Urinogenital System 



469 



THE DEVELOPMENT OF THE 

 METANEPHROS 



The development of the metanephros pre- 

 sents problems of a similar nature. The em- 

 bryonic ureter is an outgrowth of the nephric 

 duct, while the kidney tubviles differentiate 

 from the adjacent metanephric blastema. 

 Here the analysis rests mainly on experi- 

 ments with chick embryos. Interference with 

 the growth of the nephric duct at anterior 

 levels results in absence of its hinder part 

 (see Fig. 174) and in consequence the ureter 

 is also lacking. In this case the metanephric 

 tubules fail to differentiate (Boyden, '27; 

 Griinwald, '37) although a blastema appears 

 and may persist for a time. This result is 

 consistent with the generally accepted inter- 

 pretation of agenesis of the kidney (Boyden, 

 '32; Griinwald, '37, '38; Auer, '47). Congenital 

 absence of a kidney is always accompanied 

 in males by absence of the ductus deferens 

 (nephric duct) and ureter on that side. 

 Furthermore, development of double ureters 

 (a well known anomaly) results in the 

 formation of two kidneys, which may show 

 various degrees of fusion according to pro- 

 pinquity. In an early stage of this anomaly 

 each ureter has its separate blastema (e.g., 

 Wharton, '49). 



Evidently the appearance of a blastema 

 and the subsequent differentiation of tubules 

 are distinct phases. The first is independent 

 of the ureter (although if two ureters are 

 present twin blastemas may be formed; cf. 

 the eye-forming area of the neural plate), 

 but the ureter must be present for the differ- 

 entiation of tubules. This conclusion accords 

 with the fact that metanephric potency has 

 not been demonstrated in parts of the chick 

 blastoderm, bvit development readily occurs 

 in explants of the metanephric region after 

 the ureter has formed (see Seevers, '32). The 

 ureter is held to be essential. 



It is reported that tissues other than the 

 ureter (nephric duct, nervous tissue) may 

 induce differentiation in the metanephric 

 blastema, but the tubules formed are atypical 

 and of mesonephric type (Gruenwald, '42, 

 '43; see p. 465). 



THE ORIGIN AND DEVELOPMENT OF 

 THE OVIDUCT 



The embryonic oviduct (Miillerian duct) 

 develops much later than the nephric duct; 

 however, the two structures are closely re- 

 lated in origin and mode of development. In 

 certain fishes and amphibians the ostium 



is considered to arise from one or more 

 persistent pronephric tubules, or their ne- 

 phrostomes. The duct reaches the cloaca by 

 the backward growth of a cord of cells from 

 the ostial primordium (for reviews consult 

 Goodrich, '30; Brachet, '35; Willier, '39; 

 Gallien, '44). In amniotes the ostium is said 

 to be formed by invagination of a special 

 area of the coelomic epithelium, near the 

 anterior pole of the mesonephros, and the 

 duct is again formed by backward growth. 

 The resemblances are obvious but in amni- 

 otes the role of pre-existing nephric rudi- 

 ments is not generally admitted. Neverthe- 

 less, origin of the ostium from pronephric 

 remains, or from funnel-like structures re- 

 sembling nephrostomes, has been reported in 

 certain mammals (see von Winiwarter, '10; 

 Brambell, '27; Burns, '41). Recently, also 

 the old problem of the relationship of the 

 developing oviduct to the nephric duct has 

 been re-examined. In both avian and mam- 

 malian embryos the growing tip of the duct 

 is almost inseparably applied to the wall of 

 the nephric duct, from which it may derive 

 material (Gruenwald, '41). Altogether, a 

 nephric origin for the Miillerian duct is 

 probable. 



The main problems in the development of 

 the oviduct are (1) the origin and nature of 

 the ostial primordium and (2) the mode of 

 extension of the duct to the cloaca. On the 

 first point experimental evidence is limited* ; 

 with respect to the second it is clear that 

 development of the oviduct depends on the 

 presence of the nephric duct. In chick em- 

 bryos in which the posterior part of the 

 nephric duct is lacking as a result of opera- 

 tion at anterior levels (Griinwald, '37) the 

 ostium is present, plus a segment of the 

 oviduct corresponding exactly to the surviv- 

 ing portion of nephric duct. Although devel- 

 opment had been initiated it did not proceed 

 beyond the termination of the nephric duct. 

 Further evidence is found again in studies 

 of renal agenesis. Absence or partial develop- 

 ment of the nephric duct is accompanied by 

 corresponding deficiencies in the female gen- 

 ital tract (Griinwald, '38, '41; Auer, '47). 

 The nature of the dependence is not estab- 

 lished. The nephric duct may be merely a 

 guide — an essential feature of the path along 



* According to unpublished observations of the 

 author, early extirpation of the pronephros (see 

 Fig. 176), with absence of the nephric duct, is fol- 

 lowed long afterward by failure of either oviduct 

 or ostium to appear on the operated side. Formation 

 of the ostium seems to depend in some way on the 

 presence of the pronephros, cf. Gallien ('44). 



