1891.] VARIATION OF THE LEPORINK STERNUM. 163 



this minute half-developed cartilaginous seventh sternebra and, as it 

 were, blot it out. Thus there would be left the more famihar 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 some- 

 what 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 xiphi- 

 sternum being counted as one), and that afterwards, for some 

 unknown reason, the seventh pair of ribs became detached and grew 

 forward over the ventral surfaced 



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 the 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 impro- 

 bable that it may do so. 



Another variation of some slight interest occurs in the xiphi- 

 sternum : 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 subject\ 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 with the Cetacean sternum, 



^ There seems to be au inclination on the part of all the ribs to do this, their 

 articulation with the sternum being much more ventral than dorsal. 



•^ Cf. Turner, " Description of a Cleft Sternum," Journ. Anat. & Phys. p. 103 

 (1879). 



II* 



