The process of formation of the mesoblast in Loligo, as observed by Mr Ray Lankester (_Annals and Magazine of Natural History_, February, 1873), is still more modified. Here the mesoblast arises independently of the blastoderm, and by a process of free cell-formation in the yolk round the edge of the blastoderm. If Oellacher's observations in reference to the origin of formative cells are correct, then the modes of origin of the mesoblast in Loligo and the chick would have nothing in common; but if the formative cells are in reality derived from the white yolk, and also are alone concerned in the formation of the mesoblast, then the modes of formation of the mesoblast in the chick would be substantially the same as that observed by Mr Ray Lankester in Loligo. 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.