716 MISS B. LINDSAY ON THK AVIAN STERNUM. [JuilC IG, 



Fig. 2. Slioulcler-girdle of late 5 days' chick, showing relations of c/. = clavicle 

 and CO/". = coracoid. The coracoid has a flattened shape ; the scapula 

 is removed. 

 3. The same, in gifu. 



4 6. Shoulder-girdleof two 5 days' chicks (right and left). Showing clavicle 

 adjoined tj coracoid, scapula quite free ; ventral surface. Erratum : 

 R. 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 dotted line, (.3) elongated 

 clavicle : also (4) rectus attached to clavicle ((/. fig. 9). 



7, 8. Eight coracoid of Castiarixs, sliowing the foramen or incisura cora- 



coidea. which in Strufhio lies near the inner margin of the coi-ac-oid. 

 The area internal to it is probably a precoracoid (not identical with that 

 of Struthio) ; rf\ fig. 1, also woodcut diagram of shoulder-girdle of 

 Chick. CO. 

 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': rl, external rectus continuous with peL'to- 



ralis major ; r2, main .sheet of rectus ; r3, internal thin sheet of reutus; 

 s=supraooracoideus (or pect. minor). 



11. The same, showing attachment of r 3 on tlie dorsal side of the sternum. 



12. Tlie same, showing attachment of r3 nearer the margin, at a later stage. 

 13,14. The same, ribs of 7 days' embryo; 13 shows ;«', traces of the 



primitive intercostalcs eiterui, dissected out. 

 1.5. Dromcsus, 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 



Pl.ite XLV. 



Fig. 1. Chick, 6-7 days' incubation, showing rectus (r.) continuous with supra- 

 coraooideiis {p. min.) ; x2i. 



2. Embryonic sternal ribs of late 6 days' chick, clcve}i in number, the early 



form of tlie posterior processes is aLso shown. 



3. Late 6days" chick, .showing?H', primitive b^mds of the iutercostales externi, 



and wedge-shaped n))per part severed from rectus band by growth of 

 steinum — removed from X. Parts of the rectus=r. and r.^.; X4. 



4. .5. Occasional separate centre, 7c, for keel in 7 days' chick. 



6. The wedge shaped rectus band, together with (/«') three anterior muscle- 



bands (primitive intercostalcs externi), of late G days' chick. 



7. The same at a rather later stage (very early 7 day.s' chick) ; x4. 



8. Last trace of the three anterior muscle-bands, in 7 days' chick ; x4. 



9. Normal sternum of 6 days' chick, showing coramencemeut 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 anttrior ribs, 

 and the muscle-bands belonging to each, turned over. 



11 Five days' chick, showing (.r) lateral bands of muscle actached 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 dif- 

 ferentiating mesoblast alternate with the ribs, possibly indicating a 

 complex series of costal centres in the sternum com)3arabIe with wliat 

 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. c.s., mass of sternum, showing depression for attach- 



ment of coracoid; /i; = keel; »i' = three rudimentary muscle-bands; 

 hy. = thick folds of hypoblast of the throat, which afterwards form the 

 thyroid glands. 



15. Gannet, Stage 3 ; dorsal aspect. Z,-=keel; Z and Z' = hgaments. 



16. 17. Gannet, Stage 4. L, ligament. 



18. Posterior margin of sternum of birdjust hatched (x 1). 



