The most critical stage of development for the embryo’s nervous system is the third and fourth weeks of pregnancy. Starting with the uterus, let’s enlarge the area where the embryo has implanted. Here’s the embryo implanted in the uterus on day 14. You can see that it lies within the wall of the uterus, and is covered by a single layer of cells.

Take a moment to get oriented. There’s the yolk sac, which makes blood cells for the embryo, and the amnion, which surrounds and protects the embryo, and the blood vessels that will help form the placenta. Also notice that the embryo is connected to the uterus by a small connecting stalk, which will eventually become the umbilical cord.

If we rotate it to the left we’ll get a better view of the back of the embryo, where we can see the brain and spinal cord develop.

On day 14, the embryo looks like a little disc. The first part of the nervous system that forms is an indentation called the neural groove. Over the next seven days, the groove deepens as the cells around it form ridges called neural folds.

By day 27 we see that the neural folds wrap around the neural groove and form the neural tube. The neural fold will become the spinal cord. Those bundles of cells that look like building blocks are called somites. They form the vertebral column, or backbone. They also help form the ribs, and the muscles of the neck, arms, and legs.

Now let’s watch it again without interruption…

If we rotate the embryo again to the left, we can see the areas that will eventually become the brain and spinal cord. The embryo’s nervous system is particularly vulnerable at this stage of development, so an expectant mother should be careful about avoiding any substances that could potentially harm it.