G. Carl Huber 41 



a distance on its front and back surfaces, in which case the lower S-curve 

 and S-middle piece would be recognized in a reconstruction about as 

 soon as in sections. In both cases there develops from the renal 

 vesicle, b}^ a deepening of the cleft and by a growth of the structure, a 

 part which we have known as the S-middle piece and a portion known 

 as the lower S-curve. The former develops as a short tubular segment 

 of irregular cylindrical shape, the latter as a double-walled saucor- 

 shaped structure, with concavity upwards and presenting, as seen from 

 above or below, either an oval or more rounded or irregular triangular 

 shape, with a distinct border at which the two layers of the structure 

 are continuous. In the former case these parts are differentiated at a 

 relatively later period than in the latter, when the form of the anlage is 

 compared with its internal structure, as seen in sections. Before the 

 S-middle piece is completely differentiated, the border or edge of the 

 lower saucer or shell-shaped structure extends for a variable distance 

 on to the S-middle piece and recedes gradually from this as it be- 

 comes more clearly defined. So far as my observations go, from a 

 time when the S-middle piece may be considered as fairly well defined, 

 it can be traced as such into the older developmental stages and is not 

 taken up into the lower S-curve as this changes from a saucer-shaped 

 structure to one having a deeper concavity and presenting the shape of 

 an irregular hemisphere, as is stated by Stoerk. This appears to be 

 the case in tubular anlagen in which the border of the lower S-curve or 

 saucer-shaped structure extends for an appreciable distance on to the 

 S-middle piece, but this I interpret as due to a relatively late differ- 

 entiation of the S-middle piece and not as showing that it is being taken 

 up into the lower S-curve as in tubular anlagen, where the S-middle 

 piece differentiates relatively early, this appears to form a definite 

 tubular segment, which is not lost in its further development. As a 

 matter of fact, not all of the lower S-curve, as seen for instance in a 

 tubular anlage such as is shown in D of Figs. 3 and 4, differentiates 

 into the double-walled concavo-convex structure, which forms, in its 

 further development, a Bowman's capsule; but this point shall be dis- 

 cussed more fully in presenting the further stages in the development. 

 I see, therefore, no reasons for recognizing a primary S and a secondary S, 

 as is done by Stoerk, my observations, as stated, loading me to think that 

 the different parts of a tubular anlage which would correspond to his pri- 

 mary S-stage,so far as differentiated, continue as such into the stage which 

 he has designated as the secondary S-stage, the various parts — upper 

 S-curve, S-middle piece, and lower S-curve — being in the latter more 

 clearly defined than in the former; especially is tliis true of the S-mifldle 



