Figure 13_d_ is about one hundred and twenty-five sections posterior to Figure 13_c_. Although not so small as the sections that follow it, this section is considerably smaller in area than the one last described. The amnion (_a_), which was not represented in the last three figures, is very evident here. The spinal cord (_sc_) is considerably smaller here than in the preceding figure, while the notochord (_nt_) is not only relatively but actually larger than in the more anterior regions. Beneath the notochord is the aorta (_ao_), now a single large vessel. The mesoblast on each side of the body is here differentiated into a distinct muscle plate (_mp_). These muscle plates have very much the appearance of the thickened ectoderm seen in the younger stages of development. At about its middle region (_i. e._, at the end of the reference line _ec_) each muscle plate is separated from the overlying ectoderm by an empty space; this space is still more marked in some other series. Ventral to the aorta, and supported by a well marked though still thick mesentery (_ms_), is the intestine. It is a small, nearly cylindrical tube with thick walls; the splanchnic mesoblast which surrounds it is more dense than the general mass of mesoblast; it was somewhat torn in the section and is so represented in the figure. The urinary organs have made considerable progress since the last stage. In the figure under discussion they are seen as a group of tubules on either side of the aorta. The tubule most distant from the middle line, on each side, is the Wolffian duct (_wd_). It extends through the posterior two thirds of the embryo and varies in diameter at different points; it is usually lined with a single layer of cubical cells which contain large nuclei. The Wolffian bodies (_wt_) are a mass of slightly convoluted tubules that may be traced throughout the greater part of the region through which the Wolffian duct extends. These tubules also vary somewhat in diameter, but they are usually of greater caliber than the duct. No actual nephrostomes are to be seen, though the occasional fusion of a tubule with the peritoneal epithelium, as is seen on the left side of the present figure, may represent such an opening. Figure 13_e_ is about one hundred and forty sections posterior to the section just described. The embryo is here very slender, so that the contrast between this and the first figure (13_a_) of this stage is remarkable. Except in size, this section does not differ greatly from the preceding. The spinal cord, notochord, etc., are smaller than before, but are of about the same relative size. The mesentery (_ms_) in the section drawn was torn across, so that the intestine is not represented. Medial to the Wolffian duct is a tubule (_wt_), which seems to be the same as those which were called Wolffian tubules in the preceding stage, but which may be the beginning of the ureter.