From fig. 6 it is clear that a groove has already been formed in the medullary streak, a fact which was not obvious in the fresh specimen. In the second place the mesoblast is thickened both under the groove and also more especially in the medullary folds at the sides of the groove; but shews hardly a sign of the differentiation of the notochord. So that it is clear that the medullary streak is not the notochord, as was thought to be the case by the authors above mentioned. In the third place there is no adhesion between the epiblast and the mesoblast. In all the sections I have cut through the medullary groove I have found this feature to be constant; while (for instance, as in Pl. 1, figs. 7, 9, 17) all sections through the primitive groove shew most clearly an adhesion between the epiblast and mesoblast. This fact is both strongly confirmatory of the separate origins of the medullary and primitive grooves, and is also important in itself, as leaving no loophole for supposing that in the region of embryo there is any separation of the cells from the epiblast to form the mesoblast. By this time the primitive groove has attained its maximum growth, and from this time begins both absolutely to become smaller, and also gradually to be pushed more and more backwards by the growth of the medullary groove. The specimen figured in Pl. 1, fig. 18, magnified about ten diameters, shews the appearance presented by an embryo of twenty-three hours. The medullary groove (_mc_) has become much wider and deeper than it was in the earlier stage; the medullary folds (_A_) are also broader and more conspicuous. The medullary groove widens very much posteriorly, and also the medullary folds separate far apart to enclose the anterior end of the primitive groove (_pr_). All this can easily be seen with a simple microscope, but the sections taken from the specimen figured also fully bear out the interpretations given above, and at the same time shew that the notochord has at this age begun to appear. The sections marked 13-17 pass respectively through the lines with corresponding numbers in fig. 18. Section 1 (fig. 13) passes through the middle of the medullary canal. In it the following points are to be noted. (1) That the epiblast becomes very much thinner where it lines the medullary canal (_mc_), a feature never found in the epiblast lining the primitive groove. (2) That the mesoblast is very much thickened to form the medullary folds at _A_, _A_, while there is no adherence between it and the epiblast, below the primitive groove. (3) The notochord (_ch_) has begun to be formed, though its separation from the rest of the mesoblast is not as yet very distinct[8]. Footnote 8: In the figure the notochord has been made too distinct.