12 Dcvclopmont and Shape ul' L'rinil'evous Tubules 



cat; this animal lie did not study, but it ])resonts no special features. 

 Schreiner considers very fully the anlage and early developmental stages 

 of the kidney of the rabbit. A brief review of his account may here be 

 given. In the rabbit, the renal evagination arises in the 31st segment 

 (about the 11th day, Kolliker, Minot) from the dorso-median wall of the 

 Wollhan duct near its termination in the cloaca and grows into a tissue 

 known as the nephrogenic tissue, which soon surrounds its blind end as 

 a cap of hemispheric shape. As concerns the renal evagination, 

 Schreiner's and my own ohservations confirm those of other investigators, 

 wlio, since the appearance of Kupffer's contribution, have studied the 

 anlage of the kidney in amniota. Opinions differ, however, as concerns 

 the histogenesis of the tissue generally known as the renal blastema here 

 designated as the nephrogenic tissue and as to the role it plays in the 

 development of the kidney. The general statement may be made that 

 observers who regard the kidney tubules as developing by a process of 

 budding, interpret the nephrogenic tissue as of mesencliymal origin, 

 destined to form the capsule, interstitial connective tissue and vascular 

 sheaths. Concerning it Minot states that " the histogenesis of the 

 mesenchymal portion of the kidney is almost unknown." The majority 

 of those observers who accept a separate origin for the kidney tubules have 

 traced their origin to a special tissue (nephrogenic tissue, renal blastema) 

 the histogenesis of which may here be considered, although in doing so 

 it will be necessary to anticipate certain facts which will l)e considered 

 more fully later. This seems justified, since the tissue in question 

 makes its appearance before the renal evaginations are present. 



Braiin, In his accouct of the development of the urogenital system of reptiles (lizards), 

 described irregular cords of cells (Nierenzellenstrang, generallj' known as Braun"s 

 cords), whose origin he traced to the coelomic epithelium and which were thought 

 to be the anlage of the uriniferous tubules. Minot regards these cords " as merely the 

 beginning of the condensed mesenchyma of the renal anlagen," and Schreiner regards 

 them as identical with the cell-mass designated by him as nephrogenic tissue. Wie- 

 dersheim. in his account of the de\e](>iiment of the urogenital apparatus of crocodile 

 and turtle embryos, confirms in part Braun's observations, differing from him in 

 that he regards the caudal end of the mesonephros as the seat of origin of the cell- 

 masses or cords from which the kidney tubules are developed. He recognizes further 

 a condensed mesenchyme which surrounds the metanephric duct and which he char- 

 acterizes as staining more deeply than the surrounding mesenchyme with which it 

 blends. This proliferating cell-mass, which, as he states, is also found in other 

 amniot embryos can uot alone be regarded as the " metanephros blastema." as it 

 in itself does not give origin to the strictly glandular parts of the kidney, but the 

 interstitial connective tissue and the vessel sheaths. Sedgewick, who gave a very clear 

 account of the anlage of the mesonephros and metanephros as observed in chick 

 embryos, traces the origin of the Wolfiian tuluiles to a cell-mass designated by him the 

 Wolffian blastema, which has its oi'igin in the intermediate cell-mass. As described by 

 him, the Wolffian blastema extends as far back as the 84th segment, that is, to the 

 opening of the Wolffian duct into thp cloaca. This Wolfl^an lilastema breaks up into 

 the Wolffian tubules as far back as the 30th segment. He states further that " be- 

 hind this point it remains for some time almost or quite passive and ultimately gives 

 rise to the epithelium of the permanent kidneys. In consequence of this. I have 

 called that part of the WolflBan blastema between the 31st and 34th segments the kidney 

 blastema,"' and again " that it is important to notice that this kidney blastema develops 

 in exactl.v similar manner to the Wolffian blastema. It is not until the fourth day. 

 when the ureters have appeared that it is possible to draw a line between the two." 

 '■ In yet older embryos, in which the ureter is more fully developed and overlaps 

 the hind end of the "Wolffian body, the kidney blastema has quite broken off from the 

 Wolffian body and invests the anterior end of the ureter." I have quoted thus fully 



