Section VII 



CHAPTER 6 



Urinogenital System 



R. K. BURNS 



The urinary and genital systems of verte- 

 brates are related in the adult only in having 

 certain external passages in common; in 

 development, however, most of the internal 

 organs of reproduction are derived from 

 parts of the primitive nephric system. The 

 history of the remarkable transformations 

 involved has long been familiar from com- 

 parative morphological studies, but only in 

 recent years have advances in theory and 

 experimental techniques permitted rapid 

 progress in analyzing the mechanisms of 

 control and integration. As the morphological 

 precursor of many genital structures, the 

 nephric system must be given first considera- 

 tion. 



THE NEPHRIC SYSTEM 



In many vertebrates the nephric system 

 develops as three distinct entities — proneph- 

 ros, mesenephros and metanephros — which 

 appear successively in a regular temporo- 

 spatial order (Fig. 167). However, in certain 

 cyclostomes and primitive amphibians the 

 system is essentially continuous, with little 

 regional specialization (for a recent review 

 see Fraser, '50) . The units of the system at all 

 levels develop from the intermediate meso- 

 derm. The pronephric and more anterior 

 mesonephric tubules have a simple meta- 

 meric disposition, arising from discrete 

 nephrotomes; posteriorly the tubules differ- 

 entiate within an unsegmented cord of 

 nephric material — secondary and tertiary 

 elements, etc., developing as buds from the 

 primary units, or from residual nephrogenic 

 tissue in association with diverticula from 

 the nephric duct. [For general accounts 

 see Hall ('04); Gray ('32, '36); Hamilton 

 ('52). For the relationship between meso- 

 nephros and "definitive kidney" in amphib- 

 ians see Gray ('32) and Fraser ('50).] The 

 metanephros of amniotes develops entirely 

 in the latter fashion. Its nephrons differenti- 

 ate within a blastema, more or less continu- 



ous with the nephrogenic cord, in relation 

 with the vireteric diverticulum. Thus, the 

 tubules at all levels appear as independent 

 primordia and only secondarily unite with 

 the duct system. The nephric duct is laid 

 down as the duct of the pronephros, serves 

 subsequently as mesonephric duct, and as 

 such gives rise to the ureter. The problems 

 presented are concerned with (1) the history 

 and progressive localization of the nephro- 

 genic materials prior to the appearance of 

 definitive nephric primordia, and (2) the 

 nature of the integrative forces which co- 

 ordinate the later development of the various 

 parts of the system and relate them to the 

 regional environment. 



THE TOPOGRAPHY OF NEPHROGENIC 



AREAS IN EARLY STAGES OF 



DEVELOPMENT 



The first problem to be considered con- 

 cerns the topographic localization of the pro- 

 spective nephrogenic materials at successive 

 stages of development, leading up to the 

 appearance of discrete nephric primordia; 

 the related and concurrent problem of the 

 progressive determination of these materials 

 is dealt with elsewhere (Holtfreter and Ham- 

 burger, see Section VI, Chapter 1 ) . In early 

 development most embryonic organ systems 

 are not precisely localized, and after removal 

 of prospective organ-forming areas extensive 

 reorganizations are possible. Division of the 

 amphibian egg may be followed by essen- 

 tially normal development of its parts, in 

 which nephric structures show the same 

 regulative capacities as other systems. Bi- 

 lateral regulation of the nephric system 

 may even occur in dwarf embryos derived 

 from lateral pieces of the early gastrula 

 (Holtfreter, '38). At the beginning of gas- 

 trulation, however, there is a definite con- 

 centration of the prospective nephrogenic 

 material in the posterior region of the mar- 



462 



