When an embryo develops, what controls when certain organs start to develop?
The majority of development is controled via reactions to different hormones and other cells. For example, cells on the outside of a zygote will develop into the placenta due to hormones in the uterus/fallopian tubes. Cells on the inside will develop into embryotic cells. As the spinal column develops first, I’d hazard a guess that the structures that develop after this react based on their position in relation to the developing spine. Zygote cells naturally divide at a regular pace but do not cause the zygote to grow in size – the egg cell is generally the largest cell in the body and is divided to make the smaller cells.
