36 Development and Shape of TJriniferous Tubules 



results from a slight readjustment of the cells constituting this part of 

 the vesicle as well as by their proliferation. In the further develop- 

 ment, this cleft enlarges by deepening and by extending laterally so as 

 to involve more than the outer wall of the vesicle. In this way the lower 

 and outer part of the original vesicle becomes separated from the main 

 part of the vesicle and presents the appearance of a lip, which projects 

 upward toward the periphery of the kidney. Into this lip-shaped por- 

 tion the lumen of the vesicle extends. While these changes in form and 

 structure Avhich affect primarily the lower part of the vesicle are taking 

 place, the upper part of the vesicle elongates, sometimes more and some- 

 times less, growing toward the respective collecting tubule, more cor- 

 rectly its ampulla; at the same time, a slight depression appears in the 

 uppermost part of the inner wall of the vesicle, that in apposition with 

 the wall of the collecting tubule, which emphasizes the curvature of the 

 upper part of the vesicle toward the collecting tubule. In A of Fig, 3 

 is shown the first indication of this second depression in the wall of the 

 renal vesicle at a point marked by a cross. The end result of these 

 changes in form and structure on the part of the renal vesicle are rep- 

 resented in B of Figs. 3 and 4. It may be seen that by a deepening of 

 these clefts, the more prominent one in the outer wall, the less promi- 

 nent one in the upper portion of the inner wall of the original vesicle, 

 and by a growing of its upper end toward the collecting tubule, the 

 vesicle has been altered in its shape so as to present in sagittal sections 

 and at times also in reconstructions the form of an S. In the sagittal 

 section of this renal vesicle, or, as we shall now know it, the tubular anlage 

 (the word uriniferous being understood) is cut longitudinally and 

 through its middle in the section sketched for nearly its entire length, 

 the upper portion of the anlage deviating a very little from this plane 

 of section. The reconstruction of this tubular anlage, B of Fig. 4, does 

 not show this S-shape as clearly as certain of the sections of it. The 

 upper bend of a tubular anlage of about this stage presents a circular 

 and very narrow lumen. In the tubular anlage figured, the lumen of its 

 upper bend appears clearly only in the section sketched. In frontal sec- 

 tions of tubular anlagen of this stage, in which this upper bend would 

 appear in cross or slightly oblique section, the existence of a small lumen 

 in this portion of the anlage may be clearly made out, the part itself 

 being round or nearly so. It constitutes, therefore, a tubular structure, 

 its wall being made up of a single layer of columnar cells with oval 

 nuclei. Frontal sections of a tubular anlage of this stage further show 

 that its lawer bend does not represent a structure having a tubular form, 

 but is flattened from above downward, presenting in such sections the 



