At about day 10 blood-filled lacunae appear in the syncytiotrophoblast which ultimately fuses at day 12 to form the lacunar network, the primordia of intervillous spaces of the placenta, syncytiotrophoblast erodes the endometrial capillaries and maternal blood flows into these capillaries(sinusoids) giving rise to the primordial uteroplacental circulation. The rapidly proliferating cells of cytotrophoblast produce cellular extension that grows into the syncytiotrophoblast giving rise to the primary chorionic villi, mesenchyme soon grows into these primary villi giving rise to secondary villi. The mesenchymal cells soon differentiate into capillaries and blood cells, the villi at this stage are termed as tertiary villi they soon become connected with embryonic heart through vessels that differentiate in the mesenchyme. Cytotrophoblastic cells of the chorionic villi form a cytotrophoblastic shell that surrounds and attaches the chorionic sac to the endometrium. Villi that attach to the maternal tissues through the cytotrophoblastic shell are termed as stem villi from which the branch villi project.