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



continuity with the ventral extremity of the first rib of the cor- 

 responding side, and is separated from its fellow by the pericar- 

 dial cavity. From this point each sternal band extends in the 

 posterior direction as an unbroken cord of cells as far as the level ^ 

 of the seventh pair of ribs. They diverge markedly, and are 

 widely separated by the body cavity. The band tends to assume 

 a position ventro-lateral from the tips of the ribs ; the bands and 

 the tips of the ribs are, however, in immediate continuity. 



In the remaining embryos of the series which we examined 

 no marked difference in the findings was noted until the embryo 

 13 mm. long, no. 175, was reached. Here we find a stage very 

 suggestive of that described in the 18 mm. pig and the 13 mm. 

 cat. The pericardial cavity extends far forward, and the sternal 

 bands are consequently still more widely separated at their ante- 

 rior extremities. The sternal bands could not be traced farther 

 in the posterior direction than the lev^el of the fifth ribs. The 

 ventral tips of the ribs are now non-cartilaginous, and if there be 

 any continuity between them and the sternal bands, it is very 

 slight — too slight in fact to suggest a derivation of the bands from 

 the ribs. Neither the clavicles nor the median sternal anlage 

 could be detected. It is quite possible, however, that the fail- 

 ure to find them was due to the fact that a considerable number 

 of sections had been lost, practically an entire slide full, from the 

 very region in which one would expect to find these structures 

 had they been present in the embryo. 



Finally, in the embryo 10.5mm. long, no. 109, we find the sternal 

 band extending in the posterior direction only as far as the level 

 of the fourth rib. The cells composing the bands are less com- 

 pactly arranged than in the preceding stages, and the bands are 

 less sharply differentiated fror'^j the surrounding tissues; tney are, 

 however, uninterrupted throughout. Moreover it is evident that 

 they are not continuous with the tips of the ribs, but are connected 

 with them only by loose mesenchymal tissue (fig. 8). 



Thus it appears that in principle the early stages of the develop- 

 ment of the sternum are practically identical in the three forms 

 which we have studied; the only difference concerns the relation 

 between the median anlage of the sternum and the blastema of 



