22 Development and Shape of Uriniferous Tubules 



lobules, as will be stated more fully later.) The repeated division of 

 the branches of the primary renal pelvis, as here briefly sketched, results 

 in systems of branched, relatively straight tubules, which extend from the 

 pelvis of the developing kidney to the periphery of the organ and are 

 recognized by authors generally as the anlagen of the straight collecting 

 tubules. Such tubules are lined throughout by epithelium having rela- 

 tively clear protoplasm and round or oval nuclei, the arrangement of 

 which differs somewhat in the earlier branches from that found in the 

 later branches. In the former, the larger tubules, those nearer the 

 pelvis, the epithelium is for the greater part of embryonic life of a 

 pseudostratified type, while in the smaller tubules, those nearer the per- 

 iphery, the epithelium is simple columnar. The division of the end 

 branches of these tubular systems continues without much variation from 

 the manner here described to about the time of birth or until the final 

 number of collecting tubules has been formed. Hamburger states that 

 the division of the collecting tubules terminates in the human embryo in 

 the fifth month of foetal life, in the pig in embryos about 14 cm. in 

 length, in the rat and mouse at about birth. The details of the devel- 

 opment of the collecting tubules Avill not be entered upon here, as the 

 necessary data are not yet in my possession. This requires a much more 

 extensive series of reconstructions at different stages of development 

 than I have been able to make, and not for one animal, but for a series 

 of animals, as my limited observations indicate that, while the general 

 plan of development of the straight collecting tubules is the same for 

 different mammals, variations in detail are to be looked for. . This may 

 be seen also from Hamburger's account, who briefly considers this ques- 

 tion, basing his observations on teased preparations. For the purpose of 

 this account, which is concerned with the development and shape of the 

 uriniferous tubules proper, the brief statement will suffice that the 

 straight collecting tubules are developed by a process of continuous 

 growth and of repeated divisions of the ends of the tubules which grow 

 from the primary renal pelvis. These divisions take place at the per- 

 iphery of the organ (or of the primary lobes, when present) immediately 

 under the capsule. It has been stated that with each division of the 

 ampullar enlargement of an end branch of a system of developing col- 

 lecting tubules, the inner zone of the metanephrogenic tissue likewise 

 separates into parts which surround the ends of the resulting branches. 

 During the development of the kidney, this tissue proliferates by mitotic 

 division. At all stages of development it is found surrounding the 

 ampullar enlargements of the end branches of the developing collecting 

 tuhules as a layer consisting of one or two rows of cells, possessing oval 



