Which villi is seen between 15th to 20th day after fertilization?

Ans. is 'c' i.e., Secondary * Chorionic villi sprout from the chorion after their rapid proliferation in order to give a maximum area of contact with the maternal blood. These villi invade and destroy the uterine decidua while at the same time they absorb nutritive materials from it to support the growth of the embryo.* During the primary stage (primay villi formed on the 13th day following fertilization), the chorionic villi are small, nonvascular, and contain only the trophoblast. During the secondary stage (secondary villi formed on 16th day following fertilization) the villi increase in size and ramify, while the mesoderm grows into them; at this point the villi contain trophoblast and mesoderm.* During the tertiary stage (tertiary villi completed on 21st day) the branches of the umbilical vessels grow into the mesoderm; in this way, the chorionic villi are vascularized. At this point, the villi contain trophoblast, mesoderm, and blood vessels.