Embryos are generally obtained through blastomere or blastocyst biopsy. The fetal membranes are derived from the outer trophoblast layer of the implanting blastocyst. The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium. The outer layer of the blastocyst consists of cells collectively called the trophoblast. Human chorionic gonadotropin is an autocrine growth factor for the blastocyst. In mammals the blastula is referred to as a blastocyst. In preparation for implantation of the blastocyst, the endometrium undergoes decidualization. The conceptus is then known as the blastocyst. The blastomeres in the blastocyst are arranged into an outer layer called the trophoblast. Their counterparts, the proteoglycans, are found around the trophoblast cells of the blastocyst. Invasion is an even further establishment of the blastocyst in the endometrium. A blastocyst is then formed and implanted in the uterus. There is massive communication between the blastocyst and the endometrium at this stage. The zona pellucida surrounding the blastocyst breaches, referred to as hatching. Implantation is made possible through structural changes in both the blastocyst and endometrial wall.