Embryo. A four week old human embryo. Its crown- rump length, from the vertex of the skull to the mid-point of the buttocks, is about 5-8mm. In the fourth week the embryo becomes recognisable as a mammal: limb buds are visible at centre and centre right and rudimentary eyes (dark upper centre) develop in the form of stalks. The umbilical cord is seen between the head and the tail of the embryo. Within the embryo buds of tissue emerge and will later develop into the lungs, pancreas, liver and gall-bladder. It is in this period that the embryo is at the greatest risk of birth defects caused by genetic or external factors.