DEVELOPMENT OF THE STERNUM 103 



be detected, and the sternal bands could not be traced farther 

 than the level of the sixth pair ot ribs. 



From this account it appears that the early stages of the devel- 

 opment of the sternum in the cat are practically identical with 

 those observed in the pig. There are some minor differences such 

 as the fact that there are nine vertebro-sternal ribs in the cat, 

 and the further fact that there is a rudimentary clavicle in the 

 cat connected for a short time with the anterior end oi the sternal 

 anlage. The presence of this clavicle, however, does not influence 

 the median portion of the anlage, which develops in the same way 

 in both forms. 



Observations in human embryos 



It was desirable to compare the findings in the pig and cat with 

 corresponding stages of a form in which the clavicle attains full 

 development. This we were enabled to do through the kind- 

 ness of Dr. Mall, who placed the human embryos of the Johns 

 Hopkins Embryological Collection at our service. The embryos 

 which we found adapted to our purpose were those represented 

 by the serial numbers 424, 17.2 mm. long; 296, 17 mm. long; 

 409, 16 mm. long; 423, 15.2 mm. long; 144, 14 mm. long; 175, 

 13 mm. long; and 109, 10.5 mm. long. In embryos of less length 

 than no. 109 we were unable positively to identify any sternal 

 anlage. 



In the embryo no. 424, 17.2 mm. long, the sternal anlage pre- 

 sents a stage practically identical with the earliest stage described 

 by Mueller. The anterior portion of the anlage consists of a 

 median mass of closely packed mesenchymal cells having the 

 form of a band in transverse sections. On each side this band 

 becomes directly continuous with the sclerogenous tissue forming 

 the medial extremity of the clavicle. Two sections (each 50 

 microns thick) posterior to this point the anlage is triangular in 

 cross section, the two sides staining more darkly than the central 

 portion and still shading off into the clavicles. Four sections 

 farther the median anlage disappears entirely, and only the lat- 

 eral portions, or sternal bands, are left. Each of these is in direct 



