14 Developinciit and Shape of Urinit'crous 'i'nljiilos 



the narrowed middle portion, there spring three pairs of branches, each 

 of which, as soon as clearly differentiated, presents a short stalk and a 

 bulbous end. The entire metanephros now has a position whicli is dorsal 

 to the hind end of the niesonephros. The metanephrogenic tissue, which, 

 during the earlier stages, formed a continuous layer entirely surrounding 

 the primary renal pelvis, presents a different disposition with the appear- 

 ance of the ])rimary l)ranelies ami tliis applies more particularly to its 

 inner zone, which, while it shows the structural appearances observed in 

 earlier stages, presents a characteristic distrilnition. On the appearance 

 of the primary l^ranches, the inner zone of the metanephrogenic tissue no 

 longer surrounds the entire primary renal pelvis, but only its enlarged 

 extremities and the bulbous ends of its branches, the narrowed middle 

 portion and the stalks of the branches being free from it. Tlie inner 

 zone of the metanephrogenic tissue is clearly recognized both l)y its 

 staining reactions and by its structural characteristics. The primary 

 renal pelvis and its branches, as also the inner zone of the metanephro- 

 genic tissue, are surrounded by a tissue that stains less deeply than the 

 inner zone and presents fewer nuclei, the outer zone of the metanephro- 

 genic tissue. The entire renal anlage is surrounded by mesenchymal 

 tissue, containing relatively few nuclei, these having a more or less con- 

 centric arrangement. For a fuller discussion of the anlage and early de- 

 velopmental stages of the renal evagination and the accompanying changes 

 presented by the nephrogenic tissue as observed in the rabbit, the reader 

 is referred to Schreiner's paper (pages 98-121). ]\[y own observations 

 confirm his results in full except as concerns the anlage of the meta- 

 nephrogenic tissue, where the necessary stages are wanting in the ma- 

 terial at my disposal. A separate discussion of the anlage and early 

 developmental stages as observed in cat, pi^, and human embryos does 

 not seem necessary here, as this would involve unnecessary repetition, 

 since the types studied present great similarity in the shape of the renal 

 anlage and of the earlier stages of its metamorphosis. This is true, not 

 only when considering types with simple non-lobulated kidneys as the 

 cat and rabbit, but also wdien comparing these with types having lobu- 

 lated kidneys as man and pig. In each type a renal evagination grows 

 in a dorsal direction into the nephrogenic tissue and in its further growth 

 differentiates into a primary renal pelvis and ureter, the former l^eing 

 enclosed and surrounded by the nephrogenic tissue, which in rabbit and 

 human embryos permits of a differentiation into an inner and outer 

 zone of nephrogenic tissue, as described by Schreiner, while in pig and 

 cat embrj^os of corresponding stages, a differentiation of nephrogenic 

 tissue into two zones is at this stage not made with certainty. (Compare 



