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Embryonic kidneys of the baboon.
Inauguration of the embryonic development of the vertebrate nephros begins within the cranial reaches of the intermediate mesoderm. The anterior portions of this mesoderm give rise to the nephrotomes which are the forerunners of the first nephrons. Since these nephrons form in the anterior position and are the first to appear, they are collectively called the pronephros of "head-kidney." Each individual nephron is unfortunately termed a pronephric tubule