MESOZOIC ERA 



191 



brains were a considerable improvement over the brain equipment of rep- 

 tiles. 



Mammalian fossils of Cretaceous age are somewhat more abundant and 

 complete than are those of Jurassic age. By the close of the Cretaceous the 

 two main groups of mammals, the marsupials and the placentals, were in 

 existence. They seem to have arisen, probably independently, from one of 

 the groups of mammals living in the Jurassic, the Pantotheria. 



EMBRYO 



WALL OF UTERUS 



CHORION- 



AMNION 

 AMNIOTIC CAvnr 



UMBILICAL CORD 



CAVITY OF 

 UTERUS 



CHORIONIC 

 VILLUS 



PLACENTA 



YOLK SAC 



CERVIX OF UTERUS 



OPENING INTO 

 VAGINA — 



FIG. 9.12. Human embryo, with placenta and embryonic membranes, in posi- 

 tion in the uterus of the mother. (Drawn by Lyman S. Rowell.) 



Marsupial and placental mammals differ in many respects. The names 

 suggest their differences in the reproductive process. The embryos of pla- 

 cental mammals undergo a relatively long period of development within 

 the uterus of the mother, being nourished by the mother through the 

 placenta (Fig. 9.12). As noted previously (p. 59), blood vessels from the 

 embryo pass through the umbilical cord to the wall of the uterus, where 



