During the fourth week, the embryo increases in size and develops a connecting body stalk from the embryonic body to the placenta. By this time, the heart is developed and beating, pumping blood to all parts of the embryo. The head and jaws are apparent, and the primitive tissue that will form the eyes, mouth, brain, spinal cord, lungs, and digestive tissue has developed. The arm and leg buds are recognizable as small swellings on lateral body walls. The developing brain and spinal cord comes from the neural tube and develops rudimentary segments called somites which will become the vertebral column.
Branchial arches develop about the neck area resembling gills.
The most precarious time of prenatal development is during the embryonic stage of development. For this reason, it is critical that the mother take extremely good care of herself if she believes she is pregnant.