Urinogenital System 



463 



ginal zone, as shown by the consistent results 

 of many vital-staining experiments (for 

 reviews see Pasteels, '42; Nieuwkoop, '47). 

 At this stage a rather sharply defined area 

 representing the pronephros lies somewhat 

 ventrolateral to the dorsal lip of the blasto- 

 pore (Pasteels, '42). When this area is 

 marked with a vital stain the color is later 

 found to be confined almost entirely to the 

 pronephros and its duct (Fig. 168). The area 



Fig. 167. Plan of development of the nephric sys- 

 tem of vertebrates. A, Origin and mode of develop- 

 ment of the nephric duct; B and C, its relations to the 

 other parts of the system; PR. pronephric units; 

 ND, nephric duct; NGC, nephrogenic cord; G, 

 gonad; MRS, mesonephric units; MET, metane- 

 phros; UR, ureter; CL, cloaca. 



in question lies at the future cephalic end 

 of a band of nephrogenic tissue which, in 

 the course of gastrulation, passes inward 

 around the lateral lip of the blastopore 

 toward its final position in the lateral trunk 

 region. 



In salamander embryos final topographic 

 localization of the pronephric material oc- 

 curs from late gastrula to middle neui'ula 

 stages. In the early neurula the material has 

 been invaginated and lies in the lateral body 

 wall, anterior to the blastopore and below 

 prospective somites 4 and 5 (Fig. 169, after 

 Yamada, '37; see also Muchmore, '51). After 

 this stage its position relative to adjacent 

 regional structures (somites, limb area, gills, 

 etc.) does not essentially change. By the 



middle neurula stage, however, as a result of 

 posterior elongation of the body axis, the 

 location is relatively further forward (Fales, 

 '35; see also Yamada, '37; Nieuwkoop, '47; 

 Muchmore, '51). At this stage transplants of 

 a limited area (Fig. 171) always produce 

 pronephros, and contiguous areas have no 

 pronephric potency (Fales). Nevertheless, 



Fig. 168. A, Position of the prospective pro- 

 nephric area in the early gastrula of the axolotl, 

 according to Pasteels ('42); B, later distribution of 

 the stain. 



Fig. 169. Position of the pronephric area in the 

 early neurula stage of the salamander, according to 

 Yamada ('37). Prn., Pronephric area; numerals in- 

 dicate the position of the future somites. BL, blasto 

 pore. 



Fig. 170. Position of the mesonephric material 

 (stippled) and the lateral plate material (lined) in 

 a salamander. A, Middle gastrula stage; B. tail-bud 

 stage (after Nieuwkoop, '47). CH, Chorda; 5, somite 

 area; YP, yolk plug. 



the definitive pronephros-forming area is still 

 not completely autonomous; the character of 

 its final differentiation, as respects the type 

 of tubule produced, and the details of tubule 

 structure, depend on its regional environ- 

 ment — a matter for later consideration (see 

 p. 465). 



The topographic localization of the pro- 

 spective mesonephric tissue has also been 

 established by vital staining. In the middle 

 gastrula stage of a salamander egg it lies 

 ventral and ventrolateral to the blastopore, 

 in the form of an open collar (Fig. 170/1, after 

 Nieuwkoop, '47). The sides of the collar are 



