conceptus

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

double-walled stage of the embryo resulting from invagination of the blastula; the outer layer of cells is the ectoderm and the inner layer differentiates into the mesoderm and endoderm

a solid mass of blastomeres that forms when the zygote splits; develops into the blastula

the blastula of a placental mammal in which some differentiation of cells has occurred