3. When successful ("/3 of zygotes fail to implant), implantation takes about 5 days, and is usually completed by the day after ovulation; just before the endometrium normally begins to 4. The trophoblast cells begin to secrete an LH-like hormone that keeps the corpus viable and secreting estrogen & progesterone, which in turn maintain the endometrium. Levels of these 2 hormones rise as pregnancy advances. 5. The which develops from the trophoblast after implantation, continues to make hCG. All tests today measure hCG levels. D. Embryonic Development: 1. It is called an embryo for the first months of development; after week it is called a 2. The inner cell mass is now a thick plate called the disc, from which the 3 germ layers (embryonic tissues from which all organs develop) arise: (outer skin"): the nervous system and epidermis of the skin will arise from this layer. b) ("inner skin"): forms the epithelial linings of digestive, respiratory and urogenital tracts, and associated glands. ("middle skin"): forms almost everything else (muscles, bones, blood, heart, vessels, cartilages, etc.) 3. Four embryonic membranes lie outside the embryo to protect and nourish it: : a sac hanging from the ventral surface of the embryo. It doesn't supply embryo with nutrients, as it does in bird eggs. Instead, it: 1) forms part of the gut (digestive tube) and 2) is the site of early b) : a thin, transparent membrane surrounding the embryo. It is filled with fluid (derived from mother's blood and fetal urine). It cushions & protects the fetus; regulates fetal # allows fetal movement (so muscles & bones can grow). Often called the "bag of waters", it ruptures just before giving birth. c) a small finger-like pouch that develops from the back of the yolk sac. It is the structural base for the cord that links the embryo to the placenta, and becomes part of the d) : derived from the . It surrounds the embryo, and becomes the embryonic part of the placenta. It has many finger-like projections called chorionic_ which dig into the endometrium and tap into maternal blood spaces (nutrients diffuse to embryo and wastes diffuse away)