Structure that forms along the lining of the uterus as the embryo implants

Blastocyst is a structure that subsequently forms the embryo during the process of fertilization.

Explanation:

In the "innermost layer of the uterine wall" there is endometrium layer, this is where the uterus does the embryo implant. The blastocyst stage is the period of embedding the "lining of the uterus".But the fertilization of egg is done in the "Fallopian tube" and is later implanted in the uterus. Hence blastocyst is the lining structure of the uterus as the implantation of embryo occurs.