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



of the sternal bands anterior to the level of the ribs. In other 

 respects there is no essential difference. The sternal bands still 

 extend as diverging, uninterrupted strands of cells along the ven- 

 tral extremities of the first seven ribs, and are in direct conti- 

 nuity with the tips of these ribs. The condition of the anlage in 

 this embryo seems to accord very well with Mueller's description 

 of the youngest stage found by her in the human embryo. So that 

 in the two embryos just described we have encountered stages 

 practically identical with the youngest stages described by those 

 who have studied the subject before us. But, as we have seen, 

 these stages are susceptible of very different interpretations, 

 and accordingly we have sought for still younger stages. 



Such a stage was found in an embryo 22 mm. long. In this 

 pig the sternal bands are still quite distinct anterior to the level 

 of the first ribs but the median portion of the anlage is less well 

 developed. There is an appreciable thickening of the mesenchyme 

 in the region mentioned, but it is not so well defined as in the 

 preceding embryo. The pericardial cavity reaches into the 

 neck and separates the first rib and sternal bands of the two 

 sides. As one proceeds caudalward through the series he is struck 

 by the fact that the ventral extremity of the first rib falls short 

 of the sternal band, and is connected with it only by embryonic 

 connective tissue. Fig. 7 shows the corresponding stage in the 

 cat. The ventral extremity of the second rib approaches the band 

 more closely, and the evidence as to continuity is not so clear as 

 in the case of the first rib; but, if there be any continuity of tissue 

 between the ventral extremity of this rib and the sternal band, 

 such continuity is very slight. The remaining ribs, however, 

 third to seventh inclusive, present the same continuity of tissue 

 between their tips and the sternal bands as was noted in preceding 

 stages. In the more cranial portion of the band, the longest 

 diameter in the cross sections is dorso-ventral in direction, whereas 

 posterior to the level of the second rib this dorso-ventral diameter 

 is much reduced, and the bulk of the anlage tends to lie ventro- 

 lateral ward from the tips of the ribs; moreover, the band becomes 

 considerably reduced in size in its posterior portion. 



