460 



THE CHANGING GENERATIONS 



The marsupials or pouched mammals. By late Mesozoic time the 

 viviparous mammals had divided into several lines, of which two have 

 survived — the more primitive marsupials and the more advanced placen- 

 tals. In both of these the embryo develops for a time as a parasite within 

 the body of the mother, but this stage is brief in the marsupials and 

 prolonged in the placental mammals. Marsupial young are born in a very 

 immature state — practically as embryos — whereupon they crawl to the 

 nipples of the mammary glands and attach themselves. The nipple swells 

 within the mouth until it can be removed only by force, and the windpipe 

 of the young animal grows up into contact with the nasal chamber so 

 that breathing is possible even while milk is being swallowed. In all but a 



Fig. 28.27. The spiny anteater or echidna. Tachyglossus, a primitive egg-laying Australian 

 mammal, with a model of its egg. (Courtesy American Museum of Natural History.) 



very few marsupials the region of the nipples is covered by a skin 

 pouch, the marsupium, and within this pouch the young complete their 

 development. 



Except for the North American opossum, living marsupials are con- 

 fined to Australia, Tasmania, and South America. The group was prob- 

 ably numerous and widespread in the late Mesozoic, although no fossils 

 except those of opossums have been found in North America and Europe. 

 Regardless of where they originated, the marsupials were evidently 

 present in Australia and South America by Cretaceous times. In South 

 America they were later overrun and largely supplanted by the higher 

 placental mammals; but submergence of the Asiatic-Australian land 

 bridge during the Cretaceous prevented most of the placentals from 

 entering Australia. The marsupials had the entire continent almost to 

 themselves until the coming of man and the animals introduced by him. 



