The skin consist of the superficial layer, the epidermis and the deep layer, the dermis.
The epidermis is derived from the ectoderm. In the 5th week, the skin of the embryo is covered by simple cuboidal epithelium. By the 7th week, the skin contains single squamous layer (periderm or epitrichium), and a basal layer. During the 4th month, an intermediate layer, containing several cell layers, is interposed between the basal cells and the periderm. During the early fetal period the epidermis is invaded by melanoblasts, cells of the neural crest origin. The dermis is derived from the mesenchyme of somatopleura and the dermatomes.

1. Ectoderm
2. Mesenchyme

1. Periderm
2. Basal layer
3. Mesenchyme

1. Periderm
2. Basal layer
3. Intermediate layer
4. Migrating melanoblast
5. Mesenchyme

Hair begins to develop during the 3rd month as an epidermal proliferation into the underlying dermis. The epithelial cells of the hair bud give rise to the hair and the epithelial root sheath. Surrounding mesenchymal cells differentiate into dermal root sheath. Peripheral cells of the epithelial root sheath proliferate to form a sebaceous gland bud. Sweat glands develop as downgrowths of epithelial cords into the underlying dermis.
Nails begin to develop at about ten weeks of gestation as thickened areas of the epidermis at the tips of the digits. Later, these nail fields extend to the dorsal surface and become surrounded by the nail folds. Cells from the proximal nail fold grow over the nail field and form keratinized nail plate, the primordium of the nail.

The mammary glands begin to develop during the 6th week as thickened strips of the ectoderm (mammary ridges) that extend from the axillary to the inguinal regions. They regress in most locations except in the area of the pectoral muscle, where they proliferate. The downgrowth of epithelial tissue continues to proliferate into 16 to 24 solid outbuddings which give rise to the lactiferous ducts. Fibrous connective tissue and fat of the mammary gland develop from the surrounding mesenchyme. The lactiferous ducts at first open into a small mammary pit.

1. Mammary ridge
2. Upper limb
3. Lower limb
4. Tail

1. Dermis
2. Epidermis
3. Lactiferous duct
4. Mammary ridge