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



of cat embryos which form a part of the Princeton Embryological 

 Collection. As the early stages of the sternum in this form agree 

 very well with those already described in the pig, we may make 

 our account brief. 



In the cat embryo 18.5 mm. long (series 21) the anterior ex- 

 tremity of the sternal anlage appears as an unpaired, median mass 

 of mesenchymal cells, the cells being arranged more densely on the 

 sides than in the center of the anlage. Just anterior to the level 

 of the ventral extremities of the first ribs the two sternal bands 

 separate out from this median mass, and can be traced as far as 

 the level of the ninth ribs. The ventral ends of the ribs are car- 

 tilaginous, but the cartilage is surrounded by a zone ofsclero- 

 genous tissue which fuses with the similar tissue composing the 

 sternal bands. Anteriorly these bands lie side by side diverging 

 very slightly until the level of the fourth ribs is reached; but pos- 

 terior to this region they are separated by the pericardial cavity, 

 and their divergence is more marked. In this posterior region 

 also the bands rapidly diminish in size; and, instead of being cres- 

 centic in outline on cross section, they are flattened out so that the 

 long diameter of their sections nearly corresponds in direction 

 with that of the ribs. The clavicles lie almost entirely anterior 

 to the anterior end of the sternal anlage. They contain cartilage, 

 but their medial ends are capped by sclerogenous tissue which 

 is prolonged medialward and caudalward so as to fuse with the 

 sides of the anterior extremity of the sternal anlage (fig. 5). 

 This connection is, however, very slight, extending only through 

 two or three sections (fig. 6). In the progress of development 

 this connection is soon lost. Thus in the embryo 25 mm. long 

 (series 22) there is no connection between the anlage of the clav- 

 icle and that of the sternum, and the clavicles lie entirely anterior 

 to the level of the sternum. 



The embryo 17 mm. long (series 36) presents the same appear- 

 ances as those just described, except that the sternal bands are 

 more divergent. 



In the embryo 16 mm. long (series 64) the clavicles are still 

 connected to the anterior extremit}^ of the sternal anlage in the 

 way previously noted. The sternum extends about twenty sec- 



