102 R, H. WHITEHEAD AND J, A. WADDELL 



tions anterior to the level of the first ribs as an irregularly oval 

 mass of cells less densely arranged in the center. The sternal 

 bands separate out just before the first ribs are reached, and ex- 

 tend caudahvard as far as the eighth pair of ribs. At first they 

 are circular on cross section, but soon become crescentic, and 

 finally are flattened out so as almost to meet each other across the 

 median plane. They are uninterrupted throughout. Under the 

 low power there is an appearance suggestive of a line of demarca- 

 tion between the sternal bands and the tips of the first and second 

 ribs, but under the high power the two structures — ribs and ster- 

 num — appear intimately united. 



In the embrj'O 14 mm. (series 37) and 13 mm. long (series 35) 

 the median portion of the sternal anlage noted in the pig makes 

 its appearance as a distinct structure just posterior to the level 

 of the clavicles. It consists of a band of cells somewhat larger 

 and less compactly arranged than the cells of the sternal bands 

 stretching across the median plane. Its lateral extremities are fused 

 with the anterior ends of the sternal bands about fifteen sections 

 anterior to the level of the first ribs. The sternal bands are widely 

 separated throughout their extent by the pericardial cavity, 

 and diverge markedly. They occupy a position in the sections 

 dorsal to the line of junction of the axilla with the body wall, 

 and extend as far as the seventh ribs. They are united with 

 the ends of these ribs. 



In the embryo 12 mm. long (series 401) the heart and peri- 

 cardial cavity are well forward in the neck, and there is no evi- 

 dence of the median portion of the sternal anlage. The clavicles 

 are present as a short bar of cells lying far out in each side of the 

 neck, the two being widely separated by the pericardial cavity. 

 The sternal bands still extend as far caudalward as the seventh 

 ribs. The ventral ends of these ribs are now composed entirely 

 01 sclerogenous tissue. The first three fail to reach the sternal 

 bands (fig. 7) ; it is uncertain if the fourth does, but the remain- 

 ing three are connected with the bands as in the older embryos. 



The embryo 10 mm. in length (series 402) which, however, was 

 from the same litter as the previous one, shows no differences with 

 respect to the sternal anlage, except that the clavicles could not 



