As the zygote travels through the fallopian tube it undergoes further early development, dividing into a bundle of cells, and is called a blastocyst at this stage. After the blastocyst implants in the uterus, the embryonic period begins and it is known as an embryo. It continues cell division, cell differentiation begins, major systems begin to develop. The embryo turns into a fetus by week 9 of pregnancy. Initially, the embryo at this stage is comprised of a mass of dividing cells (around 150 cells) that start to separate into three distinct layers.
Let’s briefly summarize what each layer transforms into in the coming months:
This is the internal layer, also known as the endoderm (endoblast). It forms the future respiratory tract and the digestive system. Glands such as the thymus, thyroid, liver, and pancreas are also formed.
This is the middle layer. It is also known as the mesoderm. The baby’s future excretory system, circulatory system, the bones, cartilage, inner skin layer, muscles, and genitalia are formed in this layer
The last layer is the outer layer. It is known as ectoderm (ectoblast). This layer helps in forming the baby’s skin, nails, and hair. Also, the baby’s nervous system and the brain is formed by this layer.
While all these changes are happening inside the mothers’ body. The embryo simply floats within the uterus, protected by the secretions of the uterus lining. The baby is still very, very tiny — only .1 to .2 mm long.