Developiiieiit of the Excretory Oijçnns of lidellostomn stoiiti Lockiiijçton. 215 



the process has been carried so far, that in some cases the lumen 

 from one tubule extends back ahnest to the lumen from the next, while 

 in other cases the one is in the act of breaking through into the 

 other, or the break has actually occurred, so that the lumen extends 

 uninterruptedly through two segments. This is shown in Fig. 11, a 

 longitudinal section of the tubules and ducts through six segments, 

 which was reconstructed on millimeter paper. 



The tubules back to the twenty-seventh segment, that is to a 

 point a little in front of the region of the pronephros of the adult, 

 still communicate with the general body-cavity, while from here caudal- 

 wards such communication has been lost. 



As in stage A, so here, the excretory system is in essentially 

 the same stage of development throughout the regions of both pronephros 

 and mesonephros of the adult, and from a study of the system itself, it is 

 impossible to tell which part is to develop into pronephros, and which 

 into mesonephros. The tubules here still stand at right angles to the 

 segmental duct, so that transverse sections of the embryos give longi- 

 tudinal sections of the tubules. Such a section is given in Fig. 12, 

 which, however, is from the older embryo of stage B. It will be 

 seen to lie at quite a distance from the body- cavity. The tubule is 

 distinctly longer than in stage A. The original coelomic pocket is 

 clearly seen, and it has the same relation to the tubule as in stage A. 

 A series shows that as a rule it is wider than in the previous stage. 

 In the angle between the tubule and the dorsal wall of the coelomic 

 pocket a condensation of the mesenchyme indicates the first step in 

 the formation of the glomerulus. 



About the sixtieth segment, that is, a little in front of the place 

 where the first tubule was closed off from the general body-cavity in 

 stage A, the tubules show signs of atrophy. The coelomic pockets 

 are still present, but the tubule itself has lost its lumen, so that the 

 pockets no longer communicate with the duct. From here back to 

 the sixty- eighth segment the degeneration becomes more and more 

 marked, and all that is left of the tubules are irregular mass 

 cells attached to the duct. The posterior end of the embryo is in- 

 jured, and the excretory system through about the last twelve segments 

 is entirely wanting. 



The first coelomic pockets lie in the fifteenth segment, that is, 

 six segments farther forward than in stage A. These pockets were 

 first observed in the more posterior part of the system in stage A, 

 where they are in the stage represented in Figs. 8 c and 8d, and it 



