. Comparative anatomy. Anatomy, Comparative. 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.
. Comparative anatomy. Anatomy, Comparative. 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.) ^WO-i. Please note that these images are extracted from sca
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Keywords: ., bookcentury1900, bookpublisherphi, booksubjectanatomycomparative
