492 



ZOOLOGY 



SECT. 



part ol this marsupium, while in the shallower anterior part lie 

 the mammary pouches. In Ornithorhynchus mammary pouches 

 are indicated only by extremely shallow depressions, and no 

 marsupium is developed. 



In the higher Mammals, when the mammary glands are first, 

 developed (Fig. 1104), a depression (mammary pouch} is formed, 

 from the floor of which branching cylindrical strands of epidermis 

 grow inwards to give rise to the glands. At a later stage there 

 is developed around the opening or openings of the mammary 

 ducts a prominence, the teat (Fig. 1104), the wall of which may be 

 formed of the mammary-pouch area alone (Marsupials, Rodents, 

 Primates), or, with greater or less reduction of the latter, mainly 

 from the surrounding integument. In the latter case, the teat 



\ C. 



FIG. 1104. Diagrams of the phylogenetic development of teats, a, primitive condition corre- 

 sponding to the condition in Echidna ; l>, Wallaby (Halmaturus) before lactation ; c, Opossum 

 (Didelphys) before lactation ; d, Opossum during lactation ; this diagram stands also for 

 Rodents and Man ; e, embryonal, and /, full-grown cow. 1, integumentary wall ; 2, mam- 

 mary area, the broken line represents the mammary pouch ; 3, milk-ducts. (After Max 

 Weber.) 



may have a wide central canal. The number and situation of the 

 teats varies in the different groups, and has been noticed in the 

 synopsis of the characters of the orders and sub-orders (pp. 478 

 to 487). 



The two genera of the Prototheria, Ornithorhynchus and Echidna, 

 differ somewhat widely from one another in general appearance. 

 The former (Fig. 1105) has the surface covered with a close, soft 

 fur, and has the upper jaw produced into a depressed muzzle, 

 not unlike the beak of a duck, covered with a smooth, hairless 

 integument, which forms a free fold or flap at the base. The 

 eyes are very small, and there is no auditory pinna. The legs 

 are short, and the five digits end in strong claws, and are con- 

 nected together by a web, so that the limbs are equally adapted 

 for burrowing and for swimming. The tail is elongated and 



