I pointed out that beneath the medullary groove the epiblast and hypoblast were not separated by any interposed mesoblast. Along the line (where the mesoblast is deficient) which forms the long axis of the embryo, a rod-like thickening of the hypoblast appears (Pl. 3, figs. 7_a_ and 7_b_, _ch_ and _ch´_), first at the head end of the embryo, and gradually extends backwards. This is the rudiment of the notochord; it remains attached for some time to the hypoblast, and becomes separated from it first at the head end of the embryo, and the separation is then carried backwards. This thickening of the hypoblast projects up and comes in contact with the epiblast, and in the later stages with bad (especially chromic-acid) specimens the line of separation between the epiblast and the thickening may become a little obscured, and might possibly lead to the supposition that a structure similar to that which has been called the "_axis cord_" was present. In all my best (osmic-acid) specimens the line of junction is quite clear; and any one who is aware how easily two separate masses of cells may be made indistinguishably to fuse together from simple pressure will not be surprised to find the occasional obscurity of the line of junction between the epiblast and hypoblast. In the earlier stage of the thickening there is never in the osmic-acid preparations any appearance of fusion except in very badly prepared ones. Its mode of formation will be quite clear without further description from an inspection of Pl. 3, figs. 7_a_ and 7_b_, _ch_ and _ch´_. Both are taken from one embryo. In fig. 7_b_, the most anterior of the two, the notochord has become quite separated from the hypoblast. In fig. 7_a_, _ch_, there is only a very marked thickening of hypoblast, which reaches up to the epiblast, but the thickening is still attached to the hypoblast. Had I had space to insert a drawing of a third section of the same embryo there would only have been a slight thickening of the hypoblast. In the earlier stage it will be seen, by referring to figs. 6_a_ and 6_b_, that there is no sign of a thickening of the hypoblast. My numerous sections (all made from embryos hardened in osmic acid) shewing these points are so clear that I do not think there can be any doubt whatever of the notochord being formed as a thickening of the hypoblast. Two interpretations of this seem possible. I mentioned that the mesoblast appeared to be primitively formed as two independent sheets, _split off, so to speak, from the hypoblast_, one on each side of the middle line of the embryo. If we looked upon the notochord as a third _median sheet of mesoblast_, split off from the hypoblast somewhat later than the other two, we should avoid having to admit its hypoblastic origin. Professor Huxley, to whom I have shewn my specimens, strongly advocates this view.