Week Four, Longitudinal Folding

The three cell layers of the embryonic disc now fold in on themselves, fusing into full length tubes that eventually emerge as major organ systems.

The Lateral Plate

The mesoderm becomes organized into two sheets of cells on either side of the notochord, known collectively as the lateral plate. The upper sheet o f the lateral plate is associated with its overlying ectoderm and is known as the somatic mesoderm because it gives rise to the soma (Greek for body). The lower sheet of the lateral plate is associated with the endoderm and is called the splanchnic mesoderm (Greek for viscera), giving rise to muscles and connective tissue of the gut.

The Mesoderm

Between the upper and lower sheets of the lateral plate is a coelom (cavity) that is the primordium (beginning) of the abdominal and thoracic cavities. The region where the somatic mesoderm and the splanchnic mesoderm layers meet medially is called the intermediate mesoderm and will give rise to the developing urogenital system.

Coelom Development

The ectoderm and somatic mesoderm layers are collectively called the somatopleure, and adjoin the amniotic cavity in the upper coelom of the embryo.

The endoderm and splanchnic mesoderm are collectively called the splanchnopleure, and adjoin the larger cavity of the yolk sac in the lower region of the embryo. Between these two distinct sections is the extraembryonic (or chorionic) coelom, which is initially continuous with the intraembryonic coelom and is lined with mesoderm cells.

Organogenesis

Having fully implanted and completed gastrulation by the end of the third week, the embryo now begins to derive nourishment from the mother through the placenta in the fourth week. Cell differentiation continues, creating germ layers that rapidly will become embryonic tissues. During the five to eight week embryonic phase, all organs and tissues of the embryo will be formed. By the beginning of the eighth week after fertilization, the embryo is starting to acquire a recognizable body structure and is called a fetus. The fetal period extends for 32 more weeks of development, concluding at birth.