From the egg to Embryo First, the zygote becomes a solid ball of cells.Then it becomes a hollow ball of cells called a blastocyst. Inside the uterus, the blastocyst implants in the wall of the uterus. where it develops into a embryo attached to a placenta and surrounded by fluid-filled membranes.
Crossing over is a biological occurrence that happens during meiosis when the paired homologs, or chromosomes of the same type, are lined up. Hope this helps :)
