30 Development and Shape of Uriniferous Tubules 



cerns this point. I have, however, accepted Schreiner's account of the 

 processes of union of the tubular anlagen and collecting tubules as the 

 most satisfactory that can be given. In Fig. 5 are reproduced a series 

 of wax reconstructions of the anlage and early developmental stages of 

 uriniferous tubules obtained from rabbit embryos; in Fig. 6, a similar 

 series obtained from eat embryos, and in Figs. 9 and 10, a primary col- 

 lecting tubule, with its branches, the renal vesicles, tubular anlagen, and 

 uriniferous tubules in early stages of development associated with them. 

 These were obtained from kidneys of human embryos of 3 cm. and 

 6.5 cm. length respectively. In A and B of Fig. 5 are represented in 

 each instance a collecting tubule with prominent ampullar enlargement 

 and a renal vesicle. In A the renal vesicle is of spherical shape, in B 

 of egg shape, the latter representing a slightly older stage, showing, 

 however, as yet no characteristic differentiation; both are distinctly 

 separated from the respective collecting tubule. In C of Fig. 5 is repro- 

 duced a tubular anlage, which had united with the collecting tubule at a 

 relatively early stage, before presenting the S-shape to which attention 

 has been called. The lumen of this tubular anlage, as seen in sections 

 of it, shows this shape more clearly than does the reconstruction. The 

 same may be said of the tubular anlage shown in A of Fig. 6, the earlier 

 stages of the series being here not reproduced. These two figures have 

 been introduced to show that the tubular anlagen do not always present 

 the same degree of development and differentiation at the time of their 

 union with the collecting tubules. Characteristic and constant differ- 

 ences in this respect I have not observed for the different animals studied. 

 Variations in the time of fusion of the tubular anlagen and the col- 

 lecting tubules within the limits shown in these figures may be observed 

 in a series of sections from developing kidneys obtained from embryps of 

 the same species of animals. Such fusion does not take place, so far 

 as my observations go, prior to a time when the renal vesicles show a 

 distinct indentation of their outer wall, the lumen thus presenting the 

 shape of a hook, but may take place before the inner upper portion of 

 the vesicles or tubular anlagen show an indentation leading to the for- 

 mation of the upper curvature; in this case the curvature develops after 

 fusion has taken place. A study of the tubular anlagen shown in Fig. 9 

 and a comparison of these with those shown in Figs. 4, 5, and 6, will 

 prove instructive, especially the two tubular anlagen seen to the left of 

 the upper ends of the two prominent diverging branches of the collecting 

 tubule. Since the latter are sketched from a different view than those 

 of the previous figures, they represent tubular anlagen just after fusion 



