48 FRANK BLAIR HANSON 



feeble, while in specimens smaller than 15 mm. no rudiment could 

 with certainty be detected. 



In a pig 24 mm. long, to follow the description of Whitehead 

 and Waddell, the sternal rudiment is an aggregation of mesen- 

 chymal cells lying transverse to the median plane of the body. 

 In cross-section it is triangular, with the apex directed ventrally, 

 and each lateral angle of the base connected with the corre- 

 sponding first rib, there being a perfect and direct continuity of 

 tissue between the rudiment of the first rib and that of the 

 sternum. Proceeding backward in the series of sections two 

 bands of mesenchymal cells separate from the mass and extend 

 as far back as the level of the seventh rib, all seven ribs being 

 connected with and shading off into the sternal bands without 

 any definite demarkation. This is the stage in which Kravetz 

 ('05) found that the first ribs did not reach the sternal rudiment, 

 and the junction of the other six ribs was too feeble to have any 

 morphological significance. Whitehead and Waddell think this 

 is not a tenable conclusion in light of the intermediate position 

 occupied by this specimen, i.e., between older stages in which 

 no doubt of the absolute continuity of ribs and sternum exists, 

 and younger stages in which they seek for new facts concerning 

 the earliest relation between these two structures. 



In a 20-min. pig the pericardial cavity extends into the neck, 

 the ventral ends of the ribs are wide apart, and, in the region 

 anterior to the level of the first rib, the sternal rudiment is present 

 and is composed of three parts : the two sternal bands and a plate 

 of more diffused mesenchymal cells connecting the two across the 

 middle line. This stage presents two facts worthy of note: 

 the sternal bands are well defined separate structures at a level 

 considerably anterior to that of the first rib, and, further, in this 

 anterior region the two sternal bands are connected by a median 

 aggregate of cellular tissue. Posteriorly, in the region of the 

 ribs, there is the same continuity between costal extremity and 

 sternal band as in the 24-mm. stage. 



Just as the older stage of Whitehead and Waddell (24-mm. 

 pig) corresponded to the youngest studied by Kravetz, so does 

 this 20-mm. stage in the pig correspond essentially to the youngest 



