466 



Special Vertebrate Organogenesis 



The several parts (glomus, nephrostomal 

 canals, tubules, common duct) have distinct 

 origins (see Field, '91; Fales, '35; Cambar, 

 '49). The glomus arises from the coelomic 

 wall and may develop independently after 

 excision of the pronephric primordium (Hov^^- 

 land, '21; Miura, '30a; Fales, '35), but an 

 accessory glomus may also develop in hetero- 

 topic positions in response to transplanted 

 tubules (Fales). Nephrostomal canals de- 



is increasingly impaired (Fales, '35). When 

 older donors are used, however, development 

 is better at all levels. Grafts to the body 

 cavity yield only disorganized tubules, but 

 again the quality of differentiation varies 

 somewhat with the age of the primordium. 

 Apparently two major factors are operative 

 at this period: an organization in the pri- 

 mordium, expressed by capacity for autonom- 

 ous differentiation, which increases with 



Fig. 174. Table showing effects of various experimental procedures on the development of the nephric 

 duct, with respect to its origin and manner of formation. Staining experiments (O'Connor, '38) indicate a 

 local origin from a primordium adjacent to and continuous with the pronephros. Asterisks indicate cases in 

 which a short segment of duct was found posteriorly, in communication with the cloaca. According to 

 O'Connor ('39, '40) such pieces are of cloacal origin. 



velop from the somatopleure underlying the 

 tubule primordia, and are said to remain 

 after removal of the primordium (Howland, 

 '21); but this is also denied (Dalcq, '42; 

 Cambar, '49). However, primordia develop- 

 ing in ectopic locations, or in the coelom, 

 usually produce tubules without nephros- 

 tomes (Fales). In later development there is 

 marked interdependence of parts, probably 

 mediated by functional influences. For ex- 

 ample, removal of the glomus, or obliteration 

 of nephrostomal canals, leads to atrophy of 

 the tubules; and reducing the number of 

 tubules results in shrinkage of the common 

 duct (Miura, '30a; Shimasaki, '30a; Fales, 

 '35). 



In the middle neurula stage transplanta- 

 tion of the pronephric primordium in the 

 orthotopic position results in normal develop- 

 ment, but at posterior levels differentiation 



age, and regulative influences essential for 

 normal growth and development, exerted by 

 the regional environment. 



ORIGIN AND DEVELOPMENT OF THE 

 NEPHRIC DUCT 



The classic view derives the nephric duct 

 from a union of the ends of the anterior 

 pronephric tubules (Fig. 167; for reviews of 

 normal development see Goodrich, '30; 

 Eraser, '50). In many amphibians, however, 

 the tubules do not appear as discrete units 

 but arise from a continuous primordium, the 

 posterior extremity of which represents the 

 duct (e.g.. Field, '91). A vital stain applied 

 to the pronephric swelling of a salamander 

 ventral to somites 3 and 4 appears subse- 

 quently only in the tubules; but if applied 

 ventral to somites 5-7 the color is later con- 



