Final answer:
The blastocyst, a ball of embryonic cells with an inner cell mass called the embryoblast and an outer layer known as the trophoblast, is the structure that implants in the uterus following its journey from the Fallopian tube.
Step-by-step explanation:
The hollow ball of embryonic cells that enters the uterus from the Fallopian tube and eventually implants is known as a blastocyst. A blastocyst is formed after the cell mass known as the morula enters the uterus and starts to secrete fluid, organizing around a fluid-filled cavity called the blastocoel. The blastocyst consists of an outer layer of cells, the trophoblasts, which will develop into the fetal portion of the placenta, and an inner cell mass, the embryoblast, which will develop into the embryo. Implantation occurs when the blastocyst embeds into the endometrium of the uterus, approximately a week after fertilization.