Journal of Nippon Medical School

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The genital ridge and the adrenocortical primordium appear in intimate relation to the mesonephros. The cranial ends of these three primordium come to one region which lies in the dorsal coelom thick epithelium on the level of the caudal part of the lung. This level is in early Streeter's horizon XIV, XV stages opposite to the 7th or 8th spinal ganglion. This level moves caudal in the periods of Streeter's hor. izon XVI, XV1l, XVII1, and in the stage of 15 to 20 mm long embryo it comes opposite to the 17th or 9th thoracic spinal ganglion. The thick epithelium, where the cranial ends of the genital ridge and the adrenocortical primordium begin, is of 2 or 3 columnar cell layers and extends lateral to the surface of the glomeruli of the mesonephros, caudal nearly to the caudal end of it. This is the genital primordium.
In the stage of Streeter's horizon XIV begins the subepithelial invasion of round cells which continues from the thick epithelium where cranial ends of mesonephros and the genital ridge lie. This cell invasion into the mesenchyme beneath the epithelium begins vague, not sharp outlined medial to the cranial end of the genital ridge on the Uotila's socalled adrenal groove. This continuation from the epithelium and vague outline of the adrenocortical primordium become free from the epithelial surface and sharp in the Streeter's horizon XVI stage embryo. At first the length of this primordium is only half or less of the length of the mesonephros, but in 15 mm-20 mm long embryos, it lengthens nearly to the caudal end of it.

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