The germinative layer  produces new cells, which migrate and form new successive layers of intermediate cells under the periderm. During the second half of pregnancy, the germinative layer becomes the stratum basale. The intermediate cell layer becomes subdivided from the inside to the outside into a stratum spinosum, stratum granulosum and stratum corneum. From about week 8, melanocytes of neural crest origin are present. Langerhans' cells migrate around week 12 from the bone marrow. Merkel cells, which develop into mechanoreceptors, are thought to arise from epidermal cells in situ.