42 FRANK BLAIR HANSON 



• 



the subject of numerous contributions for a century past, yet 

 to-day there is no general agreement upon many of the points 

 involved. 



For several years the writer has been studying the shoulder- 

 girdle region in the vertebrates. This included also a restudy of 

 the origin of the sternum, the results of which are embodied in 

 this paper. The fact that my conclusions are at variance with 

 the usually accepted theory of sternum origin only adds to the 

 interest of the undertaking. If this paper settles the points at 

 issue or stimulates further investigation upon the part of others, 

 it will in either case not have been in vain. 



One of the recognized deficiences of much of the previous work 

 is that general and far-reaching conclusions have been deduced 

 from the study of only one or two forms, and these most often 

 the more highly specialized ones. The author has attempted 

 herein to bring together corroborating lines of evidence from 

 both ontogeny and phylogeny, believing that a theory of sternal 

 origin only so demonstrated can command consideration. 



I desire to express my deep appreciation for the constructive 

 criticisms and helpful suggestions given by Prof. J. Sterling 

 Kingsley during the course of this investigation. 



II. CRITICAL ESTIMATE OF EXISTING THEORIES 



1 . Ruge 's theory of costal origin 



Ruge ('80) was the first investigator to work up and present 

 a well-developed and illustrated paper containing a theory of the 

 origin of the sternum. His fifty pages of text and twenty-two 

 figures gave his theory a commanding place in the literature. 

 Most books on human and comparative anatomy, until the 

 present day, copy his figures and accept his view that the sternum 

 arises as a product of the ventral costal cartilages. As an example 

 of this, Keibel and Mall in their two-volume Embryology give 

 the following statement concerning the origin of the sternum: 

 "The cartilage of the sternum arises mainly from the cartilage 

 of the ribs, from which it is secondarily separated by the for- 

 mation of the costosternal joints." So completely has Ruge's 



