The amniotic sac helps to protect the fertilized egg with fluid. The placenta is round and flat and is the transferer of nutients from mom to baby. A primitive face will start to form with large circles for eyes. The mouth, lower jaw and throat are developing. Blood cells are taking shape and circulation begins. Baby's tiny heart beats 65 times a minute by the end of the fourth week of life and he/she is about 1/4" long, smaller than a grain of rice.