ii EXPLANATION OF FIGURES 



PLATE III— Continued 

 Fig. 24. Sternum (lower end) of rat nine days old, showing ossification. 



Fig. 25. Section through shoulder-girdle, and sternum of an embryo of Acanthias vulgaris 

 (advanced), showing the continuous cartilaginous structure. 



Fig. 26. Section through early embryo of Acanthias vulgaris, showing the undifferentiated 

 tissue of shoulder-girdle and sternum. 



Fig. 27. Section of chick embryo (six days eighteen hours), showing the formation of the 



sternum on one side (St.) and the ribs (RR). 

 Fig. 28. The half-sternum (St.) of the same embryo more highly magnified, showing the 



difference in the tissue of the sternum and the rib cartilages (CC). 



PLATE IV 



Figs. 29-34. Sections of Rat embryos, showing the differentiation of sternum and clavicles. 

 P.st., praesternum ; V, vein ; I.H.mm., infrahyoid muscles ; S.S., synovial 

 sac. 



PLATE V 



Fig. 35. Diagram of human sternum at birth, showing the average dates of appearance of 

 centres of ossification, and the percentage of centres present and absent at 

 the time of birth. 



Fig. 36. Sternal cornua. Ossification of the first costal cartilages, in the sternum of a male 

 aged fifty-four years. (Reduced two-thirds). 



Fig. 37. Diagram to illustrate the process of fusion of the several parts of the sternum, with 

 the average dates of coalescence. 



Fie ^8 (A C"l /Varieties in the number and arrangement of the centres of ossification of 



[ the sternum. A, B, male sterna, aet. four years. C, female, four 



r\g. 39. (D, r.) I years. D, male, E, female sternum, <z^/. five years. 



PLATE VI 



Fig. 40. Sternum of a female, aet. fifty-four, showing fusion of presternum and mesosternum 

 with an arthrodial surface, for the first costal cartilage (half natural size). 



Fig. 41. Sternum of a male, aet. fifty-nine, showing fusion of first piece of mesosternum 

 with presternum, pre-mesosternal articulation opposite third pair of costal 

 cartilages, and two-and-a-half pairs of costal cartilages articulating with pre- 

 sternum (half natural size). 



Fig. 42A. Sternum of a male, aet. forty-six. Male type (half natural size). 



Fig. 42B. „ „ female, aet. thirty-eight. Female type (half natural size). 



Fig. 43A. „ „ female foetus in the seventh month, showing supra-sternal cartilages, 



fused together, but separate from the presternum (natural size). 



Fig. 43B. Sternum of a female foetus at full time, showing two supra-sternal cartilages, 

 separated from one another, but each fused with the presternum. The 

 metasternum is absent, and there are only six sternal ribs on the right side 

 (natural size). 



Fig. 44. Presternum of an adult, viewed from behind, showing a supra-sternal ossicle 

 articulating with the supra-sternal notch on the left side, and an articular 

 surface on the right side (natural size). 



