DEVELOPMENT OF THE STERNUM 95 



level of the second rib is reached the band changes shape, the con- 

 cavity of the crescent being directed lateralward. From here 

 on the anlage consists of two diverging, uninterrupted bands of 

 mesenchymal cells extending, one on each side, as far as the ven- 

 tral extremity of the seventh rib. The extremities of these six 

 ribs, as in the case of the first rib, are undergoing chondrification, 

 but each is capped by a zone of sclerogenous tissue the cells of 

 which are entirely similar to those composing the sternal bands; 

 and the anlages of the two structures, ribs and sternal bands, 

 shade off into each other without any definite demarcation. It 

 is worthy of note, however, that while the first rib joins the lateral 

 angle of the triangular portion of the anlage (fig. 1), the remaining 

 ribs join in the concavity of the crescent. This embryo thus 

 presents a stage of sternal formation quite similar to the youngest 

 embryo of Kravetz (24 mm.). Kravetz stated, however, that 

 the first rib did not reach the anlage of the sternum in his embryo ; 

 and he held, moreover, that the connection between the other 

 six ribs and the sternal band was too feeble to have any special 

 significance. To us, on the other hand, this connection seems 

 marked; and we should not feel able from an examination of this 

 stage alone to conclude that the sternal bands either are or are 

 not derived from the ventral ends of the ribs. 



A considerably younger stage was found in a pig 20 mm. long. 

 Here the pericardial cavity extends into the neck, and the ventral 

 extremities of the ribs are separated by a greater interval than in 

 the preceding embryo. A section anterior to the level of the ribs 

 shows that in this region the sternal anlage consists of three parts : 

 the sternal band on each side, and a wide band of less condensed 

 mesenchyme reaching across the median plane and connecting 

 the two sternal bands with each other (fig. 3). The latter are 

 made up of more densely aggregated cells, and consequently 

 appear darker in the stained sections. The anlage in this stage 

 differs from the appearance presented in the preceding embryo 

 mainly in two respects: 1. The sternal bands exist as separate 

 structures well in advance of the ventral ends of the first ribs. 

 2. There is a median portion of the anlage consisting of less densely 

 aggregated mesenchymal cells connecting the cranial extremities 



