The third week is characterized by early morphogenic changes as the bilaminar embryo is transformed into a trilaminar embryo, developing the three cell lineages that will eventually form every system. It is also the time of early tissue and organ differentiation of the nervous and cardiovascular systems, as well as the formation of future body cavities.
The process of gastrulation, the conversion of the bilaminar to a trilaminar embryonic disk, is initiated by the presence of the primitive streak. This raised layer on the surface of the epiblast proliferates and migrates caudally. Cranially it ends in the primitive node. From this point on, one can identify the cranial/caudal ends, the dorsal/ventral sides, and right from left. Cells then migrate between the hypoblast and epiblast to form intraembryonic mesenchyme, a loose embryonic connective tissue. Cells also migrate to the hypoblast where they become the intraembryonic endoderm. Those cells that remain in the epiblast form the future ectoderm, the middle layer becomes the mesoderm, and the hypoblast becomes the endoderm. Cells then migrate from the primitive streak to other sites where they will undergo further differentiation. Some will travel cranially to form the notochord, some laterally, to become continuous with the extra-embryonic mesenchyme, and those most cranially will form the cardiogenic mesenchyme (future heart).
Meanwhile, a solid aggregation of mesenchyme known as the notochord is growing cranially, eventually reaching from the primitive node (one end point of the primitive streak) to the bucco-pharyngeal membrane (future mouth). As the notochord joins the endoderm, it will form a temporary communication between the amniotic cavity and the yolk sac, permitting amniotic fluid to circulate in the neurenteric canal. As the notochord grows cranially, the size of the primitive streak should diminish cranially, until it eventually disappears. The notochord then plays an important role, as the primary inducer of the the neural tube.
Neurulation, neural tube formation, begins as the the primitive ectoderm is induced to differentiate into neuroectoderm. The surface ectodermal cells transform from cuboidal to columnar cells and proliferate. This initial stage is called the neural plate and occurs around day 18. As the cells begin to change shape and the number of cells decreases, a central piece of surface ectoderm folds. The formation and eventual closing of the entire length of the neural tube occurs between days 20-26. The neural tube completes closure in the fourth week, first at the cranial end, later caudally, as amniotic fluid flows freely through the tube.
During this week, the allantois forms, an outgrowth of the yolk sac into the connecting stalk. Within the mesoderm, there is differentiation and formation of longitudinal blocks of mesoderm known as the paraxial (innermost), intermediate and lateral mesoderm. Around day 21, the first pair of somites will appear (discussed in week 4). Coelomic spaces will develop in the lateral and cardiogenic mesenchyme that will fuse to form a horse-shoe shaped cavity around the embryo. This separates the two layers of lateral mesoderm into a somatic and splanchnic layer and will form the future body and gut walls, as well as create space for future body cavities (also discussed in week 4). This week also marks the beginning of the cardiovascular system as angiogenesis (blood vessel formation) and hematogenesis (blood formation) occurs around the yolk sac and the endocardial heart tubes fuse to form the primitive heart tube. The embryo will also develop its side of the circulatory system as primary chorionic villi are transformed into secondary and then tertiary villi, which contain embryonic capillaries so that arteriocapillary networks can form.