Urinogenital System 



465 



of a nephric field (see Holtfreter, '33). 

 Systematic experimentation showed that gas- 

 trula ectoderm may produce nephric tubules 

 when implanted into any part of a lateral 

 zone extending from the level of the gills 

 to the cloaca (Fig. 173). However, as with 

 embryonic fields in general, inductive power 

 is not uniform throughout this zone but is 

 strongest near the site of the pronephros. This 

 was shown by the relative frequency of neph- 

 ric differentiation in grafts placed at various 

 levels in the field. Furthermore, grafts of the 

 prospective pronephric area diminish both in 

 size and in degree of differentiation as they 

 are moved posteriorly from the normal posi- 

 tion (Fales, '35). 



Evidence of field effects may be adduced 

 with respect to other nephric regions. Gas- 

 trula ectoderm implanted in the mesonephric 

 area of the posterior nevxral plate (Fig. 171) 

 produces mesonephric tubules (Spofford, 

 '48); on the other hand, mesonephric ma- 

 terial moved forward into the pronephric 

 area forms pronephric tubules, and its rate 

 of differentiation is correspondingly accele- 

 rated (Machemer, '29). Finally, tubules of 

 mesonephric type may be induced to develop 

 precociously in the metanephric blastema 

 of the chick by the action of abnormal in- 

 ductors (Gruenwald, '42, '43). 



From such results it may be concluded 

 that: (1) a variety of foreign tissues, meso- 

 dermal and ectodermal, acquire nephrogenic 

 potencies if introduced into a nephrogenic 

 field or exposed to an appropriate inductor; 

 (2) in the differentiation of such materials 

 both the frequency and the special character 

 (pronephric or mesonephric) of the differen- 

 tiation are modified by the position of the 

 material in the field. It has been sviggested 

 (Machemer, Gruenwald, op. cit.) that capa- 

 city to produce nonspecific tubular struc- 

 tures is essentially of the same order at all 

 levels of the system, the specific character of 

 the tubule depending on local factors. The 

 appearance of sub-regions within a wider and 

 more generalized nephric field is to be re- 

 garded as an aspect of regional differentia- 

 tion in the embryo as a whole. 



It must be recognized, however, that the 

 inductive and integrative activities attributed 

 to the field may function to a remarkable 

 degree in relative isolation. Highly differen- 

 tiated pronephric structures may be obtained 

 from disorganized cell aggregates* differenti- 



* Taken from the dorsolateral lip of the blasto- 

 pore. The material may possess original pronephric 

 significance in part, or may acquire potency through 

 induction by materials from the organization center. 



ating in ectopic situations, or in vitro (Holt- 

 freter, '44). Aggregation and reintegration 

 of cell types on the basis of specific affinities, 

 and assimilation of other types by induction, 

 are postulated. In such tissue complexes it 

 is likely that the essential constituents of 

 the normal field environment are repre- 

 sented. 



Fig. 173. Influence of the host field in determin- 

 ing the differentiation of pronephros from gastrula 

 ectoderm (Holtfreter, '33). A, Sources of donor ecto- 

 derm taken from prospective neural {1) or epider- 

 mal (2) areas of the early gastrula; B, C, and D, 

 zones within which pronephric structures may dif- 

 ferentiate from such ectoderm, when transplanted 

 into host embryos of different ages. 



THE DIFFERENTIATION OF THE 

 PRONEPHROS 



With respect to the pronephros it has been 

 shown that during the gastrula stage exten- 

 sive regulations are possible, and that vari- 

 ous non-nephrogenic materials may be as- 

 similated with essentially normal results (p. 

 464). However, by middle or late neurula 

 stages (corresponding approximately with 

 final restriction of potency; see p. 463) ir- 

 reversible determinations have occurred. 

 [There has been no analysis of pattern and 

 axiation in the pronephros comparable to 

 the analysis of the limb disc (see Section VII, 

 Chapter 4).] In the frog Discoglossus, the 

 dorsoventral axis is apparently fixed by the 

 middle neurula stage, when inversion of 

 the primordium results in an inverted pro- 

 nephros (Tvxng, '35). In the late neurula. 

 division of the pronephric area is followed 

 by development of partial structures which 

 correspond with the plane of section (Holt- 

 freter, '44), and combinations of primordia 

 in middle neurula stages result in mosaic 

 organs with extra parts (Fales, '35). The 

 pronephric primordium is now essentially a 

 mosaic, and problems of a different kind are 

 introduced. To what extent are the individual 

 parts of the complex interdependent in final 

 development; and what is the role of the 

 field in their final differentiation? 



