Figure 12_e_ is nearly one fourth the length of the embryo posterior to Figure 12_d_; it is approximately in the middle region. The diameter of the embryo has been gradually decreasing until now it is very much less than in the head region. The section being behind the head-fold the entoderm (_en_) is nearly flat and the enteron is quite unenclosed. The canal of the spinal cord (_sc_) is smaller in proportion to the thickness of its walls, and the notochord (_nt_) is somewhat larger than in the preceding sections. In proportion to its extent, the ectoderm is very thick. Under the notochord the dorsal aortæ (_ao_) are seen as two large, round openings in the mesoblast. On the left side the section passes through the center of a somite and shows a small, round myocœl (_myc_). The mesoblastic layer of the amnion (_so_) is distinct throughout from the ectoblastic layer (_a_). The most important structures to be here noted are the first rudiments of the Wolffian ducts (_wd_). They are seen in the present section as lateral ridges of mesoblast projecting outward and upward toward the ectoblast, which suddenly becomes thin as it passes over them. These ridges or cords of mesoblast are as yet quite solid. They arise suddenly at about the eightieth section of the series of two hundred and may be traced through about forty sections, or one fifth of the length of the embryo. Their exact length is difficult to determine because, while their anterior ends are blunt and sharply defined, they taper so gradually posteriorly that it is hard to tell just where they end. They apparently originate anteriorly and gradually extend toward the tail. In a slightly younger embryo the rudimentary Wolffian duct could be seen as a still smaller rod of cells extending posteriorly for a few sections, from the seventy-fifth section of a series of about two hundred. In the particular series under discussion the left rudimentary Wolffian duct was about one fifth longer than the right one. Figure 12_f_ is just posterior to the head-fold of the amnion, passing, in fact, on the left side through the extreme edge of its lateral fold, which is shown as a upward bend in the ectoblast and somatopleure. The ectoblast (_ec_) shows the same remarkable thickening that was noted in the corresponding region of the preceding stage. The spinal cord (_sc_), notochord (_nt_), aortæ (_ao_), and entoderm (_en_) need no special mention. The mesoderm seems to be separated by unusually wide spaces from both ectoderm and entoderm, and is made up of rather closely packed cells except around the aortæ, where there seems scarcely enough tissue to hold these vessels in place. The body cavity (_bc_) is large, and a small myocœl (_myc_) is seen on the left.