Carnegie Stage 15: Thorax, Abdomen and Developing Kidneys
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The neural tube and notochord are located in the dorsal part of the embryo. The mesonephros is the most obvious structure in this section. It is a precursor of the kidney and filters the embryos blood for a few days before it recedes. Its duct, the mesonephric duct (Wolffian duct), does persist in the adult, especially in males it becomes the vas deferens (which drains the testis).
The forelimbs are visible in this section, they are limb buds at this stage, and mainly consist of mesenchymal tissue covered by ectoderm. The ectoderm near the tip of the limb is thickened and forms the apical ectodermal ridge, which is an important signaling center in limb development.
