58 FRANK BLAIR HANSON" 



III. THE ONTOGENY OF THE STERN I'M 



The earliest development of the sternum in a number of mam- 

 mals has been worked out by Paterson, Kravetz, Whitehead 

 and Waddell, and myself. An attempt is made to compare the 

 steps of development in ontogeny with those in the phylogeny 

 of the sternum, or in other words to make a practical demonstra- 

 tion of Haeckel's recapitulation theory as applied to sternal 

 development. 



1. The sternal bands 



One decisive result of this investigation has been to demon- 

 strate the existence of the sternal bands as independent struc- 

 tures far earlier in development than Ruge and the older workers 

 suspected. Hence a new theory of sternal origin was demanded, 

 and as above indicated, this has taken two directions: White- 

 head and Waddell do not relate the sternal rudiment genetically 

 to any preexisting structure, while Paterson identified it with 

 the coracoidal girdle of lower forms. 



The conclusive evidence against Ruge's theory of costal origin 

 led the author to examine the material of Paterson and White- 

 head and Waddell, in an effort to confirm one or other of these 

 workers or reject both, as the case might be. Paradoxically 

 enough, I have been able to corroborate Paterson's account 

 of the shark-like girdle, found by him in the rat, both in 

 the mouse and human embryos; and in the identical slides 2 

 of cat and human embryos used by Whitehead and Waddell 

 have been equally able to confirm their observations. In pig 

 embryos of 24-mm. length, Kravetz found that the first rib did 

 not reach the sternal band, and the connections of the remain- 

 ing six ribs were too feeble to have any morphological importance. 

 Whitehead and Waddell studied a 24-mm. pig in which the first 

 rib did just reach the sternal rudiment and the union of the other 

 six ribs was marked. My 24-mm. stage agrees with that of 

 Kravetz in that the first rib does not reach the band, and with 

 Whitehead and Waddell's in that the connection of the other 



2 My sincere thanks are tendered Dr. C. W. F. McClure, of Princeton Uni- 

 versity, and Dr. Oeorge L. Streeter, of The Johns Hopkins University, for the 

 loan of the series of cat and human embryo studied by Whitehead and Waddell. 



