B. Umbilical vein
C. After eight weeks
B. Two
D. To exchange materials between mother and embryo
C. Seven days after fertilization
B. The finger-like villi
C. 39 weeks
B. A zygote
C. Urea
B. It undergoes mitotic division
C. Mixed blood
B. A temporary disc-shaped organ
C. To enclose the embryo and contain amniotic fluid
C. Three
C. The embedding of the embryo in the uterine lining