During the third and fourth weeks of development, cells that migrate from the epiblast region into the primitive streak contribute to the formation of the mesoderm. This process is gastrulation. Midline mesodermal cells form the notochord. Lateral to the notochord, the mesoderm is the paraxial mesoderm which will form clumps of tissue called somites. Somites are usually visible on the surface of the embryo.
Lateral to the paraxial mesoderm is the intermediate mesoderm which will form genitourinary structures. The lateral plate mesoderm splits into two layers to form the linings of the body cavities and the coverings of the visceral organs.
In the view to the right, we are looking down on the surface of the trilaminar embryonic disc to see the various regions of mesoderm through the surface ectoderm.
Below is a cross section of an embryo.

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  • Paraxial Mesoderm

    The paraxial mesoderm becomes organized into segments known as somitomeres. Further organization of the cells results in the formation of blocks of tissue called somites. Somites begin to appear in the occipital region of the embryo beginning about day 20 of development. By the end of the 5th week, 42 to 44 pairs of somites are formed, lying on either side of the developing neural tube. They are divided regionally into:

    • 4 occipital
    • 8 cervical
    • 12 thoracic
    • 5 lumbar
    • 5 sacral
    • 8 - 10 coccygeal
    The first occipital and the last 5 - 7 coccygeal somites disappear. The remainder contribute to the development of the axial skeleton, the skull and vertebral column and related muscles.

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  • Intermediate Mesoderm

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    The intermediate mesoderm is located between the somite and the lateral plate mesoderm. It will contribute to the development of genitourinary structures such as the gonads, reproductive ducts, kidneys and ureters.

  • Lateral Plate Mesoderm

    The lateral plate mesoderm splits to form two layers. One layer becomes affixed to the outer layer of body ectoderm and is therefore referred to as the somatic mesoderm. It will form the mesothelial lining of the various body cavities. The other layer becomes attached to the yolk sac and is known as the splanchnic mesoderm. Splanchnic means pertaining to the viscera and this layer will form the outer mesothelial covering of various visceral organs. When the embryo folds in the rostral and caudal directions as well as in the lateral direction the two layers of the lateral plate mesoderm are incorporated into the body of the embryo and form the lining of the intraembryonic coelom.

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