OCR Text |
Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 693 of he ribs, i. e. is not, like that of the muscle-plates, intravertebral. Ths supercostal position it will be seen, however, from comparison with other types, is only a secondary arrangement occurring during part of their course, in consequence of the curvature of the ribs. They represent, in fact, traces of the intercostales externi of ribs in their primitive arrangement, before they acquired their present modifications in connection with the sternum. 4. Although the sternum as a whole is greatly lengthened in the course of its development, owing to the appearance of the metasternum, yet its costal portion loses one posterior attached rib ; there are six sternal ribs in the embryo, five in the adult. Further, a seventh rib, although never attached to the sternum, shortens considerably in the course of development, which implies that it was once a sternal rib. The position of the primitive intercostales externi of the two unattached posterior ribs in the four days' embryo, being, as they are, longer than the ribs they accompany, leaves little doubt that both originally entered into the formation of the primitive costal bands. 5. The series of embryos shows the gradual addition of the anterior lateral process to the costal sternum. 6. The 7 days' embryo shows an anterior part which maj7 probably be compared to the manubrium sterni of mammals. Its extent in front of the first sternal rib, at a stage when none of the parts known to be of secondary origin have as yet been added to the costal sternum, taken in connection with the existence of two anterior spinal ribs of which the second is very long, seems sufficient to prove its costal origin. Further, St. George Mivart1 records the appearance in the Ostrich of a rudimentary sternal rib, the first of six, which was not attached to the corresponding spinal rib (the 3rd) ; from the number and position of the spinal ribs he describes, namely ten, whereof the two posterior are not attached to the sternum, it appears that this rudiment corresponds with the first sternal rib, fully attached to the corresponding spinal portion, of the seven embryos here described, This affords a further reason for supposing that the process of atrophy extended in the above-named specimen to the third spinal rib has taken place already in the case of the first and second. 7. Comparison of the 15 days' embryo with preceding stages shows the addition of the metasternum to the costal sternum. Although this region, seen in all subsequent stages of the embryo, is in perfect continuity with the cartilage of the original costal sternum, yet in the adult (as appears in Plate XLII. fig. 9) only a small part of it is ever ossified. In Rhea still less of it is ossified, so that the halves of the sternum remain unittd by cartilage only. Apparently these do not, however, correspond exactly with the primitive halves ; for a fontanel, nearly closed by thinner bone, marks, in some specimens, a boundary corresponding in position with the notch that separates off the posterior lateral process of the Ostrich (cf. fig.IY.f, p. 710). 1 "Axial Skeleton of the OBtrich {Struthio camelus)," by St. George Mivart, F.B.S.: Trans. Zool. Soc. x. p. 1. PROC. ZOOL. Soc-1885, No. XLV. 45 |