Reproduction in Multicellular Animals - 391 



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Fig. 21-13. The duckbill, one of the few surviving species of monotremes, 

 which do not develop any placenta. This primitive egg-laying mammal has a 

 birdlike bill and webbed feet. (Courtesy of the American Museum of Natural 

 History, New York.) 



jority of present-day mammals are viviparous. 

 In such mammals the developing offspring 

 are retained within the uterus of the female 

 until the end of the embryonic period, and 

 the embryos are nourished during the devel- 

 opmental period by food substances derived 

 from the maternal blood stream. Essentially 

 the uterus *s a specialized portion of the ovi- 

 ducts (Fig. 21-14), which in some mammals, 

 like the rabbit, remains duplex, but in others, 

 like man, becomes simplex. 



The efficiency of the uterus as an abode 

 for the developing embryo hinges upon the 

 development of another structure, the pla- 



centa. This important nutritive organ is de- 

 rived partly from maternal tissues that grow 

 around the embryo when it becomes im- 

 planted in the uterine wall, and partly from 

 certain of the tissues of the embryonic mem- 

 branes. 



In higher mammals, the chorion and allan- 

 tois arise in much the same fashion as in rep- 

 tiles and in birds; but in mammals the 

 chorion comes into contact with the tissues 

 of the uterine wall (Fig. 21-1 ID), rather than 

 with the inner surface of the egg shell. More- 

 over, the chorion sends out a large number 

 of fingerlike outgrowths, the chorionic villi, 



UTERUS 



UTERUS 



UTERUS 



DUPLEX TYPE 

 (RABBIT) 



B1C0RNUATE TYPE 

 (CAT) 



SIMPLEX TYPE 



(MAN) 



Fig 21-14. Types of uteri in various mammals. Note that the uterus develops 

 as a specialized part of the oviduct. 



