“This is so unexpected a method of formation for the anterior part of the medullary folds that I have made use of both Figures 4 and 5. They were made from very perfect specimens, and the sections of both of them, and of the specimen from which Figure 6 was drawn, proves that the structure is what it is indicated to be in surface appearance. That is, the transverse sections posterior to the V, in the embryos shown in Figures 4 and 5, show the medullary groove and the medullary folds; the several sections passing through the apex of the V show neither groove nor folds, but only a median thickening; and in front of the point or apex of the V the successive sections discover a gradually widening groove between the arms, which is also much deeper than the shallow groove found posterior to the V. While I have not seen, and from the nature of the conditions one cannot see, the change actually proceeding from the form of Figure 5 to that of Figure 6, still the explanation given appears to be the only one possible” (17). A somewhat extended series of transverse sections of an embryo of about this age is represented in figures 6_a_-_i_. Figure 6_a_ is a section through the head-fold; it passes through the extreme anterior end of the secondary folds (_sf_) that were described, in surface view, above (Figs. 5 and 6). The section was not quite at right angles to the long axis of the embryo, so that the fold on the right was cut farther toward its anterior end than was the fold on the left. The pushing under of the head causes a forward projection of the secondary folds, so that the fold to the right appears as a rounded mass of cells with a small cavity near its center. On the left the plane of the section passes through the posterior limit of the head-fold, and shows the cells of the secondary fold continuous with the dorsal side of the ectoderm (_ec_). As pointed out above by Clarke, the secondary folds are here some distance apart, and gradually approach each other as we proceed toward the posterior. The entoderm (_en_) is here flat and takes no part in the secondary folds. In Figure 6_b_, a short distance back of the one just described, the secondary folds (_sf_) are much larger and are closer together. On the right the section passes through the extreme limit of the head-fold, so that the secondary fold of that side is still a closed circle, with a few scattered cells enclosed. On the left the section is posterior to the head-fold; on this side the secondary fold is seen as a high arch of ectoderm, with a thick mass of entoderm beneath it.