From the fifth day after fertilization onwards, the embryo is called blastocyst. At this stage, it is surrounded by a thin membrane and, around it, cells that form the placenta gather together. In the lower left part of the image, you can see the cells of the inner cell mass that will form the baby-to-be.
Embryo development from zygote to blastocyst
By Aitziber Domingo Bilbao BSc, MSc (embryologist), Andrea Rodrigo BSc, MSc (embryologist) and Sandra Fernández BA, MA (fertility counselor).
Last Update: 02/02/2017