64 THE FROG 



Exercise 76. — ^The Human Placenta and Associated Membranes. 



(q) Examine a demonstration of the discoidal human placenta 

 and attached remnants of embryonic membranes. This is what is 

 commonly known as the "after-birth." The human placenta is formed 

 by a fusion of a vascularized region of the chorion and the lining of 

 the uterus; there is no continuity between embryonic and maternal 

 blood channels. Remnants of the non-vascularized part of the chorion 

 and of the thin amnion, which are closely applied to one another but 

 not fused, will be found at the edges of the placenta. Note the um- 

 bilical cord which was severed at the time of birth. Embryonic blood 

 vessels pass between the fetus, as the human embryo is called begin- 

 ning with the third month of its development, and the chorionic 

 capillaries in the placenta by way of this cord. Sometimes the vestige 

 of the yolk sac can be found as a small bulb-like mass on a delicate 

 cord near the junction of umbilical cord and placenta. The vestige 

 of the allantois is within the umbilical cord. What are the functions 

 of the placenta and how are they accomplished? 



