confined to the 

 Mammalia, which owe their name to the possession of these organs. 

 Their phylogenetic relations are by no means clear; they must, 

 however, be considered as modified integumentary glands 

 (sebaceous glands). 



The so-called mammary pouch of Echidna 1 may be taken as a 

 point of origin of the different forms of teats. It consists of a 

 pocket-like in-sinking of the skin of the abdomen, which is 

 possibly only formed periodically ; in it the eggs or unripe young 

 appear to be protected. How the latter, in the absence of true 

 teats, take in the milk is not at present known. 



A B 



FIG. 16. A, TRUE (SECONDARY) TEAT ; AND B, PSEUDO- (PRIMARY) TEAT. 



This pouched condition repeats itself ontogenetically in every 

 Mammal by the epidermis extending inwards towards the derma, 

 and cylindrical more or less branched processes arising from the 

 base of the pouch thus formed. These processes only are the 

 proper glands, the mammary pouch being simply a part of the 

 outer surface of the skin which has sunk inwards, and thus it may 

 give rise to hairs and other integumentary structures. 



The teats may become developed in one of two ways. In 

 the first of these, the skin surrounding the pouch becomes 

 raised up, and so forms a teat perforated by a canal, into the 

 base of which the proper ducts of the gland open (Fig. 16, B). 

 In the second case, the gland surface itself becomes elevated into 

 a papilla, while the surrounding skin remains almost on a level 

 with the rest of the integument (Fig. 16, A). In the latter 



1 Whether the absence of a pouch in Oinith n rhynchus is the more primitive or 

 secondary condition is not certain : possibly one is formed periodically during "heat." 



