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Shortly after the differentiation of the mesonephric duct, through mesenchymal-epithelial transformation, the mesonephric vesicles  11 arise out of the nephrogenic cord, which represents a mesoderm condensate. Via the mesonephric tubules they connect up with the mesonephric duct 14. This differentiation takes place bilaterally in the area between the upper thoracic region (Th1) and the lumbar region (L3). A cranio-caudal gradient is also visible here. To the extent in which new mesonephric vesicles and tubules develop caudally, the cranial elements begin to atrophy so that never more than 30 pairs exist in the mesonephros. They form the excretory system that closely resembles the adult nephrons.
Medially, the mesonephric vesicle dead-ends in that it forms a funnel (Bowman's capsule). Each of these funnels surrounds a tuft of capillaries (glomerulus), which have also arisen in the nephrogenic tissue and come from lateral (visceral) branches of the dorsal aorta. They drain into the inferior cardinal vein.
The capsule with glomerulus together form a renal corpuscle. A renal corpuscle and its associated tubule are called a nephron and the functional unit an excretory mesonephric unit.
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