90 R. H. WHITEHEAD AND J. A. WADDELL 



* 



This observation led Paterson to make a study of the develop- 

 ment of the sternum in embryos of the rat. In the rat of 9 mm. 

 he finds the sternal anlage as a structure quite similar to that just 

 described in the human embryo — a median mass of cells in the 

 anterior part of the thoracic wall without bilateral subdivision 

 except in front, "where the cells are consolidated into two horns 

 which are concerned with the formation of the clavicles and sterno- 

 clavicular articulations and with the anterioi parts of the prester- 

 num." The sternal anlage is not connected with that of any of 

 the costal cartilages. In the embryo 10 mm. long the sternum 

 is still cellular. Here there is added to the single median anlage 

 of the presternum the anlage of the mesosternum in the shape 

 of two strands of cells which diverge caudally. The strands are 

 joined by the ventral extremities of the first six ribs, but, he states, 

 there is an obvious difference between the cells composing the 

 two structures. Later on the two cellular strands unite with 

 each other in the median plane forming an unpaired mass of cells 

 which is connected with the first seven ribs on each side. Ulti- 

 mately the cartilaginous sternum of the ra1 is laid down as a 

 single median band separated from the clavicles by connective 

 tissue, but in complete fusion with the first seven costal carti- 

 lages on each side. 



Paterson's observations on the human embryo are probably of 

 little value ; but his findings in the rat led him to challenge the 

 traditional account of the development of the sternum. In a 

 subsequent article^ he holds that the anlage of the sternum is at 

 first single, median in position, and directly continuous with the 

 mass of cells which is to form the shoulder-girdle on each side ; 

 and that the shoulder-girdle and the presternum are derived from 

 the same elements. This anlage of the presternum bifurcates 

 into two strands, which grow caudalward and become connected 

 secondarily with the ventral extremities of the ribs. Thus in 

 the final analysis, according to Paterson's view the sternum is 

 derived from the shoulder-girdle. 



* Paterson, A. M. 1902. The development of the sternum and shoulder girdle 

 in mammals. Brit. Med. Jour., vol. 2, p. 777. 



