. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 344 THE METANEPHROS. which, as has already been noted (p. 342), is separated from the portion that gives rise to the mesonephros. This terminal portion of the ridge forms what is termed the metanephric blastema and in embryos of 7 mm. it has come into relation with the outgrowth from the Wolffian duct and covers its free extremity as a cap. Since both the blastema and the outgrowth from the Wolffian duct take part in the formation of the uriniferous tubules, these have a double origin. The ou
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 344 THE METANEPHROS. which, as has already been noted (p. 342), is separated from the portion that gives rise to the mesonephros. This terminal portion of the ridge forms what is termed the metanephric blastema and in embryos of 7 mm. it has come into relation with the outgrowth from the Wolffian duct and covers its free extremity as a cap. Since both the blastema and the outgrowth from the Wolffian duct take part in the formation of the uriniferous tubules, these have a double origin. The outgrowth from the Wolffian duct as it continues to elongate comes to lie dorsal to the mesonephros, carrying the cap of blastema with it, and it soon assumes a somewhat club-shaped form, its terminal enlargement or ampulla form- ing what may be termed the primary renal pelvis, while the remainder represents theureter. The primary renal pelvis then gives rise to from three to six, usually four, tubular outgrowths, which may be termed primary collecting tubules, and with their formation the original cap of metanephric blastema undergoes a division into as many portions as there are tubules, so that each of the latter has its own cap of blastema. As soon as each tubule has reached a certain length it begins to enlarge at its free extremity to form an ampulla, just as did the primary renal pelvis, and from this ampulla there grow out from two to four secondary collecting tubules, a further corresponding division of the metanephric blastema taking place. In their turn these secondary tubules similarly enlarge at their extremities to form ampullae (Fig. 212, A) from which tertiary collecting tubules are budded out, accompanied by a third fragmentation of the blastema and so the process goes on until about the fifth fetal month, the number of generations of collecting tubules formed being between Fig. 212.—Diagrams of Early Stages in the Development of the Metanephric Tubules. t, Uri
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