ONTOGENY AND PHYLOGENY OF THE STERNUM 59 



six is marked. No special significance attaches to these varia- 

 tions since individual differences in development as well as in 

 measurements and amount of shrinkage gives form for consid- 

 erable variation. However, in stages of the pig 22 mm., 20 

 mm., 18 mm., and smaller, there is no doubt of the failure of 

 several pairs of the anterior ribs to meet the sternal rudiment. 



Figures 3 and 4 are of two cat embryos from the Princeton 

 Embryological Collection. Camera-lucida drawings were made 

 of the sternal bands and ventral ends of the ribs. These struc- 

 tures were then plotted upon millimeter-ruled paper, which gives 

 a graphic reconstruction made to scale. Several wax models 

 were also made from the early pig and mouse embryos. Figure 

 3, of the 12-mm. cat embryo, shows clearly that in this stage the 

 first three pairs of ribs do not extend to and unite with the ster- 

 nal bands. It is also apparent that no anterior sternal rudiment 

 or presternal rudiment is present at this age. It is undoubtedly 

 true, as claimed by Whitehead and Waddell, that in the ontog- 

 eny of the mammalian sternum the two sternal bands antedate 

 in appearance the median and anterior rudiment. However, in 

 the phylogeny of the sternum, as will be shown further on, the 

 presternum is the first to arise, and from this come the sternal 

 bars. I am unable to account for this discrepancy by any ob- 

 served facts, but think the history from phylogeny must take 

 precedence over that from ontogeny; explaining the rise of the 

 sternal bands in the mesenchyme of the mammal as the result 

 of protoplasmic memory, which dates back to the early reptilian 

 ancestor in which the presternum grew backward as two pro- 

 longations that became the mesosternum and the xiphisternum, 



In human embryos from the F. P. Mall Collection studied 

 by Whitehead and Waddell and myself, is found the best evi- 

 dence of the complete separation of ribs and sternal bars in the 

 early stages of development. In embryos 10.5 mm. and 13 mm. 

 long none of the ribs reaches the sternum, the presternum has 

 not yet appeared, and no clavicles are apparent. These stages, 

 if graphically represented, would appear similar to figure 3 of 

 the cat, except that all the ribs would be in the same relation 

 to the sternum as are the first three in the stage of the cat 

 figured. 



