Embryo. A six week old embryo seen together with the chorion. The chorion, here opened, completely surrounds the embryo. The external surface of the chorion is characterised by many tiny projections, the chorionic villi, which establish an intimate connection with the lining of the uterus (endometrium). The part of the chorion at left will give rise to the placenta. The crown-rump length of the embryo, from the vertex of the skull to the mid-point of the buttocks, is about 17- 19mm at this stage. At six weeks all the major organs such as heart, kidneys, lungs and liver have begun to form.