THE MAMMALS 



431 



will be used by the embryos until they can nurse. There are a few 

 species in which rudimentary placentae are developed, but these ac- 

 complish little in the way of nourishment. 



The opossum is one of the best known of the marsupials. It is very 

 abundant in our southern states and furnishes the main dish at many 

 a rural dinner of "possum and taters." The opossum eggs are retained 



Courtesy New York Zoological Society 



Fig. 28.5. A female kangaroo with a baby in its pouch. Like other marsupials a 

 large part of the embryonic development takes place in the pouch of the mother. How- 

 ever, in the kangaroo the young are carried in the pouch at times after they have 



completed their embryonic development . 



in the uterus until they hatch, which is only thirteen days after fertiliza- 

 tion, so the young are very tiny and in an early embryological stage of 

 development. They will be no more than half an inch long, but have 

 sharp little claws by means of which they climb up the mother's body 

 and enter the pouch on the ventral surface of her body. There they find 

 the nipples and each little opossum grabs a nipple which he sucks down 

 his throat and his mouth actually grows onto the nipple. When they 

 have completed their embryonic development they leave the pouch, but 

 clamber back in to nurse or when there is danger. 



The kangaroo and the cuddly little koala "bear" of Australia are 

 other well-known marsupials. 



