Third and Fourth weeks of development
20- Transverse presentation of embryo folding
The embryo changes from a flat disc into a three-dimensional cylinder by the end of week four. These structural changes are illustrated chronologically in three stages. Structures derived from ectoderm, the skin and the central nervous system, are illustrated in blue. Various areas of mesoderm are colored red and pink, and the developing respiratory and gastrointestinal systems are yellow, reflecting their derivation from endoderm.
Early stage ~ Day 23
The main force responsible for embryo folding is differential growth of tissues, with the ectoderm expanding at a faster rate than the endoderm.
The cross-section illustrates the neural tube and the derivatives of mesoderm: the notochord, somites, intermediate mesoderm, and lateral mesoderm.
Middle stage ~ Day 25
The embryonic disc grows more vigorously than the yolk sac; thus, the embryo becomes convex in shape. The ectoderm grows together to form the ventral midline of the body.
The medial part of the somites differentiates into the sclerotome, while the lateral mesoderm divides into somatic (body) and splanchnic (visceral) portions.
Late stage ~ Day 28
Much of yolk sac is folded inside the embryo, forming the gut tube. Mesoderm differentiation continues as the somites divide into three separate parts:
The intermediate mesoderm gives rise to kidneys and gonads. The lateral mesoderm splits into somatic and splanchnic components: