G. Carl Hiiber 19 



embryos for corresponding stages are in all essentials as here described 

 and figured for the human- embryo. In each type the inner zone of the 

 metanephrogenic tissue consists at this stage of generally two, sometimes 

 three, layers of cells, which, as other observers have stated, have the ap- 

 pearance of epithelial cells, always surrounding the ampullse of the pri- 

 mary collecting tubules or of their branches much as shown in Fig. 2. 

 The same may be said as concerns the structure and disposition of the 

 outer zone of the metanephrogenic tissue and of the capsule anlage. 



If the inner zone of the metanephrogenic tissue found surrounding the 

 right ampullar extension as shown in Fig. 2 (&') is traced in sections 

 preceding and following the one shown in the figure, it will be seen that 

 its border is not for its entire circumference an even one, but that it 

 presents a bud-like prolongation which extends for a short distance along 

 the side of the collecting tubule. In this prolongation, which is here cut 

 through its center, the cells show an arrangement in two distinct layers, 

 continuous at the end of the prolongation. I v/as led to recognize this 

 fact by Schreiner's description of similar stages. Such bud-like prolonga- 

 tions of the inner zone of the metanephrogenic tissue early acquire a 

 narrow lumen, around which the cells assume a radial position, certain 

 cells of the bud in the region of its junction with the main mass of the 

 nephrogenic tissue turning with their inner ends toward the lumen. In 

 this way the bud becomes separated from the main mass of the inner zone. 

 In the figure this process of separation is shown at its very beginning; 

 a lumen is made out with difficulty and one cell, the fourth in the inner 

 row, appears to have been fixed in the act of turning. In the further 

 differentiation of such a cell-mass, the lumen increases in size and the 

 cells surrounding it increase in length, becoming more distinctly columnar 

 in shape. The whole presents now the appearance of a small vesicle, the 

 wall of which is formed by a single layer of columnar cells, which during 

 its differentiation has completely separated from the nephrogenic tissue. 

 Such a vesicle is shown to the left in Fig. 2 (e). A number of investi- 

 gators have observed such vesicles and have interpreted them as represent- 

 ing anlagen of uriniferous tubules. They were first described and quite 

 correctly figured by Emery, who speaks of them as " vesicules renales," 

 later by Kiede, very briefly by Hamburger, Chievitz, and Ribbert, and 

 quite accurately by Herring, whose excellent photographic reproductions 

 are worthy of study, and finally by Schreiner, who gives a minute and ac- 

 curate account of their origin and structure in the different types of 

 animals studied by him. The account here given, based on sections of 

 human embryos, is readily verified in sections of rabbit, cat, and pig 

 embryos of corresponding stages. In each type small vesicles, which we 



