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The modified blastula which is characteristic of placental mammals, including humans; further cleavage of the morula produces a thin walled hollow sphere, whose wall is the trophoblast, with the embryo proper being represented by a mass of cells at one side; this second stage of embryonic development must be completed before successful implantation into the uterine wall is possible.
The outermost layer of cells, membrane which forms the wall of the blastocyst and which attaches the fertilized ovum to the uterine wall and supplies nutrition to the embryo; it facilitates implantation by eroding away the tissues of the uterus with which it comes in contact allowing the blastocyst to sink into the cavity formed in the uterine wall, and differentiates into the extraembryonic membranes surrounding the embryo; after implantation of the blastocyst in the uterine wall it divides into two layers, the chorion and the placenta. aka - trophoderm.
The mass at the embryonic pole of the blastocyst concerned with the formation of the body of the embryo; the inner cell mass will form all of the tissues of the human body, therefore, these are the cells that develop into the fetus; these inner cell mass cells are pluripotent; this means that they are able to give rise to many, but not all cell types necessary for fetal development (e.g., they are able to give rise to fetal tissues, but not placental tissue). [Note: ICM in the figure below right.]