G. Carl Hubcr 65 



columnar epitlielium. confirming thus the generally accepted view of the 

 -size and structure of the two aruis of Henle's loop. 



Peculiarities of Form Presented by the First Developed 

 Uriniferous Tubules. 



Before leaving the discussion of the earlier stages in the development 

 of the uriniferous tubules, mention may yet be made of difEerences in 

 development presented by tubules which develop from the first few genera- 

 tions of renal vesicles, when compared with those which develop from 

 the later generations. Such differences of development are expressed 

 mainly in the relative degree of development shown by the different 

 parts of the tubules which are first formed, when these are compared 

 with those which develop later. Tt should, however, at the beginning be 

 stated that such differences in development as shall here be mentioned 

 are not of such a character as to form exceptions to the statement that 

 the uriniferous tubules which develop from the various generations of 

 renal vesicles present essentially the same developmental stages as have 

 been given in the preceding pages. As has previously been stated, 

 tubules which develop from the earlier generations of renal vesicles 

 present in the S-stage and for some little time after, when their con- 

 figurations are taken as a whole, a more rounded form than do those which 

 develop from later generations of renal vesicles. This difference in form 

 when the one type is compared with the other, is also observed in tubules 

 of the respective types in which the anlagen of the different parts of the 

 nriniferous tulnile may be clearly made out in that the first developed 

 tubules present at this stage, an irreguhu-ly spherical mass when the 

 developing Malpighian corpuscle and the tubular complex are taken 

 together and considered as a whole. (See E and F of Fig. 5). The 

 tubules of siuiilar stage of development derived from later generations of 

 renal vesicles, likewise considered, form a more elongated mass. (See 

 G of Fig. 6). This, as has been stated, is in part at least due to the 

 fact that during the earlier stages of development of the kidney, the tubu- 

 lar structures are relatively far apart and are surrounded by a relatively 

 large amount of mesenchymal tissue and may thus in growth and elonga- 

 tion expand in all directions, whil(> in later stages of development the 

 tubular structures are in much closer proximity, separated by only a 

 relatively small amount of mesenchymal tissue; this juxtaposition to 

 surrounding tubules influencing the direction of their growth. In their 

 further development, tubules which are developed from tlie first few 

 generations of renal vesicles are characterized by a relatively early and 

 marked elongation of those portions of the tubule destined to form the 



