24 Development and Sliapo of Urinifcrous Tubules 



Differentiation of the Eexal Vesicles. 



We may now consider the further developinent of the renal vesicles. 

 As has been stated, these are differentiated from the inner zone of the 

 metauephrogenic tissue and, when completely separated, consist of a 

 single layer of columnar cells with oval nuclei which stain relatively 

 deeply and surround a central lumen. At this stage of their develop- 

 ment they are generally quite definitely circumscribed and have a shape 

 which varies from that of an irregularly spherical body to that of an 

 egg. They lie in close relation with the collecting tubules and their 

 ampullar enlargements, these being often slightly flattened in the region 

 of their contact with the vesicles. At least two renal vesicles develop 

 in connection with each ampullar enlargement; these do not, however, 

 present the same degree of differentiation and development, as may be 

 seen from Fig. 2, and is more clearly seen in renal vesicles which de- 

 velop in connection with the ampullar enlargements of the end branches 

 of collecting tubules which appear after the first and second division of 

 the branches, which arise from the renal pelvis. The branches resulting 

 from each successive division of a collecting tubule form with this soon 

 after their anlage a T-shaped structure; in their peripheral growth, the 

 angle between such branches becomes smaller and they form with the 

 collecting tubule a Y-shaped structure, each branch developing an am- 

 pullar enlargement surrounded by nephrogenic tissue. A renal vesicle 

 develops from the nephrogenic tissue on the outside of each branch 

 (therefore nearer the pelvis of the kidney) earlier than from the nephro- 

 genic tissue between the branches. (See also Schreiner, especially text- 

 figure 34.) The renal vesicles, soon after their separation from the 

 nephrogenic tissue, increase in size by active proliferation of their cells 

 by mitotic cell division, the vesicle elongating somewhat and coming in 

 close contact with the ampullar enlargement of the collecting tubules. 

 At the same time it may be observed that the outer wall of the vesicle, 

 that away from the collecting tubule, and especially its upper portion, 

 becomes thicker than the wall of the vesicle in contact with the collect- 

 ing tubule. This thickening of the outer wall of the vesicle was first 

 described by Schreiner and fully discussed by him. He states, and in 

 this my own observations confirm his account, that the cells of the middle 

 and upper portions of the outer wall of the vesicles elongate, the cells 

 here appearing longer when the vesicles are in sagittal section^ than in 



^ By a sagittal section of a renal vesicle is here meant a section which cuts a col- 

 lecting tubnle in longitudinal direction and parallel with its lumen and passes at the 

 same time through a renal vesicle which may develop by its side, thus cutting the renal 

 vesicle in a plane which passes through the middle or nearly the middle of its outer 

 wall. The renal vesicles seen in Fig. 2 and A of Fig. 3 are cut in sagittal section. 

 The plane of a frontal section of a renal vesicle passes perpendicularly through it and 

 would intersect the plane of a sagittal section at right angles. 



