REPRODUCTION AND LIFE CYCLES 



427 



into the chorionic cavity. Thus the allantois in reptiles and birds 

 comes to He close to the porous shell, where it is well supplied with 

 blood vessels and so readily becomes a membrane through which 

 oxygen may be secured and the various waste products of metabolism 

 eliminated. 



Placenta 



In all mammals except the egg-laying types and the marsupials, 

 who bring forth their young in an immature stage of development, a 

 new mechanism, the placenta, is evolved to supply the metabolic needs 



placenta 

 yolk $ojt 



allantois .r> 

 chorion... 



<xmn\on.. 



amniotic 



cavity 



fillecCVitlrj 



ocrnniotiC' 



fluid. 



Fallopian tube 



Cavitvof 

 tfie uterus 



mucus plug 



muscular volls oj^lctefifS- 



Diagrammatic sagittal section of human uterus. What devices do you find for 



protection and nourishment .^ 



of the embryo. Other important changes are associated with the 

 formation of this structure. In the first place the developing embryo 

 reaches the uterus and becomes implanted in the uterine mucosa at 

 about the time of gastrulation. The amnion is formed and serves the 

 same protective function as in the lower types, while the chorion is 

 intimately associated with the maternal tissue lining the uterus and 

 so becomes concerned with respiration, excretion, and nutrition. 

 Blood vessels invade this modified chorion, extending from it down the 



