In section, the cells of the hypoblast appear generally very thin and spindle shaped, but the outlines brought out by the silver nitrate shew that they are much expanded horizontally, but very irregular as to size, varying even within a small area from 1/4000 - 1/400 of an inch in the longest diameter. At about the twelfth hour they are uniformly smaller a short way from each extremity of its longer axis than over the rest of the blastoderm. It is, perhaps, fair to conclude from this that growth is most rapid at these parts. At this time the hypoblast, both in sections and from a surface view after treatment with silver nitrate, appears to end abruptly against the white yolk. The surface view also shews that its cells are still filled with highly refractive globules, making it difficult to see the nucleus. In some cases I thought that I could (fig. 3, _a_) make out that it was hour-glass shaped, and some cells certainly contain two nuclei. Some of the cells (fig. 3, _b_) shew re-entrant curves, which prove that they have undergone division. The cells of the epiblast, up to the thirteenth hour, have chiefly undergone change in becoming smaller. In surface views they are about 1/4000 of an inch in diameter over the centre of the pellucid area, and increase to 1/2000 of an inch over the opaque area. In the centre of the pellucid area the form of the epiblast cells is more elongated vertically and over the opaque area more flattened than was the case with the original upper layer cells. In the centre the epiblast is two or three cells deep. Before going on to the further changes of the blastodermic cells it will be well to say a few words in reference to the origin of the mesoblast. From the description given above it will be clear that in the chick the mesoblast has an independent origin; it can be said neither to originate from the epiblast nor from the hypoblast. It is formed coincidently with the latter out of apparently similar segmentation cells. The hypoblast, as has been long known, shews in the chick no trace of its primitive method of formation by involution, neither does the mesoblast shew any signs of its primitive mode of formation. In so excessively highly differentiated a type as birds we could hardly expect to find, and certainly do not find, any traces of the primitive origin of the mesoblast, either from the epiblast or hypoblast, or from both. In the chick the mesoblast cells are formed directly from the ultimate products of segmentation. From having a secondary origin in most invertebrates the mesoblast comes to have, in the chick, a primary origin from the segmentation spheres, precisely as we find to be the case with the nervous layer in osseous fishes. It is true we cannot tell which segmentation-cells will form the mesoblast, and which the hypoblast; but the mesoblast and hypoblast are formed at the same time, and both of them directly from segmentation spheres.