blastocyst: the blastula of a placental mammal in which some differentiation of cells has occurred
blastula: early stage of an embryo produced by cleavage of an ovum; a liquid-filled sphere whose wall is composed of a single layer of cells; during this stage (about eight days after fertilization) implantation in the wall of the uterus occurs
thyrotropin-releasing hormone: hormone released by the hypothalamus that controls the release of thyroid-stimulating hormone from the anterior pituitary