464 



Special Vertebrate Organogenesis 



undergoing involution around the lateral 

 lips. (At this stage the pronephric material 

 has for the most part passed in.) Mesonephric 

 potency, however, is not restricted to this 

 band. After extirpation there is extensive 

 reconstitution of the nephrogenic and lateral 

 plate material (Nieuwkoop). 



In neural plate stages the mesonephric 

 material is still in process of involution. Two 

 positions have been demonstrated independ- 

 ently (Fig. 171). One lies just lateral to the 

 neural fold, a little anterior to the blasto- 

 pore (Fales, '35; Nieuwkoop, '47); but at a 

 slightly earlier stage an area within the 

 posterior neural plate also produces meso- 

 nephros (see Spofford, '45). The latter area 



close to the mid-line, a little behind the node 

 (Fig. 1725; for a review and analysis see 

 Rudnick, '44). 



Fig. 171. Position of pronephric and mesonephric 

 areas in the middle neurula stage of Amblystoma, 

 modified from Fales ('35). The pronephric area is 

 stippled; the adjacent areas indicated lack nephric 

 potency. M^, position of prospective mesonephric 

 material in the hinder part of the medullary plate, 

 according to Spofford ('45). M~, position of mesone- 

 phric material already invaginated and shifted 

 laterally toward its definitive position (Fales, '35; 

 Nieuwkoop, '47). 



evidently represents in part the uninvoluted 

 portion of the nephrogenic band; the former 

 consists of material which has been invagin- 

 ated. After involution is complete, elonga- 

 tion of the body axis shifts the mesonephric 

 material forward to its definitive position 

 (Fig. 1705). At this stage 90 per cent of 

 grafts of the intermediate mesoderm posterior 

 to the ninth somite give rise to well devel- 

 oped mesonephric tubules (Humphrey, '28a). 

 In chick blastoderms of the head-process 

 stage (comparable to the early neurula of 

 amphibians) mesonephric potency is found in 

 a limited area which includes the anterior 

 part of the primitive streak and the node 

 (Fig. 172^, after Rawles, '36; see also Willier 

 and Rawles, '35). When portions of this area 

 are transplanted to the chorio-allantoic mem- 

 brane, mesonephros develops (in association 

 with gonad and adrenal cortex). Nephro- 

 genic capacity presumably resides in the 

 mesodermal constituent; however, all three 

 germ layers are included in such trans- 

 plants. Somewhat earlier, in the definitive 

 streak stage, a prospective nephric area lies 



Fig. 172. A, Location of the prospective mesone- 

 phros- and gonad-forming areas in the chick blasto- 

 derm of the head-process stage, adapted from 

 Rawles; the area within the broken line represents 

 mesonephros, the cross-hatched area gonad and 

 adrenal. B, Location of nephrogenic material (pre- 

 sumably mesonephros) at the stage of the definitive 

 primitive streak, after Rudnick ('44); left side: epi- 

 blast — the material is moving toward the primitive 

 groove; right side: mesoblast, showing position of 

 material already involuted and moving laterally. 



THE EXISTENCE OF NEPHRIC FIELDS 



Although topographically restricted areas 

 representing pronephros or mesonephros can 

 be defined in amphibian eggs as early as 

 gastrula stages, nephrogenic potency is not 

 limited to such areas until much later. In 

 fact, capacity to produce nephric tubules is 

 widely distributed in the gastrula, in keeping 

 with the pluripotency of both ectoderm and 

 mesoderm at this stage (for a discussion see 

 Holtfreter and Hamburger, Section VI, Chap- 

 ter 1). Almost any part of the ectoderm, 

 epidermal or neural, may produce nephric 

 tubules when transplanted into the pro- 

 nephric region (Holtfreter, '33; Mangold, 

 '24) or when exposed to the influence of 

 "trunk organizer" (Holtfreter '36) ; and 

 nephrogenic tissue inserted into the blasto- 

 coele may induce nephric tubules in any 

 overlying ectoderm (Holtfreter, '33). Nephric 

 tissue has been obtained also from various 

 regions of the mesoderm which normally give 

 rise to other tissues, for examples, the archen- 

 teric roof (Holtfreter, '36) and somite ma- 

 terial (Yamada, '37; Muchmore, '51). 



The fact that various tissues not normally 

 nephrogenic become so when placed in a 

 nephrogenic region, or brought in contact 

 with nephrogenic tissue, led to the concept 



