The yolk sac (1) is seen attached to the embryo (3) by a stalk (2). Blood vessels in this stalk (2) carry nutrients (food) from the yolk sac to the embryo (3). At this stage of development the embryo is very small, about the same size as the yolk sac. The embryo is attached to the placenta (4) by a developing umbilical cord.
Above is a unique 3-D image of a six week developing embryo. It is four weeks after conception or six weeks after the first day of the last menstrual period. The right version highlights the developing parts.
