Ovulation is the process wherein the ovary releases the ovum which is picked up by the fallopian tubes to be transferred to the uterus and during this transit the ovum gets fertilized in the tube. After fertilization, the outer layer of the egg gets thickened so as not to allow any other sperm inside. Mother and child do not share the same blood; their circulations are seperated by the chorion and that is why human maternal-fetal circulation
is called haemo-chorionic. At conception, the single fertilized egg, called the zygote, transits into the uterus and during this process it divides and it is the 16-celled blastocyst
that gets implanted into the wall of the uterus. Yolk sac
forms early in pregnancy
and is visible in scans at about 5 weeks. It is also true that fetal blood formation initiates in the liver followed by the spleen and at 21 weeks in the bone marrow
, wherein mostly myelocytes are produced until birth. Good luck.