. A text-book of embryology for students of medicine [electronic resource]. Embryology; Embryology. DEVELOPMENT OF THE KIDNEY AND URETER. 237 atrophic for reasons that will appear hereafter. In certain vertebrates that are of higher type than those in which the pronephros functionates, such as adult amphibians and fishes, the Wolffian body persists throughout life as an organ of urinary secretion. In birds and mammals, however, its functional activity is but temporary, since it is supplanted, before the end of fetal life, by the permanent kidney. In man it disappears relatively early, retrogre
. A text-book of embryology for students of medicine [electronic resource]. Embryology; Embryology. DEVELOPMENT OF THE KIDNEY AND URETER. 237 atrophic for reasons that will appear hereafter. In certain vertebrates that are of higher type than those in which the pronephros functionates, such as adult amphibians and fishes, the Wolffian body persists throughout life as an organ of urinary secretion. In birds and mammals, however, its functional activity is but temporary, since it is supplanted, before the end of fetal life, by the permanent kidney. In man it disappears relatively early, retrogression beginning in the eighth week and the Malpighian bodies having almost disappeared by the fifth month. The presence of the meso- nephros as a temporarily functionating organ in birds and mammals, while it is a permanent structure in certain lower members of the vertebrate series, exemplifies the embryo- logical principle elsewhere referred to, that the higher types pass through stages during their development that are per- manent in some of the forms below them in the scale of evolution. The Metanephros or Permanent Kidney.—While the Wolffian body is temporarily functionating as a kidney, a structure is developing from the lower, cau- dal end of the Wolffian duct which is to form the permanent oro;an. It has been stated that the Wolffian duct opens into the cloaca. From the dorsal aspect of this duct, near its cloacal end, a small diverticulum, the kidney evagination (Fig. 118 and Plate VIL, 1), grows forth and soon pm 12L_Diagram to show exten. lengthens into a tube which sion and branching of kidney evagi- , •, -i -i . i nation and separation of its stalk grows head ward, dorsomesial from the Wolfflan duct. Ui primitive to the Wolffian duct, penetrating ureter; p, pelvis of ureter; WD, . ° Wolffian duct; Bl, bladder ; us, uro- into the nephrogenic tissue or genitaisinus; a, cloaca; q, gut. mesonephric blastema (p. 236). The cephalic end of the tube dilates somewhat t
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