44 FEANK BLAIR HANSON 



originally derived from the same (italics mine) element; a primi- 

 tive band of cellular tissue which crosses the midline. " Paterson 

 has no interest in deriving the sternum from the shoulder-girdle 

 or vice versa; his contention being, first, that the sternum is not 

 a product of the costal cartilages, and, second, that it is yielded 

 from a common, continuous, mesenchymatous element which 

 gives rise to the shoulder-girdles and the sternum. 



Paterson ('02) compares this continuous cellular element in 

 the rat to the girdle in the elasmobranchs. He exhibited before 

 the British Medical Association his sections of rat embryos side 

 by side with embryos of Acanthias vulgaris to demonstrate that 

 ''essentially the same method of development occurs in the dog- 

 fish and in the rodent. But a marked difference is produced in 

 the process of development. Instead of a jointed and highly 

 differentiated structure such as is characteristic of mammals, a 

 simple continuous bar of cartilage is formed, across the middle 

 line and below the heart, which gives rise laterally to the primi- 

 tive shoulder-girdle." 



Paterson ( '00) also points out, and gives several figures in sub- 

 stantiation, that the parts of the sternum opposite the costal 

 attachments remain longest in a cellular condition. His point 

 being, of course, that if the sternum were ossified from the ribs, 

 these regions should ossify first, and not last as is actually the 

 case. 



This comparison of the girdles in the shark and rat embryos 

 is very suggestive. Many other structures of present-day mam- 

 mals may be traced directly back to homologous structures in 

 the elasmobranchs, and since in the cartilaginous girdle of the 

 shark we have all the necessary material and in proper position 

 for differentiation into scapulae, coracoids, and sternum, we might 

 even upon a priori grounds expect to find in the higher groups of 

 vertebrates an embryonic stage in which the rudiment of the 

 girdles and sternum might be represented by such a "contin- 

 uous bar .... reaching across the middle line " as Paterson 

 found in the rat. 



