However, if you're asking about the first structures to develop, it would be the blastocyst, which forms around day 5 after fertilization. The blastocyst is a hollow ball of cells with an inner cell mass that will eventually develop into the embryo itself.
Here's a breakdown of the early stages of embryonic development:
1. Zygote: The fertilized egg cell.
2. Cleavage: The zygote undergoes rapid cell division without increasing in size.
3. Morula: A solid ball of cells.
4. Blastocyst: A hollow ball of cells with an inner cell mass.
The blastocyst implants in the uterine wall, and from the inner cell mass, the following structures will start to develop:
* Ectoderm: The outer layer that gives rise to the skin, nervous system, and sensory organs.
* Mesoderm: The middle layer that gives rise to muscles, bones, blood vessels, and the circulatory system.
* Endoderm: The inner layer that gives rise to the lining of the digestive system, lungs, and other internal organs.
These three layers are the foundation for all the organs and tissues of the developing embryo.
