56 Development and Sliapc of Uriniferous Tubules 



out and the respective Malpighian corpuscles are nearly developed; one of 

 these is on the rear side of the model as sketched and shows only in part, 

 between the two prominent diverging branches. The relation of these 

 structures to each other and to the collecting tubule is shown with sufficient 

 clearness in the figure to obviate fuller discussion. In Fig. 10, is shown 

 a similar reconstruction for a human embryo of 6.5 cm. length. The 

 long diameter of the kidney from which this reconstruction was made 

 measures 5mm. after fixation. The collecting tube reconstructed presents 

 five and in some branches six successive dichotomous divisions. Of the 

 four branches resulting from the second division, only one was recon- 

 structed in full, the others are represented as cut soon after they are 

 separated. That the collecting tubules grow in length along their entire 

 extent as the kidney develops is shown by the fact that a greater distance 

 intervenes between the successive branches than is shown in the former 

 stage; the angle at which the branches meet is for the older stage more 

 acute than for the earlier stage. During this growth in length of the 

 collecting tubules, they obtain a smaller diameter and consequently a 

 smaller lumen ; especially is this true of the later generations of branches. 

 In connection with the one large branch here fully reconstructed, there are 

 found in the model five renal vesicles and 14 tubular anlagen and urinifer- 

 ous tubules in various stages of development, only a portion of which 

 could be represented in a sketch presenting one view of the model. The 

 complexity of this model is such that a reproduction of it can give only 

 in a general way the shape, size, and relations of the various structures 

 modeled and this can be shown in one figure quite as clearly as in several 

 figures drawn from different aspects of the model. In Fig. 11 is repro- 

 duced one of the most typical sections of the series from which the model 

 shown in the former figure was made ; it falls in a plane which passes 

 through about the middle of the model. It will serve to show the struc- 

 ture and cellular differentiation of certain representative tubules and 

 tubular anlagen shown in the reconstruction ; a comparison of the 

 two figures will enable the identification of the respective parts, as 

 represented in each figure, as they are drawn to the same scale. In the 

 section reproduced, a collecting tubule was cut through its entire length 

 and may be seen to end in characteristic ampullar enlargement. The 

 epithelium lining the collecting tubules is here shown as a single layer of 

 columnar cells which in the primary collecting duct presents the appear- 

 ance of a pseudostratified epithelium. In each of the two models (Fig. 

 9 and 10) all of the renal vesicles, tubular anlagen and uriniferous 

 tubules, with the exception of one uriniferous tubule, are associated with 

 or attached to the ampullar enlargement of the end branches of the 



