18 Development and Shape of Uriniferous Tubules 



relatively large oval nuclei which stain rather deeply and have a radial 

 position with reference to the ampulla and tubule. In the preparation 

 figured they are arranged in two rows quite clearly defined. This inner 

 zone of the metanephrogenic tissue presents a sharp demarcation toward 

 the ampulla and tubule, emphasized in the preparation sketched by reason 

 of the fact that the ampulla and tubule were slightly contracted during 

 the process of hardening. Herring has called attention to similar appear- 

 ances presented by his preparations. In the relatively thin sections into 

 which my entire material was cut, the distinct demarcation between the 

 inner zone of the metanephrogenic tissue and the epithelium of the pri- 

 mary collecting tubules and their ampullar enlargements may nearly 

 always be readily made out, if the plane of the section passes through the 

 lumen of the tubules and their ampullar enlargements, thus giving a cross 

 section to their epithelial walls. 



Such appearances as described by Sedgewick, when he states " Some of the larger 

 cohimnar cells of the kidney tnbnles become branched, the processes being continnovis 

 with the processes of the " branched cells of the kidney blastema. In fact, every 

 stage between a columnar lining cell of the tubule and a branched cell of the blastema 

 Is visible " — I have not observed. The same may be said of the observations of 

 Riede, who regards the epithelium of the renal evagination and of the primary col- 

 lecting tubules as contributing to the formarion of tlie renal blastema. It is only in 

 relatively thick sections or in sections which pass obliquely through the epithelial 

 walls of a primary collecting tubule without including the lumen, in which the nephro- 

 genic tissue overlaps the tubular epithelium that a distinction between the two tis- 

 sues is not readily made. Herring, who speaks of a cap or mantle of cells lying 

 over the end of each ampulla, speaks of a sharp line of demarcation between the 

 tube and the cap or mantle of cells. Ribbert describes groups of cells which surround 

 cap-like the ends of collecting tubules. These groups consist of two or three layers 

 of epithelium-like cells which may be separated from collecting tubules on the one 

 side and from " renal blastema " on the other. Schreiner, for all the types studied by 

 him, describes and figures a distinct demarcation between the inner zone of the 

 metanephrogenic tissue and the primary collecting tubules (Nierengangiiste). 



Immediately surrounding the inner zone of the metanephrogenic tissue 

 there is a tissue in which the cell boundaries are also indistinct, with rela- 

 tively large, round or oval nuclei, which show no definite arrangement, c, 

 and which do not stain so deeply as the nuclei of the inner zone. This 

 constitutes the outer zone of the metanephrogenic tissue. This zone 

 blends with a tissue which is recognized as mesenchymal tissue. In the 

 mesenchymal tissue, there is recognized a layer in which the majority of 

 the nuclei show an elongated oval form and fairly regular arrangement, 

 their long axis being parallel to the outer boundary of the kidney anlage, 

 entirely surrounding it. In this layer is recognized the anlage of the 

 capsule of the kidney, d. External to it, there is seen a mesenchyme 

 containing relatively few nuclei, not sketched in the figure. The entire 

 renal anlage measures in the embryo from Avhich Fig. 2 was taken almost 

 exactly 1 mm. in length. In the stained sections it is readily recognized 

 with the naked eye as a small area, staining more deeply than the sur- 

 rounding tissue and having a bean or kidney shape. The appearances 

 presented in sections of the developing kidney of rabbit, cat, and pig 



