IN THE MONOTEEMATA 3 



seriatim the development of the mammary 

 apparatus in the three principal groups of 

 the Mammalia, the Monotremata, the Didelphia 

 (Marsupialia), and the Placentalia (Mono- 

 delphia), and in so doing I shall always begin 

 by recalling in a few words the typical 

 characteristics of the mammary organs in the 

 adults of these three groups. 



The Monotremes comprise, as is well known, 

 only the genera Echidna and Proechidna, both 

 terrestrial, as well as the Ornithorhynchus 

 anatinus, which is excellently adapted for an 

 aquatic life. They are forms which in many 

 of their characters recall the reptiles, and 

 which also resemble them in being oviparous. 

 They have on either side of the abdomen 

 a well-developed mammary gland, consist- 

 ing of about 100-150 separate gland tubes, 

 attaining an average length of 25 milli- 

 metres ; their secretion serves as nourishment 

 for the young. The individual tubes of the 

 mammary glands are very much like the sweat 

 glands in structure, so much so that Gegen- 

 baur derived them from these glands. The 

 ducts terminate close together on an area, 

 the so-called gland area, which in the 



