This stage is about one fourth longer than the preceding. The medullary canal is enclosed throughout its entire length, though it appears in surface view (Fig. 11) to be open in the posterior half (_mc_) of the embryo. An enlargement of this apparently open region at the extreme posterior end (_pg_) is probably caused by the remains of the primitive groove or the neurenteric canal, and a slight opacity at the same point may be caused by the primitive streak. The anterior end of the neural tube is bent in a ventral direction (_v_), as in the preceding stage. The somites (_s_) now number fifteen pairs; they are somewhat irregular in size and shape. The head-fold is not so striking a feature as in the preceding stage. The head-fold of the amnion (_a_) now covers nearly two thirds of the embryo. The heart (_ht_) is seen as a dark, rounded object projecting to the right side of the neural canal, just anterior to the first somite. The vitelline blood-vessels are just beginning to form, but are not shown in the figure. The depression of the anterior region that was noted in the preceding stage has advanced so far that a considerable part of the embryo now projects forward under the blastoderm. In some cases it is almost concealed in a dorsal view; in other cases it may easily be seen through the transparent membranes, especially after clearing. In opening eggs of this stage one is at first apt to underestimate the size of the embryos, since the anterior part of the embryos cannot be seen until after they are removed from the yolk and are viewed from the ventral side. The embryo from which the series of transverse sections of this stage was made, while of the same state of development as that shown in Figure 11, was more fully covered by the blastoderm than is shown in the surface view in question. Figure 11_a_ passes through the tip of the head. Dorsal to the embryo is the ectoderm and a thick mass of yolk (_y_). The amnion (_a_) is seen as an irregular membrane which entirely surrounds the head. The medullary canal (_mc_) is entirely closed except at the extreme anterior end, which is bent downward so that the opening is on the ventral side. The nervous (_nl_) and epidermal (_ep_) layers of the ectoderm are in contact throughout, but are clearly distinguishable because of the difference in the compactness of their cells. In Figure 11_b_ is represented a section, behind the preceding, which passes through the posterior tip of the turned-under anterior end (_mc′_). Here the medullary canal is closed both above (_mc_) and below (_mc′_). The amnion (_a_) has about the same appearance as in the more anterior section, but there is here a considerable space, filled with mesoblast (_mes_), between the nervous (_nl_) and epidermal (_ep_) layers of ectoderm.