If you were to see your little one this week, you might think you were growing a tadpole, not a baby, in there! Don't worry though. It's normal for your embryo to look more like a fish than a human this week. In fact, where before your embryo was a mass of cells, that tadpole form is your little one's first actual shape. And speaking of little, by now, the embryo is about the size of a sesame seed. But don't let a small stature fool you! Your embryo has officially developed three layers. The outermost, the ectoderm, is forming the beginnings of a neural tube, which will become your baby's brain, spinal cord, nerves, and backbone. The middle layer, or mesoderm, is busy developing a heart and circulatory system. That little heart will divide into chambers and even begin to beat and pump blood this week. Meanwhile, your embryo's inner layer, called the endoderm, will eventually house organs like lungs and intestines. In addition, your baby's placenta will also begin to develop now. And it is through the placenta, and fingerlike structures called villi, that your embryo will receive nourishment during your pregnancy. For more on your little one's development, check back in your sixth week!