40 Development and Shape of Urinifcrous Tubules 



tures are more crowded and separated by a relatively small amount of 

 interstitial tissue; consequently the tubular anlagen which develop at 

 this period, when they are considered as a whole, do not present a rounded 

 form, but a form which is elongated from above downward. This may 

 be seen when comparing C and D of Fig. 4, and B and C of Fig. 6, with 

 the two anlagen shown to the left of the two prominent branches as 

 shown in Fig. 9, and no doubt explains, in part at least, the difference in 

 form of early stages of tubular anlagen as given in Stoerk's figures and 

 the majority of those here represented. I am led to assume, both from 

 a study of his figures and from the statements which he makes, that it 

 is advisable to study the anlage and early developmental stages of the 

 uriniferous tubules in as young embryos as possible, because the ele- 

 ments of the neogenic zone are here farther apart; that his reconstruc- 

 tions are made from ver}^ young embryos, while my own were made from 

 relatively older stages — the series shown in Fig. 4 from a human embryo 

 of the seventh month. Besides such differences in shape as here indi- 

 cated, there are observed in tubular anlagen which show essentially the 

 same stage of development, especially when these are seen in sagittal 

 section, minor differences in shape involving the extent of the curvature 

 of different parts and the relative size and shape and degree of differ- 

 entiation of the lower S-curve, the saucer-shaped expansion of the 

 S-shaped tubular anlage, when this is compared with its other parts. 

 Of the series of models in my possession showing the S-stage of develop- 

 ment of the uriniferous tubules, no matter whether we speak of those 

 made from different species or of those from the same species, no two 

 are exactly alike or even nearly so. They all show certain character- 

 istics of form which enable a classification as to stage of development, 

 but differ when compared in detail. This is more particularly true as 

 concerns the form and size of the lower S-curve and its relation to the 

 S-middle piece. The lower S-eurve is developed primarily, as will be 

 remembered, not by an invagination of the outer wall of the renal 

 vesicle, but by a cleft which develops in a thickening of the outer wall. 

 The extent of this cleft varies. It may not, as has been stated, extend, 

 when first recognizable, on to the front and back surfaces of the renal 

 vesicle and may be quite deep before the lower part of the vesicle, that 

 part which will develop into the lower S-curve, has by extension of the 

 cleft to any appreciable extent been separated from the part of the renal 

 vesicle just above it, the part which will form the S-middle piece. Such 

 a structure in reconstruction does not present the form of an S, but may 

 do so in sagittal sections. On the other hand the cleft which appears in 

 the outer wall of the vesicle may, almost from its beginning, extend for 



