There is no line of demarcation between the typical medullary groove region of Figure 9_l_ and the equally typical primitive groove region represented in Figure 9_m_. As was noted in the preceding stage, the medullary folds are quite continuous with the folds of the primitive streak, and the medullary groove with the primitive groove; so that, unless we take the dorsal opening of the neurenteric canal as the point of separation, there is no line of division between these structures. The entoderm (_en_) and the lateral regions of the ectoderm (_ec_) and mesoderm (_mes_) in Figure 9_m_ are about as they were in Figure 9_l_, but in the middle line is seen a compact mass of cells forming the primitive streak (_ps_), with the shallow primitive groove (_pg_) on the dorsal side. The cells on each side of the primitive groove and for a short distance below it are compact, as is shown in the figure, but as we pass ventrally and laterally they become looser and more angular to form the lateral sheets of mesoblast (_mes_), very much as is the case in the chick and other forms. For a few sections posterior to the one shown in Figure 9_m_ the primitive streak may be seen, then it disappears, and only the ectoderm and entoderm remain as thin sheets of tissue above the yolk. STAGE VII FIGURES 10 AND 10_a_ (PLATES XV., XVI.) Although of practically the same size as the preceding, this stage has advanced sufficiently in development to warrant a description. The medullary folds are apparently still slightly open for the greater part of their length, though they are evidently fused together in the head region, except at the extreme end. Transverse sections, however, of Figure 12, in which the medullary folds, from the dorsal aspect, seemed open (_mg_) as in Figure 10, have shown that these folds are fused throughout their length. The first cerebral vesicle (_v′_) is clearly indicated as an enlargement of the anterior end of the nervous system, and a slight enlargement (_v″_) posterior to the first probably represents the second cerebral vesicle. There are now eight pairs of somites (_s_). The head-fold (_h_) now shows in both dorsal and ventral views, appearing in the former, when viewed by transmitted light, as a lighter, circular area on either side of the body, just posterior to the hinder edge of the amnion. The head-fold of the amnion (_a_) has extended about twice as far backward as it did in the preceding stage. Owing to the opacity caused by the medullary folds being in contact along the middle line, the notochord is no longer visible in surface views. The head at this stage begins to push down into the yolk in a strange way that will be described later. STAGE VIII FIGURES 11-11_k_ (PLATES XVI., XVII., XVIII.)