view as a faint, linear opacity extending along the middle line from the head-fold to the primitive streak. Two sagittal sections of this stage are shown in Figures 8_b_ and 8_c_. The embryo from which the sections were made was apparently somewhat crooked, so that it was not possible to get perfect longitudinal sections. For example, in Figure 8_b_ the plane of the section is almost exactly median in the extreme posterior and middle regions, but is on one side of the middle line elsewhere. This explains the enormous thickening of the ectoblast in the region of the head, where the section passes through one of the medullary folds (_mf_) at its thickest part; and also explains the fact that the ectoblast is thinner in the middle region (_ec_), where the section passes through the medullary groove, than it is farther toward the blastopore where the section cuts the edge of the medullary folds. The outlines of the middle and extreme posterior regions of the ectoblast are much more irregular and ragged than is shown in the figure. The plane of the section passes through the notochord (_nt_) in the posterior region, but not in the anterior end of the embryo, where a layer of mesoblast (_mes_) is seen. The great size of the blastopore (_blp_) is well shown, as is the beginning of the foregut (_fg_). Comparison of this figure with the more anterior transverse sections and with the dorsal surface view of this stage will make the rather unusual conditions comprehensible. Figure 8_c_ is cut to one side of the median plane, distal to the medullary folds. Being outside of the medullary folds, the ectoderm (_ec_) is thinner and less dense than in Figure 8_b_; anteriorly it is pushed down and back as the head-fold, and posteriorly it becomes thin where it forms the dorsal boundary of the primitive streak (_ps_). The foregut (_fg_), as would be expected, is not so deep as in the median section (8_b_). The most striking feature of the section is the presence of five mesoblastic somites (_s_). Each somite, especially the second, third, and fourth, is made up of a mass of mesoblast whose cells are compactly arranged peripherally, but are scattered in the center, where a small myocœl may be seen. A series of transverse sections of the embryo shown in Figures 8 and 8_a_ is represented in Figures 8_d_-_j_. Figure 8_d_ is through the anterior end of the embryo; the posterior edge of the amnion is cut only on one side (_a_). The medullary folds (_mf_) are shown as two distinct masses of tissue, separated by a considerable space from each other, both dorsally and ventrally; they are underlaid by the ectoderm of the head-fold, through which the section passes. A mass of yolk (_y_) is shown at one side of the section.