294 J- B- Haycraft, 



it to the pelvis of the Kidney, and one or perhaps two others running 

 deep into the cortex and becoming the convoluted tubule of a 

 Malpighian body more deeply situated. 



Older Embryo Rabbits. 



It may be well to finish the development of the organ in rough 

 outline leaving out for the present the details of the process for sub- 

 sequent consideration. By the 25 th or 26 th day the papillary part 

 of the Kidney is well seen, and there are a larger number of tubules 

 opening into the pelvis due to the process of evagination already 

 alluded to. There are several sets of Henle's loops, the oldest set of 

 all can be traced into the papillary region, the youngest are seen at 

 the cortex. In this region there are some five or six sets of Malpighian 

 bodies and urinary tubules, those near the medulla are well developed, 

 and quite at the cortex the youngest ones are budding off the primary 

 renal vesicles. All the tubules, even those that were first formed are 

 still seen as offshoots of the vesicles. 



Tubules evaginateti to form the Pelvis. 



It may not unreasonably be asked, on what evidence does the 

 statement rest that evagination of the tubules takes place? The evi- 

 dence is as follows: the number of tubules opening into the ureter 

 thi'ough the pelvis increases during development and each tubule — 

 this was tested by most careful examination — passes up and ends 

 by its branches in the primary renal vesicles. There is no evidence 

 that new tubules sprout out from the pelvis, or grow towards it from 

 the cortex, so that except by evagination the increase in the number 

 of tubules cannot possibly be accounted for. Moreover the process of 

 evagination can actually be traced out, for in the embryo the ends of 

 the tubules are seen to become dilated and the epithelium lining these 

 dilatations assumes the character of the epithelium lining the pelvis. 

 The formation of new urinary tubules from the renal vesicles occurs 

 in the rabbit until birth, and probably for some time during extra- 

 uterine life. In the newly born and milk rabbit the primary i-enal 

 vesicles may be seen, but these disappear, Ihe Kidney increasing in 



