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Show 716 MISS B. LINDSAY ON THE AVIAN STERNUM. [June 16, Fig. 2. Shoulder-girdle of late 5 days'chick, showing relations of cl.= clavicle and cor. = coracoid. The coracoid has a flattened shape ; the scapula is removed. 3. The same, in situ. . . 4 5. Shoulder-girdle of two 5 days'chicks (right and left). Showing clavicle adjoined to coracoid, scapula quite free ; ventral surface. Erratum : B." and L. are reversed in the diags., and sc. is put for cl. in fig. 5. 6. Six days' chick, showing (1) absence of interclavicle, (2) fused coraco-scapular plate, removed from position of doited line, (3) elongated clavicle ; also (4) rectus attached to clav icle {cf. fig. 9). 7, 8. Eight coracoid of Casuarius, showing the foramen or incisura cora-coidea, which in Struthio lies near the inner margin of the coracoid. The area internal to it is probably a precoracoid (not identical with that of Struthio) ; cf. fig. 1, also woodcut diagram of shoulder-girdle of Chick. C.C. 9. Uria troile: 7 days' embryo, showing clavicles closed before the sternum and giving attachment to rectus muscle, r. 10. Uria troile: about 15 days': r\, external rectus continuous with pectoralis major ; r2, main sheet of rectus; r3, internal thin sheet of rectus; s=supracoracoideus (or pect. minor). 11. The same, showing attachment of r 3 on the dorsal side of the sternum. 12. The same, showing attachment of rS nearer the margin, at a later stage. 13, 14. The same, ribs of 7 days' embryo; 13 shows m', traces of the primitive intercostales externi, dissected out. 15. Dromaus, showing relations of clav., precor., and /, for. cor., in young bird. C.C. 16. Coracoid of adult bird: /, foramen ; g, posterior border of coracoid. C C PLATE XLV. Fig. 1. Chick, 6-7 days' incubation, showing rectus (r.) continuous with supra-coracoideus {p. min.); x2£. 2. Embryonic sternal ribs of late 6 days' chick, eleven in number, the early form of the posterior processes is also shown. 3. Late 6days' chick, showing??*', primitive bands of the intercostales externi, and wedge-shaped upper part severed from rectus band by growth of sternum-removed from X- Parts of the rectus=r. and r.l.; x4. 4, 5. Occasional separate centre, Jc, for keel in 7 days' chick. 6. The wedge shaped rectus band, together with {m') three anterior muscle-bands (primitive intercostales externi), of late 6 days' chick. 7. The same at a rather later stage (very early 7 days' chick); X 4. 8. Last trace of the three anterior muscle-bands, in 7 days' chick ; x4. 9. Normal sternum of 6 days' chick, showing commencemeut of external process, and {t) thin region towards median line : cf. fig. 8. 10. Abnormal sternum of 6 days' chick, showing a process of the sternum anterior to the coracoid, four rudiments apparently of anterior ribs, and the muscle-bands belonging to each, turned over. 11. Five days' chick, showing (.T) lateral bands of muscle attached to lower jaw ; upon the complete closure of the pericardial cavity these unite to form the median band seen in figs. 3, 6, and 7. 12. Sternum of 5 days' chick (broken by needle), showing masses of differentiating mesoblast alternate with the ribs, possibly indicating a complex series of costal centres in the sternum comparable with what Albrecht calls " hemisternebrals " in mammals ; and four anterior ribs attached to the vertebral column, the posterior ones free. 13. Gannet, Stage 1. r=rectus muscle. 14. Gannet, Stage 2. cs., mass of sternum, showing depression for attachment of coracoid; & = keel; m' - three rudimentary muscle-bands; hy. = thick folds of hypoblast of the throat, which afterwards form the thyroid glands. 15. Gannet, Stage 3 ; dorsal aspect. &=keel; I and Z' = ligaments lb, 17. Gannet, Stage 4. L, ligament. 18. Posterior margin of sternum of bird just hatched ( x 1). |