64 FRANK BLAIR HANSON 



4. That in early stages of the cat, pig, mouse, and human 

 embryos, the sternal bands exist as well-defined, separate, 

 mesenchymatous entities, prior to their union with the costal 

 cartilages, thus indicating their independence of, and the sec- 

 ondary nature of their relation to, the ribs. 



[V. THE PHYLOGENY OF THE STERNUM 



Paterson's comparison of the " continuous bar .... 

 across the middle line" in the rat with the cartilaginous scapular 

 arch of Acanthias, suggested the idea of following this structure 

 found by Haswell, and later independently by Parker, in the 

 middle ventral line of the shark Notidanus and identified by 

 them us a presternum, up through the various groups of verte- 

 brates to see how nearly it could be carried up in a phylogenetic 

 series to the rodents, in which Paterson thought he had detected 

 it again. For this purpose recourse was had to all available 

 figures extant in the literature, and especially to that monu- 

 mental monograph on the shoulder-girdle and sternum, by 

 Parker ('68). From these sources a series of figures has been 

 adapted, beginning with Parker's ('91) figures of the presternum 

 in the shark, and including one figure from the Ganoids, and 

 one each from the Teleosts and Dipnoi; then numerous figures 

 from the Amphibia, Reptilia, Birds, and Mammals. Such a 

 search through the literature, though wearisome, has rewarded 

 the labor far beyond any expectations. 



It is believed that the evidence presented in this phylogenetic 

 survey would alone go far toward convincing any one of the 

 truth of the conclusions arrived at in this paper, even though 

 it were not preceded by the corroborating evidence of the sec- 

 tion on the ontogeny of the sternum. It is the hope of the 

 author that it may lead to a general agreement as to the homology 

 and origin of the mammalian sternum. 



1. Fishes 



It is pretty clearly demonstrated by Haswell ('84) and also 

 by Parker ('91) that at least in one shark there is a presternum 



