Many people have mistaken ideas about how a growing embryo eats and breathes in the uterus.

From the earliest stages of its development, the growing embryo requires nutrition and oxygen, and a disposal system for the waste products of its own metabolism. All of this is accomplished by the placenta, which allows the growing embryo to eat and breathe while in the mother's uterus.

To get some perspective on how the placenta began, let's go back to Day 8. This hollow ball of cells moving through the uterus is the blastocyst, searching for an implantation site. Here you see its outer layer beginning to extend out and implant in the uterine lining, searching for the uterine blood vessels that would nourish it throughout the pregnancy.

As it went deeper, a single layer of cells from the mother's uterine lining surrounded it, so that it would be protected from harm. On Day 9, as it grew larger and more complex, the blastocyst became an embryo. Here it's about the size of a pinhead.

Also on Day 9, the outer layer of the embryo developed spaces called lacunae. The lacunae filled up with blood from the mother's uterine lining.

On Day 13, small projections from the embryo's chorionic layer reached out into the uterine lining. The chorionic layer is one of the membranes that surround the embryo and help it implant.

On Days 15 through 21, blood vessels began to form beneath this chorionic layer.

Around Day 21, the embryo's blood stream and the mother's blood stream were in such close contact that nutrients and oxygen could cross from mother to embryo. This was how the embryo first got its food and air from the mother, and technically this is when the placenta began to function.

Let's magnify this area so you can see what we're talking about. Here you see a vein and an artery from the embryo in close contact with the blood in the mother's uterine lining. Inside the blood vessels, you can also see red blood cells, which carry oxygen.

The two blood streams are separated by a thin collection of tissues in the placenta called the blood barrier. This barrier permits small particles like nutrients and oxygen to pass from the mother to the embryo, and allows waste products to pass from the embryo back to the mother. The blood barrier also prevents many large or potentially harmful particles from entering the embryo's blood stream. Notice that the red blood cells do not cross from the mother's blood stream to the embryo's.

You may be wondering how a mother's blood cells could be harmful to her growing baby, and why it's important to keep the two blood streams separate. If the mother's blood type is RH negative, and her embryo's blood type is RH positive, then the mother's antibodies would treat the embryo as an invading foreign organism, and try to destroy it.

Now you can see why the placenta and its blood barrier are important for supplying the growing embryo with nutrition and oxygen, removing its waste products, and preventing harmful substances from getting into its blood stream.