Urinogenital System 



467 



fined to the duct (O'Connor, '38), which 

 presumably has a distinct origin from inter- 

 mediate mesoderm at this level* (see also 

 Fig. 168; Pasteels, '42). Transections through 

 the pronephric region at the neurula stage 

 indicate approximately the same localization 

 (Holtfreter, '44). 



More difficult to determine is the manner 

 in which the duct reaches the cloaca. On 

 this point confhcting views have long existed. 

 The classic theory envisages independent 

 caudal growth of a bud of tissue arising at 

 the pronephric level (Fig. 167), but an 

 alternative view (Field, '91, and others') 

 postulates formation in situ by incorporation 

 of local materials at each successive level 

 (hence the term "segmental duct"). The 

 problem has been investigated experiment- 

 ally by three methods: (1) excision of the 

 pronephric primordium before outgrowth of 

 the dvict; (2) interference with the growth 

 of the duct at various levels; (3) the use of 

 vital staining to identify the duct materials. 

 The results are summarized in Fig. 174. In 

 brief, it appears that removal of the pri- 

 mordium, or interference with the tip of 

 the duct, prevents development posterior to 

 the point of intervention (Fig. 175); but if 

 the growing tip is left intact, development 

 continues. Stain applied to the duct rudiment 

 at the point of origin (as noted previously) 

 appears subsequently throughout its length, 

 but not in any adjacent tissue (O'Connor, 

 '38). Conversely, if the prospective meso- 

 nephric material is stained prior to the for- 

 mation of the duct, the latter is subsequently 

 found to be vmstained (Spofford, '48). The 

 evidence supports the view that the duct 

 develops by caudal growth of an original 

 primordium rather than by local accretion. 

 An exception must be made with respect to 

 the posterior end of the duct. A short piece — 

 often less than one somite in length — ^is 

 sometimes found in contact with the cloaca, 

 in the absence of the main part of the duct 

 CMachemer, '29; O'Connor, '39, '40; Holt- 

 freter, '44; Tvmg and Kvi, '44; van Geertrviy- 

 den, '46). This piece is considered by O'Con- 

 nor to be of cloacal origin. 



However, the territory traversed by the 

 duct is not without influence on its develop- 

 ment. In normal development the duct grows 

 backward in a narrow groove between the 

 ventral edges of the somites and the lateral 

 mesoderm, and covered by ectoderm. Ob- 

 structions placed across this pathway usually 



* In anuran embryos the duct primordium lies 

 at the level of somite 5 according to Dalcq ('42) 

 van Geertruyden ('42), and Cambar ('49"). 



prevent further development (see Fig. 174); 

 or if the tip of the duct is rotated it usually 

 fails to grow far into strange territory (Tung 

 and Ku, '44). On the other hand, if the 

 pathway is reoriented or dislocated in various 

 ways, the duct may deviate considerable 

 distances in order to reenter and traverse 

 it (Holtfreter, '44; Tung and Ku). The duct 

 is able to traverse the path in either direction, 

 or two ducts may be made to do so in oppo- 

 site directions (Tung and Ku). When seg- 

 ments of the duct are excised, regeneration 



Fig. 175. A, Absence of the nephric duct in a 

 chick embryo (left side) after preventing the back- 

 ward growth of the primordium by transection at 

 the 12-somite stage; B, failure of the mesonephric 

 tubule to develop on the operated (right) side in the 

 absence of the nephric duct — only a formless 

 blastema is present (after Waddington. '38); ND, 

 nephric duct; BL, blastema; T, mesonephric tubule. 



proceeds along the pathway from either or 

 both ends, (Howland, '26; Tung and Ku). 

 Evidently no irreversible polarizations are 

 involved, either of the pathway or the duct. 

 The properties of the path favoring develop- 

 ment of the duct are perhaps in large part 

 mechanical, but the importance of physio- 

 logical factors has also been stressed (Holt- 

 freter, '44). 



In amphibians the nephric ducts terminate 

 in a pair of cloacal diverticula (cloacal 

 horns) which are histologically distinct from 

 the duct (Field, '91; O'Connor, '40); in 

 avian embryos vmion is with the urodeum. 

 Studies of this relationship have yielded dif- 

 ferent results. In amphibians cloacal horns 

 develop normally in the absence of the duct, 

 and may even elongate, replacing its hinder 

 end (O'Connor, '39, '40). Elimination of the 

 duct in chick embryos results in reduction 

 and abnormal differentiation of the urodeum 

 (Boyden, '24). The duct readily unites with 

 grafts of cloacal tissue placed along its path 



