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Show 162 MR. R. H. BURNE ON THE [Feb. 1/, apparent difference being that the seventh was smaller than the others. This seventh sternebra may thus be regarded as arising from the seventh pair of ribs, just as the other sternebrse do from the ribs in front, and as having a like morphological value with the former. In the next stage (7 cm. long, fig. J, p. 161) the sternum had begun to ossify, centres of ossification being present in the xiphisternum and in all the sternebrse except the seventh ; this structure, however, was still clearly present, although only represented in cartilage. The seventh sternebra, in comparison with those remaining, is of the same size as in the previous stage. In sterna of this age the seventh pair of ribs have already begun to approximate ventrad of their sternebra, their lower extremities reaching nearly halfway to the middle line. Thus far the growth of the sternum has been regular and uniform, with the exception of the lack of ossification in the seventh sternebra; but about this time the sternebrse begin to grow unequally, and consequently to manifest slight differences in their proportions (figs. K, L). The gap between the sixth and seventh pairs of ribs is, at this stage, usually shorter than in the earlier ones; but occasionally these ribs are separated by a gap (fig. L ) , indicating that the seventh sternebra is still in process of growth. In none of these specimens was there, however, any sign of ossification in the seventh sternebra, although in all the other sternebrse it was much more pronounced than before. The points of the seventh pair of ribs had in all cases approached much nearer the middle line than at the earlier stages. In Rabbits just born (14-15 cm.) the most noticeable feature is the apposition of the lower extremities of the sixth and seventh pairs of ribs, the latter having by this time met in the middle line. None which I examined showed any trace of an ossific centre between the sixth sternebra and the xiphisternum. It has been seen that in all 3 cm. embryos the seventh costal segment is formed from the seventh pair of ribs, in the same way as are the other costal segments from their respective ribs, and therefore it has morphologically the same value as each of those more normally present; it is, however, somewhat smaller than the other segments. W e may therefore assume that in all cases the seventh sternebra is invariably present at an early age and that it differs only from its fellows in size. All segments of the sternum appear now to grow uniformly ; for in all 7 cm. embryos the seventh sternebra is relatively of the same size as in the previous stage; but it is still cartilaginous, all the others being now ossified. So far all the sterna of the same age are much alike; but in 11 cm. embryos the seventh sternebra is usually shorter in comparison with its fellows than in the earlier stages, although in some cases it is still of moderate size. Those in which it was t-hort I take to be the more normal, the seventh sternebra having developed with the rest to a certain extent, and then, when the embryo was about 8 or 9 cm. long, having ceased to grow: the xiphisternum and sixth sternebra, as they increase in size, grow over |