THE UROGENITAL SYSTEM 



433 



three pairs of mesonephric tubules occur in 

 human embryos, it is obvious that, unUke 

 the pronephros, the mesonephros is not 

 metameric in its origin. While the anterior 

 nephrotomes are segmented, the most pos- 

 terior mesonephric tubules are derived from 

 a continuous unsegmented nephrogenic cord 

 or intermediate cell mass. At their first 

 appearance, the anlagen of the mesonephric 

 tubules are budded off as solid spherical 

 cell masses, which secondarily attain con- 

 nexion with the primitive duct, now become 

 the mesonephric duct. 



The differentiation of mesonephric 

 tubules takes place thus: The spherical 

 cell-masses elongate towards the meso- 

 nephric duct and at the same time acquire 

 a lumen. While the medial portion of the 

 anlage enlarges into a vesicle, the Bowman's 

 capsule, the lateral end unites with the 

 mesonephric duct and finally opens into it. 

 Each vesiculated enlargement is converted 

 into a renal corpuscle by the ingrowth of 

 a knot of capillaries derived from the dorsal 

 aorta. A sharp bend of the mesonephric 

 tubule near the Bowman's capsule divides 

 it into a distal secretory portion and a 

 collecting portion proximal to the duct. 

 These changes, in conjunction with the 

 growth of capillary networks derived from 

 postcardinal and subcardinal veins and 

 the associated enlargement of the genital 

 folds, cause the combined organs to bulge 

 into the body cavity as urogenital folds. 



Most of the mesonephric tubules de- 

 generate, only twenty-six pairs remaining in 

 a 20 mm. embryo. In the male, some of 

 these are converted into the efferent ductules 

 of the testes, and in this sex the mesonephric 



Fig. 358. — Drawings showing the development 

 of the mesonephric tubules in the pig. d, meso- 

 nephric duct; t, mesonephric tubule; cap., glomerular 

 (Bowman's) capsule. (From Patten's "Embryol- 

 ogy of the Pig," based upon figures by McCallum 

 and Lewis.) 



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leteraU 



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ventral 



