ONTOGENY AND PHYLOGENY OF THE STERNUM 85 



3. The body of the sternum ends at 'the level of the fourth 

 ribs. The fifth pair is attached to its lower end. The fifth, 

 sixth, and seventh pairs meet one another and fuse. 



Kirchner ('98) and Adolphi ('05) also describe sterna in large 

 numbers and find many cases of variation in the posterior tho- 

 racic ribs much in accord with the above. The results of these 

 papers on the adult sterna of the human subject, together with 

 similar observations on the lower forms, which are pertinent to 

 our thesis are four : 



1. Ribs posterior to the level of the sternum may grow ven- 

 trally and meet by fusion or articulation in the midline 

 (Chamaeleo) . 



2. Ribs at the level of the sternum may articulate firmly with 

 it, as is the usual case in man. 



3. Ribs may pass on the ventral side of the posterior end of 

 the sternum and fuse in the midline much as they do when farther 

 back where no sternum is present (Chamaeleo, man). 



4. All ribs may fail to meet the sternum at any point without 

 affecting the full development of that structure (Amphibia, 

 Chirotes, Manatus, human embryo). 



Plate 12, figures 43 to 49, inclusive, is designed to give at a 

 glance a series of shoulder-girdles representing the main groups 

 of vertebrates, in an effort to show how from the elasmobranch 

 to the rodent, either in the embryo or throughout life, there is an 

 intimate relation in all between shoulder-girdle and sternum, and 

 in some it is demonstrated that they are for a time one struc- 

 turally continuous element. In some (shark, lissotriton, lizard, 

 and monotreme) this relation is maintained throughout life; in 

 others (marsupial, mouse, rat, and man) it is of short duration in 

 embryonic life. In all, however, it indicates a phylogenetic 

 relationship between the shoulder-girdle and sternum in all stages 

 of vertebrate evolution that cannot possibly be duplicated or 

 even remotely approximated by a similar comparison of sterna 

 and costal cartilages. 



In most adult Amniotes the sternum is in close relation with 

 both ribs and shoulder-girdle; in some, however, the connection 

 with either ribs or shoulder-girdle may be lost (Chirotes, fig. 30, 



