Answer the following question given this paragraph: 

A fully developed placenta, like the one in Figure below , is made up of a large mass of blood vessels from both mother and fetus. The maternal and fetal vessels are close together but separated by tiny spaces. This allows the mother’s and fetus’s blood to exchange substances across their capillary walls without the blood actually mixing.


Q: A fully developed placenta is made up of a large mass of what?


A:
blood vessels