In the next stage (C II) we find that the involution to form the alimentary canal has commenced (_al_), but that it is of a very different character to the involution in the previous case. It consists in the growing inwards of a number of cells from the point _x_ (C I) towards the segmentation cavity. The cells which grow in this way are partly the blue cells and partly the smaller yellow ones. At first this involuted layer of cells is only separated by a slit from the remainder of the lower layer cells; but by the stage represented in C II this has widened into an elongated cavity (_al_). In its formation this involution pushes backwards the segmentation cavity, which finally disappears in the stage C III. The point _x_ remains practically stationary, but by the general growth of the epiblast, mesoblast and hypoblast, becomes further removed from the segmentation cavity in C II than in C I. On the opposite side of the embryo to that at which the involution occurs the epiblast cells as before, grow round the lower layer cells. The commencement of this is already apparent in C I, and in C II the process is nearly completed, though there is still a small mass of yolk filling up the blastopore. The features of this involution are in the main exaggerations of what was supposed to occur in the previous animal. The asymmetry of the involution is so great that it is completely one-sided and results, in the first instance, in a mere slit; and the whole process of enclosing the yolk by epiblast is effected by the epiblast cells on the side of the egg opposite to the involution. The true mesoblast and hypoblast are formed precisely as in the previous case. The involuted cells become separated into two layers, one forming the dorsal epithelium of the alimentary canal, and a layer between this and the epiblast forming the mesoblast. There is also a layer of mesoblast accompanying the epiblast which encloses the yolk, which is derived from the smaller yellow cells at _y_ (C I). The edge of this mesoblast, _m´_, forms a thickened ridge, a feature which persists in other vertebrates. It is a point of some importance for understanding the relation between the mode of formation of the alimentary canal in the Frog and other vertebrates to notice that on the ventral surface the cells which are to form the epithelium of the alimentary canal become distinguished as such very much later than do those to form its dorsal epithelium, and are derived not from the involuted cells but from the primitive large yolk-cells. It is indeed probable that only a very small portion of epithelium of the ventral wall of the mid-gut is in the end derived from these larger yolk-cells. The remainder of the yolk-cells (C III, and C II, _yk_) form the yolk mass and do not become directly formed into the tissues of the animal. In the last stage I have represented for the frog, C III, there are several features to be noticed.