The Frog is the vertebrate whose development comes nearest to that of Amphioxus, as far as the points we are at present considering are concerned. But it will perhaps facilitate the understanding of their relations shortly to explain the diagrammatic sections which I have given of an animal supposed to be intermediate in its development between the Frog and Amphioxus. Plate 5, fig. B I, represents a longitudinal section of this hypothetical egg at the close of segmentation. The lower pole, coloured yellow, represents the part containing more yolk material, and the upper pole, coloured blue, that with less yolk. Owing to the presence of this yolk the lower pole even at the close of segmentation is composed of cells of a different character to those of the upper pole. In this respect this egg can already be distinguished from that of Amphioxus, in which no such difference between the two poles is apparent at the corresponding period (Plate 5, fig. A I). The segmentation cavity in this ovum is not quite so large proportionately as in Amphioxus, and the encroachment upon it is due to the larger bulk of the lower pole of the egg. In fig. B II the involution of the lower pole has already commenced; this involution is (1) not quite symmetrical, and (2) on the ventral side (the left side) the epiblast cells forming the upper part of the egg are growing round the cells of the lower pole of the egg or lower layer cells. Both of these peculiarities are founded upon what happens in the Frog and the Selachian, but it is to be noticed that the change from the lower layer cells being involuted towards the epiblast cells, to the epiblast cells growing round the lower layer cells, is a necessary consequence of the increased bulk of the latter. In this involution not only are the cells of the lower pole pushed on, but also some of those of the upper or yellow portion; so that in this as in all other cases the true distinction between the epiblast and hypoblast does not appear till the involution to form the latter is completed. In the next stage, B III, the involution has become nearly completed and the opening to the exterior or blastopore quite constricted. The segmentation cavity has been entirely obliterated, as would have been found to be the case with Amphioxus had the stage a little older than that on Plate 5, A III, been represented. The cavity marked (_al_), as was the case with Amphioxus, is that of the alimentary canal.