No very important changes take place in the actual forms of the cells during the next few hours. A kind of fusion takes place between the epiblast and the mesoblast along the line of the primitive streak forming the axis-string of His; but the line of junction between the layers is almost always more or less visible in sections. In any case it does not appear that there is any derivation of mesoblast cells from the epiblast; and since the fusion only takes place in the region of the primitive groove, and not in front, where the medullary groove arises (see succeeding paper), it cannot be considered of any importance in reference to the possible origin of the Wolffian duct, &c., from the epiblast (as mooted by Waldeyer, _Eierstock und Ei_, Leipzig, 1870). The primitive groove, as can be seen in sections, begins to appear very early, generally before the twelfth hour. The epiblast spreads rapidly over the white yolk, and the area pellucida also increases in size.
From the mesoblast forming at first only a small mass of cells, which lies below the primitive streak, it soon comes to be the most important layer of the blastoderm. Its growth is effected by means of the formative cells. These cells are generally not very numerous in an unincubated blastoderm, but rapidly increase in numbers, probably by division; at the same time they travel round the edge of, and in some cases through, the hypoblast, and then become converted in the manner described into mesoblast cells. They act as carriers of food from the white yolk to the mesoblast till, after the formation of the vascular area, they are no longer necessary. The numerous cases in which two nucleoli and even two nuclei can be seen in one cell prove that the mesoblast cells also increase by division.
The growth of the hypoblast takes place in a very different way. It occurs by a direct conversion, cell for cell, of the white yolk spheres into hypoblast cells. This interpretation of the appearances, which I will describe presently, was first suggested to me by Dr Foster, from an examination of some of my specimens of about thirty-six hours, prepared with silver nitrate. Where there is no folding at the junction between the pellucid and opaque areas, there seems to be a perfect continuity in the silver markings and a gradual transition in the cells, from what would be undoubtedly called white yolk spheres, to as undoubted hypoblast cells (vide Pl. 1, fig. 5). In passing from the opaque to the pellucid areas the number of white yolk spherules in each cell becomes less, but it is not till some way into the pellucid area that they quite cease to be present. I at first thought that this was merely due to the hypoblast cells feeding on the white yolk sphericles, but the perfect continuity of the cells, and the perfect gradation in passing from the white yolk cells to the hypoblast, proves that the other interpretation is the correct one, viz. that the white yolk spheres become directly converted into the hypoblast cells. This is well shewn in sections (vide Pl. 1, fig. 4) taken from embryos of all ages from the fifteenth to the thirty-sixth hour and onwards. But it is, perhaps, most easily seen in embryos of about twenty hours. In such an embryo there is a most perfect gradation: the cells of the hypoblast become, as they approach the edge of the pellucid area, broader, and are more and more filled with white yolk sphericles, till at the line of junction it is quite impossible to say whether a particular cell is a white-yolk cell (sphere) or a hypoblast cell. The white-yolk cells near the line of junction can frequently be seen to possess nuclei. At first the hypoblast appears to end abruptly against the white yolk; this state of things, however, soon ends, and there supervenes a complete and unbroken continuity between the hypoblast and the white yolk.