embryo 



J ■'umbilicus 



yolk sac 



Figure 7-19. Amnion and placental development of the pig. A, 1 6 to 1 7-somite embryo and extra- 

 embryonic membranes seen in sagittal section; B, 4 to 5-week foetus in its membranes; C, diagram- 

 matic detail of placental relationship to maternal tissues. (After Nelsen, 1953) 



chorionic folds 



areolae of villi (fit into secretion-filled 

 pockets of uterine wall) 



,™-. ^.„ cotyledon or areola of branching > 



5^ -^J-^. ',t.'J^ 



primary placenta 

 x^'f^r-A^r^ with compound villi 



compound villi 



secondary placenta 



Figure 7-20. Placental types observed in mammals. A, diffuse with chorionic folds as scattered 

 areolae of simple villi (pig); B, cotyledonory with scattered large areolae or cotyledons of branch- 

 ing villi (cow); C, discoidal with a single large oreolus of compound villi (human, bear); D, double 

 discoidol (monkey, gorilla); E, zonory formed by o belt of compound villi around the middle of the 

 elongate foetal sac (dog, cat, seal); F, incomplete zonary (raccoon). 



PLACENTA 



213 



