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:

  1. Sclerotome forms the vertebral column and ribs,
  2. Myotome forms the limb and trunk muscles,
  3. Dermatome gives rise to the dermis. 

The intermediate mesoderm gives rise to kidneys and gonads. The lateral mesoderm splits into somatic and splanchnic components:

  1. Somatic (body) mesoderm forms serous membranes that line the body cavity. The coelom or ventral body cavity, is surrounded by lateral mesoderm.
  2. Splanchnic (visceral) mesoderm forms serous membranes that surround viscera & gives rise to heart and blood vessels.