56 FRANK BLAIR HANSON 



To sum up, then, there are extant at the present time in the 

 literature three opposing theories as to the origin of the sternum 

 in the Mammalia. The oldest and most generally accepted of 

 these is that proposed by Ruge in 1880, which in substance 

 states that the sternum is a direct derivative of the ventral ends 

 of the costal cartilages. 



In 1900 and more fully in 1902 and 1904, Paterson was led to 

 doubt the validity of Ruge's theory, claiming that there was an 

 earlier history than that of which Ruge was aware. Paterson 

 derived the presternum from the same element which gives rise 

 to the shoulder-girdle, describing a continuous cellular element 

 crossing the midline in the rat. He derived the sternal bands 

 from this presternum as backward prolongations, which later and 

 secondarily are fused with the ventral ends of the ribs. 



Whitehead and Waddell ('11) agree with Paterson that Ruge 

 did not have the earliest stages, and that his theory is therefore 

 untenable, but they disagree with Paterson as to the interpre- 

 tation of these early stages. They deny any connection or re- 

 lation between sternum and shoulder-girdle, believing that both 

 presternum and sternal bands arise 'in situ.' 



This discussion of the literature is one of selected papers 

 which supports one or other of the different theories of sternal 

 origin and is fairly representative of the literature. However, 

 only a few papers are mentioned in comparison with the vo- 

 luminous literature extant. The author has collected a bibliog- 

 raphy of about one hundred titles on the sternum, but has con- 

 sidered it necessary to treat only a few of the more prominent 

 ones in this connection, with the assurance that those omitted 

 contain nothing new or affect the situation as outlined here. 



It has added greatly to the confusion existing between these 

 opposing theories that most of the more important papers 

 (Ruge's excepted) are very inadequately illustrated. Paterson 

 ('02) does not give a single figure in this paper and his figures in 

 the 1900 paper are small and inadequate. Important stages are 

 described in the Whitehead and Waddell paper, but those upon 

 which they base their chief conclusions are not supported by any 

 figures. If we had clear-cut drawings of Paterson's continuous 



