During that week the embryo keeps developing; on one side, a round head, thicker than the midpiece, is formed. On the opposite end, the tail is formed. Its size increases nearly 50%.
By the fourth week, the three embryonary layers will be formed, through the process of gastrulation. From that week, major changes in the development of its internal organs will take place; the main organs will develop from each one of the layers.
The blood vesicles that connect the early umbilical cord to the placenta are formed in the mesoderm. That strucuture will feed and provide oxygen to the embryo.
Composed of cells of the mesocord, the notochord is formed. It is a rod-shaped, flexible body whose main duty is to provide support until the spinal cord is completely formed.
The notochord, moreover, induces the formation of the central neurvous system, by facilitating the formation of neural folds in the ectoderm that will coalesce and become the neural tube.
The ectoderm, therefore, originates the neural tube, that, before the fifth week is over, creates the spinal cord, the head is enlarged, and the brain vesica is formed. The neurulation process is thereby completed.