THE EARLY DEVELOPMENT OF THE MAMMALIAN 



STERNUM 



R. H. WHITEHEAD and J. A. WADDELL 

 From the Anatomical Laboratory of the University of Virginia 



EIGHT FIGURES 



In the text-books of human anatomy it is usually stated that 

 the sternum is formed by the union in the median plane of two 

 longitudinal cartilaginous bars, which are derived on each side 

 from a fusion of the ventral extremities of the embryonic ribs; 

 and that, after this union is established, the costal cartilages are 

 segmented off from the sternum with the formation of costo- 

 sternal articulations. This account is based chiefly upon the 

 studies of Ruge^ in 1880, although these had to do almost entirely 

 with the sternum after chondrification had begun, that is to say, 

 with a comparatively late stage of development. 



The accuracy of this account, with respect to some of its details 

 at least, was first questioned by Paterson.^ This author describes 

 the sternal anlage in a human embryo 'in the second month' 

 as consisting of an aggregation of mesoblastic cells in the median 

 line of the anterior (cranial) part of the thoracic wall. There 

 is no indication, he says, of bilaterality in the mass. The ventral 

 ends of the cla\'icles and ribs are composed of cartilage. The 

 first three ribs 'join' the cellular sternum, the fourth and fifth 

 join those above them, while the sixth and seventh have free 

 pointed ends. The meaning of the word 'join' is not entirely 

 clear, but subsequent use of the term in the article indicates that 

 the author does not intend to imply fusion of these structures. 



1 Ruge, G. 1880. .Untersuchungen ueber Entwicklungsvorgaenge am Brust- 

 bein und an der sterno-clavicular Verbindungen des Menschen. Morph. Jahrb. 

 Bd. 18. 



^ Paterson, A. M. 1900. The sternum, its early development and ossification 

 in man and mammals. Jour. Anat. and Physiol, vol. 33. 



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