End of first month. During the second week of fetal development, lacunar spaces develop between cells of the syncytiotrophoblast, paicularly in the region of the embryonic pole as the conceptus invades the endometrium. Endometrial capillaries in this region become dilated and engorged with blood to form sinusoids. The syncytial cells direct erosion of the endothelium of the maternal capillaries, allowing maternal blood to enter the lacunae and bathe the syncytial cells. During the second week, core will then differentiate to form capillaries and blood cells by the end of the third week (teiary villus). primary villi consist of projections of syncytial cells surrounding a core of cytotrophoblast cells (answer b). During the third week (answer c), the villus core is invaded by mesodermal cells to form a secondary villus. Cells of the mesodermal Those vessels become connected to the fetal circulation early in the fouh week establishing the functional uteroplacental circulation.