98 R. H. WHITEHEAD AND J. A. WADDELL 



of the anlage encountered in the previous embryos, so that the 

 sternal anlage now consists of the two lateral portions, or sternal 

 bands, alone. These can be traced from a point about 10 sec- 

 tions of 15 microns each anterior to the level of the ventral ex- 

 tremities of the first ribs to the level of the seventh ribs. Each 

 band, while it presents about the same structure as in the pre- 

 ceding embryos, is smaller, the aggregation of the cells composing 

 it is less dense, and the band is not so well defined from the sur- 

 rounding mesenchyme. This is particularly true as to the more 

 posterior portion. Still the thickening is quite perceptible, and 

 is uninterrupted throughout. In cross sections the band occupies 

 a position dorsal to the junction of the axilla with the lateral 

 wall of the body, between the pectoral muscle laterally and the 

 pericardial cavity medially. Into this region the ventral ends of 

 the ribs project. They are now entirely free from cartilage, 

 and the cells composing them are very densely packed together, 

 so that the tips of the ribs are very deeply stained in the sections. 

 The first and second ribs fail to reach the sternal band, but the 

 succeeding five are fused with it, and offer the same evidences 

 of continuity with it shown in preceding stages (fig. 4). 



In embryos 16 and 15 mm. long the bands were still present, 

 but less clearly defined from the surrounding tissues. In the 16 

 mm. pig the bands stopped caudally at the level of the fourth 

 ribs; while in the 15 mm. embryo they could be traced no further 

 than the third rib. In both cases the first and second ribs failed 

 to reach the band. In embryos of less length than 15 mm. we 

 were not able to detect any sternal anlage with certainty. There 

 could be found a thickening of the mesenchyme in the region 

 occupied by the band in embryos 15 and 16 mm. long, but it was 

 too poorly defined to allow us to differentiate it from surrounding 

 tissues. Judging from the behavior of the ventral extremities 

 of the first and second ribs in somewhat older stages, w^e think 

 it probable that a stage exists in which no rib is connected directly 

 with the sternal band, but we were unable to detect such a stage. 



To recapitulate : We first find the anlage of the sternum as a 

 blastema occupying a region on each side of the body dorsal to 

 the juncture of the axilla with the lateral bodj^ wall between the 



