60 FRANK BLAIR HANSON 



2. The anterior median sternal rudiment 



In the mouse, rat, and human embryos occurs a stage in 

 which a mesenchymatous girdle appears, in shape and rela- 

 tions comparable to the pectoral girdle of the shark. Figures 

 5 and 6 show this girdle in the mouse and human embryos. It 

 is composed throughout of mesenchyme cells, and the structural 

 development of each part may be followed in later stages. In 

 the mouse girdle the two dorsally extending wings on either side 

 are the rudiments of the scapulae; the medial and ventral ex- 

 tensions are the coracoids and clavicles; the enlarged portion in 

 the ventral midline is the fundament of the presternum. This 

 is conclusive evidence that the presternum is intimately asso- 

 ciated with the shoulder-girdle in the earliest ontogenetic stages 

 in the mammals, just as they are also phyletically bound to- 

 gether in the evolution of the vertebrate shoulder-girdle (infra). 



The mesenchymatous material extending from the scapulae to 

 the presternum (fig. 5) is the track in which the clavicles will 

 soon develop. In the human embryo (fig. 6, cl.) this has already 

 commenced on one side. According to Gegenbaur and his fol- 

 lowers, the core of the clavicles is the old cartilaginous precora- 

 coid of the Amphibia. If it be true that the clavicles do have a 

 precoracoidal core of cartilage, as Gegenbaur thought, here is the 

 coracoidal extension in the human embryo reaching the prester- 

 num in the ventral midline, just as it does in Hexanchus, Amphibia, 

 Reptilia, Aves, Monotremes, and fetal Marsupialia (infra). 



Gegenbaur's clavicle containing a precoracoidal cartilaginous 

 core has been attacked in several papers by Broom, who denied 

 the presence of any cartilage in the earliest stages of the clavicle. 

 However, Broom admits that cartilage does appear at a later 

 stage in the development of the clavicle, and it may be assumed 

 that cartilage appearing either as a clavicular basis (Gegenbaur) 

 or at some later stage (Broom) would in this region in highest 

 probability be coracoidal tissue. This position is strengthened 

 when it is recalled that in the Anura a precoracoid actually 

 functions as the core of the dermal clavicle. Huntington 3 is 



3 From a private communication containing Huntington's views on several 

 shoulder-girdle problems, kindly prepared and sent to the author October 30, 1918. 



