By the end of the second week, the blastocyst has completed implantation and can now be thought of as an organism of twos: a bilayered embryonic disk, with two cavities and two layers of trophoblast. It will have extensive extraembryonic structures, including an amniotic cavity and amnion, yolk sac, connecting stalk, and chorionic sac.
The syncytiotrophoblast continues to invade the stroma of the endometrium, inducing the decidual reaction, in which the uterine cells around the implanted embryo accumulate glycogen and lipids and swell. This may serve an immunological and a nutritive role for the developing embryo. The syncytiotrophoblast begins to produce human chorionic gonadotrophin (hCG) at the end of the week. This is the basis of the pregnancy test.
While the embryo is implanting at the embryonic pole, a cavity begins to form in the inner cell mass, known as the amniotic cavity. A layer of amnion forms around it, derived from the epiblast layer of the embryonic disk. The rest of the inner cell mass has now formed a flattened disk, composed of two layers, a thicker, columnar epiblast (amniotic side), and a smaller, cuboidal hypoblast (exocoelomic side), that will become the future endoderm.
The exocoelomic cavity and membrane form the primary yolk sac, which is thought to play a role in nutrition, despite the absence of a yolk. Cells from the primary ectoderm will migrate to surround the amniotic cavity, bilaminar disk and yolk sac, forming the extraembryonic mesoderm.
Simultaneously, lacunae, or spaces, are forming in the syncytiotrophoblast, through which maternal blood can access the embryo to provide nutrients and oxygen. Both arterial and venous branches communicate with the lacunae, forming the beginning of a uteroplacental circulation. Around day 12, these lacunae link up to form networks, giving the syncytiotrophoblast a "spongy" appearance. Endometrial sinusoids are gradually eroded by the invasive syncytiotrophoblast so that maternal blood can flow freely into the embryonic space.
Meanwhile, the fluid-filled spaces are opening up in the extra-embryonic mesoderm, which will then link up to form the extra-embryonic coelom, completely surrounding the amnion and yolk sac (except at the connecting stalk). At this time the primary yolk sac shrinks and is replaced by a smaller secondary (definitive) yolk sac, formed from extra-embryonic endodermal cells that migrated to the primary yolk sac from the hypoblast.
By the end of the week, primary chorionic villi will form as extensions of cytotrophoblast grow into the syncytiotrophoblast, creating protrusions into the endometrium. The extra-embryonic coelom (chorionic cavity) also separates the extra-embryonic somatic mesoderm (lining the trophoblast layer and amnion, future gestational sac) from the extra-embryonic splanchnic mesoderm (surrounding the secondary yolk sac). Finally, there is a thickening of the hypoblast at one end, indicating the prechordal plate (future mouth) of the embryo. The bilayer embryonic disk surrounded by the amniotic cavity and yolk sac is then suspended by the connecting stalk, completely surrounded by a large, fluid-filled extraembryonic coelom (primordium of chorionic cavity) which is surrounded by the chorion.