28 



COMPARATIVE ANATOMY 



Monotremes possess no teats, and the milk probably passes 

 along the hairs, which in this region are arranged in bunches, and 

 is then licked off by the young animal. The gland is compressed by 

 a strong sphincter muscle. In Echidna, a mammary or marsupial 

 pouch which is primarily paired and becomes unpaired secondarily, is 

 early formed for the reception of the young, and the gland-masses 

 open into two depressions of the ventral integument where the 

 bunches of hair are situated (Fig. 17). These depressions may be 

 called mammary pockets, and are of especial interest as they repre- 

 sent the first stage in the development of the various forms of teats 

 present in all other Mammalia, in many of which distinct indica- 

 tions of the Monotreme 

 condition are met with. 

 The marsupial pouch 

 of the Marsupialia i& 

 probably homologus 

 with that of Echidna. 



Thus a similar 

 mammary pocket is 

 formed in the embryos 

 of Marsupials and pla- 

 cental Mammals by 

 the epidermis extend- 

 ing inwards towards 

 the dermis, and cylin- 

 drical, more or less 

 branched processes a- 

 rising from the base 

 of the pocket thus 

 formed (Fig. 17, B r 

 A). These processes 

 only are the glands 

 proper, the mammary 

 pocket being simply a 

 part of the outer sur- 

 face of the skin which 

 has sunk inwards, and 

 thus it may give rise to 

 hairs and other integu- 

 mentary structures. 



The teats may be- 

 come developed in one 

 of two ways. In 



the first of these, the skin surrounding the pouch becomes 

 raised up to form a circular rampart, and thus gives rise to a teat 

 perforated by a canal, into the base of which the ducts of the 

 gland open (Fig. 17, B, -#). In the second case, the gland sur- 

 face itself becomes elevated into a papilla, while the surrounding 



FIG. 17, B. DIAGRAMMATIC REPRESENTATIONS OK 

 THE EARLY DEVELOPMENT OF THE LEADING 

 TYPES OF MAMMARY GLANDS. (Modified from 

 Gegenbaur. ) 



A, first or undifferentiated (mammary pit) stage; 

 B, stage of the false teat ; (7, stage of the true 

 teat ; v, v, rim (or rampart) of the glandular 

 area ; /, .7, glandular area ; <//, mammary glands ; 

 d, mammary canal. 



