ONTOGENY AND PHYLOGENY OF THE STERNUM 49 



stage in man found by Mueller ('06), yet the interpretations of 

 these stages by the three authors are totally at variance. 



The next stage was a 22-mm. pig, longer than the preceding 

 by 2 mm., but distinctly a younger stage in so far as develop- 

 ment of these parts was concerned. The sternal bands and con- 

 necting bridge of mesenchymal cells are still anterior to the 

 first ribs; the pericardial cavity reaches far into the neck and 

 separates the sternal band and first rib of each side. Passing 

 caudally in the series of sections, the first rib falls just short of 

 reaching the sternal band; the extremity of the second rib ap- 

 proaches more nearly to the sternal band, and the remaining five 

 present the same continuity as before. 



In the next younger stage, the 18-mm. pig, the heart is far for- 

 ward in the neck, with the consequent wide separation of the sternal 

 bands in this region; the median connecting portion is now absent; 

 the remaining parts of the sternal bands were traced from a point 

 150 ^ anterior to the level of the first ribs back to the level of 

 the ventral extremity of the seventh; the first two pairs of ribs 

 do not reach the sternal bands, but the other five are firmly 

 fused with it. 



The earliest stages in which any sternal rudiment could be 

 detected were in 15- and 16-mm. pigs. The bands are not very 

 clearly defined and stop at the level of the third and fourth 

 ribs, respectively; again the first and second ribs fail to reach the 

 sternal bands, and it is the opinion of the authors that, "judging 

 from the behavior of the ventral extremities of the first and second 

 ribs in somewhat older stages, we think it probable that a stage 

 exists in which no rib is connected directly with the sternal 

 bands, but we were unable to detect such a stage." 



The cat embryos studied by Whitehead and Waddell (from the 

 Princeton Embryological Collection) ranged in size from 25 mm. 

 to 10 mm. From the description given, they are essentially the 

 same as the several corresponding stages of the pig. In the 

 12-mm. cat the first three ribs clearly did not reach the sternal 

 bands, the fourth was uncertain, while the three posterior pairs 

 made the connection. Since, from the account given, the early 

 stages in the pig and cat are practically identical, nothing further 

 need be said of the sternum in the cat. 



