What Are Different Levels of Embryonic Development?
Image Citation: Oza, Raman (artist). (2016). Fetus Placenta Umbilical Cord [digital art]. Retrieved from
We know how a sperm cell fertilizes an egg cell, and we know that the fertilized embryo eventually becomes a baby nine months later, but what happens in between? How do we get from a single cell the size of a pinhead to an infant?
There are three main levels of embryonic development. Those levels are called the germinal stage, the embryonic stage, and the fetal stage.
The germinal stage begins immediately after conception (or the meeting of the sperm and the egg) and lasts for around two weeks. After conception, the fertilized egg is referred to as a “zygote”. The single-celled zygote divides rapidly to form a ball of cells called a “blastula”.
About a week after conception the blastula implants in the wall of the uterus. This allows it to connect to the placenta, which will facilitate the exchange of oxygen, nutrients, and waste between the mother and the blastula.
The embryonic stage lasts from two weeks to about two months after conception. During this phase, the developing ball of cells is referred to as an “embryo”. The focus of the embryonic stage is for the embryo to continue growing and to form major organs like the heart and lungs.
From two months after conception until birth, the embryo enters the fetal stage. In this stage, the embryo is now referred to as a “fetus”. Everything needed to make an inch-long embryo into a full human baby occurs during this stage.
Major organs continue developing and fully functional organ systems appear. Bones and muscles form, giving the fetus the ability to move inside the uterus. The brain doesn’t grow to its full size until the last couple months of development to prevent our abnormally large human brains from taking up too much space.
At birth, the baby is separated from the placenta from which it derived oxygen and nutrition for nine months. By then it has developed all the necessary organs, a full human brain, and a respiratory and digestive system that can breathe and consume food apart from the mother.