the gestation sac is approximately 3mm in diameter and the embryo is just big enough 2b seen by the naked eye. EMBRYO Development...your embryo wil develop witin a bag fluid in ur womb.the ball of cells makin up the embryo grows into three distinct layers.the innermost layer called endoderm will become lungs liver and intestines.the middle layer become called the meroderm become bones makin up the skeleton muscles and also the heart and blood vessels.outermost layer called the ectoderm will form the brain