34 Development and Shape of Uriniferous Tubules 



mirror pictures to those here given and show the same relations to the 

 collecting tubules. They show similar inner and outer, front and back 

 surfaces as above described, these designations having reference to the 

 relations of any tubular anlage to the collecting tubule to which it is 

 attached, independent of the fact that such anlage may be placed to the 

 right or left of the collecting tubule. With this definition of the ter- 

 minology to be used, we may now proceed to a fuller consideration of 

 the S-shaped stage and state that the upper S-curve represents a tubule 

 with narrow lumen having an outward curvature and extends without 

 definite boundary into the S-middle piece, which is also of cylindrical 

 shape, representing a short tubular segment also with narrow lumen and, 

 like the upper S-curve, is composed of a single layer of cells with oval 

 nuclei. When first recognized, it has a nearly horizontal position; but 

 as development proceeds and the S-shape of the tubular anlage becomes 

 more pronounced, it loses this horizontal position and inclines upward 

 toward the collecting tubule. This portion of the anlage is not so clearly 

 defined as the other parts and the relative time of its differentiation 

 varies somewhat, as will be more fully stated later. The lower S-curve, 

 as has been stated, is not of cylindrical shape, but from the time of its 

 anlage is flattened from above downward and presents the form of a 

 double-walled saucer or shallow bowl with concavity directed upward, 

 the cavity which extends into this portion being likewise flattened and 

 presenting a similar shape. The cells forming the upper wall of this 

 structure are of columnar shape, those forming the lower wall of cubical 

 shape or even more flattened, depending on the stage of development. 

 The saucer-shaped structure becomes narrowed as the region where it 

 joins the S-middle piece is approached and here gradually changes (in 

 the most typical anlagen) from a flattened to a nearly cylindrical struc- 

 ture. The place of junction of the two parts is indicated by a sharp 

 bend in the lumen of the tubular anlage as this is seen in sagittal sections 

 and may be indicated by a nearly equally sharp bend when the anlage 

 is seen in reconstructions. The lower end of the upper S-curve, with 

 a portion of the S-middle piece, rests in the concavity of the saucer- 

 shaped portion representing the lower S-curve so that only a narrow 

 space separates these parts. In sagittal sections this space appears as a 

 curved cleft and contains, soon after its anlage, a delicate strand of 

 mesenchymal tissue. (See C of Fig. 3.) In Z) of Figs. 3 and 4 is 

 shown a slightly older tubular anlage, which differs from the former in 

 that the place of union between the tubular anlage and the collecting 

 tubule is no longer recognized and in the more pronounced curvature 

 of the anlage as a whole. This is especially clearly seen in the section 



