Yolk Sac in Early Pregnancy: What You Need to Know

Question
I am 9 weeks pregnant. I had an ultrasound last week where my doctor pointed out the yolk sack (it is round and appears next to the fetus). At the time, I assumed this was the early stage of the placenta. Am I right? I am unable to find out in my pregnancy books or the internet. Thank you!

Answer
The yolk sac is not part of the placenta (which forms at around 11 weeks gestation). As the embryo develops, the single-layer ectoderm differentiates into a two layered plate. Endodermal cells appear beneath the ectoderm on the side toward the center of the blastocele. The endoderm forms the roof of the yok sac in the same way that the ectodermal disk forms the floor of the amnionic sac. The rest of the yolk sac wall consists of mesoblast. The yolk sac is never functional and does not grow larger than 1-1.5cm in diameter. The fluid  within the yok sac soon disappears and the sac atrophies during the fourth month of pregnancy. It persists as a barely recognizable fibrous strand within the umbilical cord.