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TEXTBOOK OF ZOOLOGY 



fuse in some forms) to form this saclike organ, the placenta. The 

 chorion is richly supplied by blood from the fetus while the uterine 

 layer is similarly supplied by maternal blood. Although the em- 

 bryonic and maternal parts of the placenta may be almost interwoven 

 in mammals like the human, there is no actual circulation of maternal 

 blood through the vessels of the fetus. The exchange of materials 

 (nutrition, respiration, excretion) is by diffusion through the tissues 

 here in this intimate relation. As in the case of circulatory connec- 



Yofk Sac 



Muscu/ar Layers 

 of Uterus // 



Basal Plate- 

 of Placenta 



Umbilical Cord- 



Amnion 



Chorion & Decidua- 

 Capsularis 



Fig. 425. — Sectional view of a fetus in normal position in the uterus, showing 

 also the intimate relations of embryonic membranes and uterine wall. The chorion 

 is the outer embryonic membrane. (Modified after Ahlfeld. ) 



tions, there are no nerves passing from parent to fetus. The stalklike 

 extension from the abdomen of the fetus to the placenta is the um- 

 hilical cord. It carries the umbilical arteries and vein, extending from 

 the chorion to the fetus; the allantoic stalk; and, a vestigial yolk 

 stalk from the intestine. The Tiavcl of the adult is the scar where the 

 body wall has closed in about the umbilical cord at the point of its 

 severance at birth. The fetus is suspended within the amniotic cavity 

 which is filled with the watery, lymphlike amniotic fluid. 



