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Show 1891.] VARIATION OF THE LEPORINE STERNUM. 163 this minute half-developed cartilaginous seventh sternebra and, were, blot it out. Thus there would be left the more familiar form of sternum with no trace of anything between the sixth sternebra and the xiphisternum. The following, as compared with the above, are the stages in metamorphosis of the permanently seven-segmented type. After the embryo has reached a length of 7-8 cm. the seventh sternebra, instead of ceasing to grow, as in the more familiar form, continues to do so for a longer or shorter time. The growth of this sternebra seems to stop at different ages in different individuals ; at a somewhat later stage ossification sets in, more or less strongly, forming in some cases a large bone (fig. A ) , in others merely a minute nodule (fig. E ) ; in others, again, ossification never occurs at all, and the sternebra persists as cartilage (fig. F). From the foregoing it may be justly inferred that at some past date the Leporine sternum consisted of eight sternebrse (the xiphisternum being counted as one), and that afterwards, for some unknown reason, the seventh pair of ribs became detached and grew forward over the ventral surface1. The place of the seventh sternebra having been thus usurped by the ribs which gave rise to it, the latter (degradation following disuse) apparently began gradually to disappear, until there was realized the condition in which we now find it, i. e. that of maximum absorption. These facts are not only interesting in themselves, but they throw light upon the origin of tbe xiphisternum. For if it be granted that each sternebra arises from the pair of ribs immediately behind it, the sixth sternebra would be the derivative of the sixth pair of ribs, the seventh when present of the seventh pair, and therefore the eighth (xiphisternum) of the eighth. The Leporine xiphisternum would thus appear to be not (as the study of it in ordinary would suggest) different in origin from that of Man, but, on the contrary, homologous with it-at any rate as far as concerns its origin from the eighth pair of ribs. I have not been able to ascertain whether the ninth pair of ribs takes any part in its formation as it does in that of Man, but it seems not improbable that it may do so. Another variation of some slight interest occurs in the xiphisternum : this structure is occasionally seen to be forked, a fonta-nelle being contained between the prongs (cf fig. A ) . This split recalls in a minor degree the phenomenon of the " cleft sternum " well known to occasionally occur in the human subject2. The groove in the seventh sternebra in fig. F is probably an example of the same failure to unite, but to a still less marked degree. Both these specimens will bear comparison witb the Cetacean sternum, 1 There seems to be an inclination on the part of all the ribs to do this, articulation with the sternum being much more ventral than dorsal. 2 Cf. Turner, " Description of a Cleft Sternum," Journ. Anat. & Phys. p. 103 (1879). 11* |