162 MR. R. H. BURNE ON THE [Feb. 1 7, 



apparent difference being that the seventh was smaller than the 

 others. This seventh sternebra may thus be regarded as arising 

 from the seventh pair of ribs, just as the other sternebrse do from 

 the ribs in front, and as having a like morphological value with the 

 former. 



In the next stage (7 cm. long, fig. J, p. 161) the sternum had begun 

 to ossify, centres of ossification being present in the xiphisternum 

 and in all the sternebrse escept the seventh ; this structure, however, 

 was still clearly present, although only represented in cartilage. 



The seventh sternebra, in comparison with those remaining, is of 

 the same size as in the previous stage. In sterna of this age the 

 seventh pair of ribs have already begun to approximate ventrad of 

 their sternebra, their lower extremities reaching nearly halfway to 

 the middle line. Thus far the growth of the sternum has been 

 regular and uniform, with the exception of the lack of ossification in 

 the seventh sternebra ; but about this time the sternebrse begin to 

 grow unequally, and consequently to manifest slight differences in 

 their proportions (figs. K, L). The gap between the sixth and 

 seventh pairs of ribs is, at this stage, usually shorter than in the 

 earlier ones ; but occasionally these ribs are separated by a gap 

 (fig. L), indicating that the seventh sternebra is still in process 

 of growth. In none of these specimens was there, however, any 

 sign of ossification in the seventh sternebra, although in all the 

 other sternebrse it was much more pronounced than before. 



The points of the seventh pair of ribs had in all cases approached 

 much nearer the middle line than at the earlier stages. 



In Rabbits just born (14-15 cm.) the most noticeable feature is 

 the apposition of the lower extremities of the sixth and seventh pairs 

 of ribs, the latter having by this time met in the middle line. None 

 which I examined showed any trace of an ossific centre between the 

 sixth sternebra and the xiphisternum. 



It has been seen that in all 3 cm. embryos the seventh costal 

 segment is formed from the seventh pair of ribs, in the same way as 

 are the other costal segments from their respective ribs, and there- 

 fore it has morphologically the same value as each of those more 

 normally present ; it is, however, somewhat smaller than the other 

 segments. We may therefore assume that in all cases the seventh 

 sternebra is invariably present at an early age and that it differs only 

 from its fellows in size. 



All segments of the sternum appear now to grow uniformly ; for 

 in all 7 cm. embryos the seventh sternebra is relatively of the same 

 size as in the previous stage ; but it is still cartilaginous, all the 

 others being now ossified. So far all the sterna of the same age are 

 much alike; but in 11 cm. embryos the seventh sternebra is usually 

 shorter in comparison with its fellows than in the earlier stages, 

 although in some cases it is still of moderate size. Those in which 

 it was short I take to be the more normal, the seventh sternebra 

 having developed with the rest to a certain extent, and then, when 

 the embryo was about 8 or 9 cm. long, having ceased to grow : the 

 xiphisternum and sixth sternebra, as they increase in size, grow over 



