4 MAMMAEY APPARATUS OP THE MAMMALIA 



case of the adult Echidna would measure 

 about 6 millimetres in length and 1| milli- 

 metres in breadth. Each duct opens at the 

 base of a long hair, the mammary hair, as I 

 may call it for brevity. Nipples, such as 

 all the other Mammalia show, are absent. 

 As a rule the gland area is nearly on a level 

 with the skin, it may even be sunk a little, 

 or, on the contrarj^, a little raised. 



About the time of pregnancy, the female 

 of the Echidna develops a so-called incuba- 

 torium on the median line of the abdomen in 

 the form of a little pouch, which has a diam- 

 eter of about 4 centimetres (Fig. 1, where, 

 however, it is represented much too large). 

 In this pouch, which includes both gland 

 areas, first the egg and then the young are 

 carried for some time by the mother. When 

 the lactation period is over, it disappears com- 

 pletely. 



In the case of Ornithorhynchus (Fig. 2), it 

 is only natural in view of the aquatic habits of 

 this creature that the incubatorium should be 

 absent, and this absence, it is important to 

 note, is accompanied by a difEerent develop- 

 ment of the skin muscle (panniculus carnosus) 



