Observations on the development of the excretory system in Turtles. 695 



the duct at its tip. On the left, of course, it is the S reversed. 

 Sometimes a second bend is found in the upper half. In the fifteenth 

 somite there are four tubules on each side; the second on the right 

 and the third on the left show a further ditferentiation. Here the 

 tubules on each side are doubled, the lower is the same as those seen 

 before, generally with the second bend, and just above is a smaller 

 one. This doubling continues as a tubule or rudiment through the 

 rest of the organ. In the sixteenth somite there are four double 



tubules on each side and in the third on the left side a small glomer- 

 ulus is seen in the upper tubule, but this is the only one seen in 

 the smaller tubules at this stage. Somite XVII has five double 

 tubules; the glomerulus of the primary tubule is dwindling, and the 

 cells of the tubules are smaller. Somite XVIII has four, and somite XIX 

 six double tubules. In these somites the glomerulus has not yet 

 appeared. In the twentieth somite the four double tubules are rudi- 

 mental. In somite XXI the five double rudiments can no longer be 

 called tubules. In somite XXII the rudiment is sometimes single 

 but more distinct and double apparently where tubules would form if 

 they had a chance. Sections three and four of somite XXV show the 

 last remnants of the mesonephric basis. The opening of the duct into 

 the cloaca extends over sections three to eleven of this somite. At 

 section six the ureter opens into the duct. Beyond this point I 

 counted twelve somites, ten in the tail, making thirty-five in all. The 

 kidney lobe has become quite large and its endothelial wall consists 

 of long flattened cells. 



In an embryo of stage XVI (Plate 49, Fig. 69) further development 

 is seen. Here there are very constantly three tubules on a side, one 

 over the other, and three glomeruli. Sometimes there appear to be 

 more. The duct here lies against the wall of the mesonepbros with 

 the cardinal vein just above, and the tubules can be found opening 

 into the duct at any point not so covered. Beyond this stage the 

 tubules become so numerous, crowded and twisted, that it would be most 

 difficult to count them. The sagittal section seen in the camera 

 drawing on Plate 50 also shows its structure in this stage. 



