Your baby-to-be is now about the size of a baby pea and has millions of cells, all of them busy replicating and creating more and more complex structures. With the placenta still forming, the embryo depends on the yolk sac for nourishment. Inside the yolk sac, tiny structures called “blood islands” form. These will become the first blood cells and the first blood vessels. A two-chambered heart, not much more than a tube twisted back on itself, has formed. Amazingly, it starts to beat about 21 days after fertilization—the first organ in the embryo to begin functioning. Cells enclose the neural tube, suggesting the beginning of a spinal column. The facial features start to form, and the cells that will become your baby’s eyes and ears appear.