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The surface of the embryo retains the single layer of ectoderm.


The epitheloid character of the dermatome disappears as a result of cellular proliferation. Small islands of epitheloid cells are dermatome remnants sometimes present in the more caudal segments.

As the dermatome breaks up, many mesenchymal cells are produced, which spread beneath the expanding ectoderm.


A thickened band of ectoderm called the mammary ridge becomes evident along the lateral body wall. It extends in a longitudinal manner from the upper to the lower limb bud (Fig. 6–1A).

Source: Atlas of Human Embryos.