12 days
Maternal blood in lacunae of syncytiotrophoblast
Lacunar circulation
Uterine cavity
Langman’s Medical Embryology, 8th ed., TW Sadler Lippincott, Williams & Wilkins (Philadelphia) 2000, p. 52
Embryo completely embedded
Pre-streak
Lacunae fuse with maternal blood sinuses = lacunar circulation