G. Carl Huber 



17 



two parts as the ampulla becomes flattened and the separation becomes 

 complete as the buds or lateral branches develop, each becoming in this 

 way capped by an inner zone of metanephrogenic tissue. The stage here 

 reached is the one shown in the reconstruction reproduced in Fig. 1. 



Renal Vesicles. 

 In Fig. 2 is shown a portion of a longitudinal section of a developing 

 kidney of a human embryo 18 mm. in length and presenting essentially 

 the same development of primary renal pelvis and branches as that in B of 

 Fig. 1 ; this shows a longitudinal section of a primary collecting tubule, a, 

 the distal end of which shows an ampulla, slightly flattened and present- 

 ing on each side a lateral extension recognized as the anlage of a branch. 

 The section from which the sketch was made does not pass through the 





Fig. 2. From a longitudinal section of a developing kidney of human 

 embryo (No. 4), 18 mm. in length, x 233. A, primary collecting tubule 

 with ampulla; h. &', inner zone of metanephrogenic tissue; c, outer zone of 

 metanephrogenic tissue; d, anlage of capsule; e, renal vesicle. 



center of the tubule. It is owing to this that its wall has the' appearance 

 of being composed of stratified epithelium. In other sections of this 

 tubule, as also in sections of primary collecting tubules at corresponding 

 stages from cat, rabbit, and pig embryos, it may be seen that their wall is 

 composed of a single layer of cylindrical cells, varying somewhat in length, 

 tlie nuclei of which are not always in the same plane. In the tissue sur- 

 rounding the tubule and ampulla here figured, the following may be seen : 

 Immediately surrounding the lateral extensions of the ampulla and ex- 

 tending for a short distance along the tubule there are recognized groups 

 of cells, 6, &', which may be separated from the surrounding cells. These 

 groups of cells we may know as the inner zone of the metanephrogenic 

 tissue. The cells of this tissue are indistinctly bounded and possess 



