In Figure 20_i_ the size and complexity of the figure are due, it will be easily understood, to the fact that the plane of the section passed through the curve of the body, thus practically cutting the embryo in two regions--an anterior, where the lungs (_lu_) and liver (_li_) are seen, and a posterior, where the Wolffian bodies (_wt_) are present. The spinal cord and the surrounding structures have almost the same characteristics at both ends of the figure, except that the primitive spinal column is rather more distinct in the posterior end of the section. The posterior cardinal veins (_pc_), Wolffian ducts (_wd_), and Wolffian bodies (_wt_) are also prominent structures of this end of the figure, the last being made up of a great number of tubules. The extreme anterior ends of the Wolffian bodies are seen in the other half of the section in the upper angles of the body cavity, dorsal to the lung rudiments (_lu_). Filling most of the body cavity (_bc_) and making up the greater part of the middle of the figure are the liver (_li_), now a very large organ; the stomach (_i′_), also quite large; the pancreas (_pan_), a small body lying near the stomach; and the lungs (_lu_), which here consist of several thick-walled tubes, surrounded by lobes of mesoblast. The other features of the figure need no special mention. Figure 20_j_ is through the base of the posterior appendages (_pa_), in which the cartilages are already being outlined by condensations of mesoblast. The intestine (_i_) is cut in two regions--at a more anterior point, where it is seen as a small, circular hole surrounded by mesoblast and hung by a narrow mesentery, and through the cloacal region, the larger and more ventral cavity, into which the Wolffian ducts (_wd_) open a short distance caudad to this section. The blood-vessels present a rather curious appearance. A short distance anterior to this point the aorta has divided into three, or it might be said that it has given off two, large branches. These two branches, one on either side near the posterior cardinal vein, pass toward the ventral side of the embryo on each side of the cloaca and end at about the region represented by the present figure. The small portion of the aorta that remains after the giving off of the two branches just described continues, as the caudal artery (_ca_), into the tail; it is a small vessel just under the notochord, and gives off small, paired branches at regular intervals toward the vertebral region. The posterior cardinal veins (_pc_), posterior to the openings of the Wolffian ducts into the cloaca, unite to form a large caudal vein lying just ventral to the caudal artery. STAGE XVIII FIGURE 21 (PLATE XXVII.)