DEVELOPMENT OF THE STERNUM 99 



pectoral muscle laterally and the pericardial cavity medially. 

 At first the blastema is found only in the more anterior portion 

 of this region, reaching as a column of cells from a point well 

 anterior to the level of the first rib as far as that of the 3rd rib on 

 each side in embryos 15 mm. long. It extends rapidly in the 

 caudal direction, so that in the embryo 18 mm. long it reaches as 

 far as the level of the ventral end of the seventh rib. It is unin- 

 terrupted throughout its entire extent. Into the region occupied 

 by these sternal bands the ventral ends of the growing ribs pro- 

 ject, and soon fuse with the sternal anlage. At first they are 

 not cartilaginous, but are composed entirely of sclerogenous tis- 

 sue in embryos 18 mm. long. In the earliest stages the first and 

 second ribs do not reach the sternal anlage ; it could not be deter- 

 mined whether or not this is also true of the more posterior ribs 

 of the series. However, at the time the embryo is 20 mm. long 

 the ventral ends of the first seven ribs are firmly fused with the 

 sternal band on each side. At first, owing to the projection of 

 the heart into the neck, the two sternal bands are widely separated 

 throughout their entire extent; but as the heart sinks into the 

 thoracic cavity, the bands are allowed to approach each other, 

 the approximation being greatest in the neck, until their anterior 

 extremities finally meet and fuse. Coincident with the sinking of 

 the heart, there appears a third division of the sternal anlage in 

 the shape of a transverse bridge of mesenchymal cells in the ven- 

 tral wall of the neck which unites the anterior extremities of the 

 sternal bands. As the latter approach each other, this median 

 anlage becomes incorporated with them, and thus assists in the 

 formation of that part of the sternum which lies anterior to the 

 level of the first pair of ribs. At no stage could we detect any 

 evidence of continuity between the sternal anlage and any ele- 

 ment of the shoulder girdle. 



Observations in cat embryos 



Our study of the development of the sternum in the cat was 

 made possible by the courtesy of Professor C. F. W. McClure, 

 who gave us the opportunity of examining the beautiful series 



