DEVELOPMENT OF THE STERNUM 93 



MATERIAL EMPLOYED 



We have studied first the pig, thinking that the appearances 

 at the cranial end of the anlage would be less complicated in this 

 form owing to the absence of the clavicles; and then have followed 

 this with an examination of corresponding stages in the cat, in 

 which animal the clavicle is a rudimentary bone not articulating 

 with the sternum. Finally we have studied the development of 

 the sternum in human embryos, the clavicle here reaching its 

 full development. 



Observations in pig embryos 



"We shall begin the account of our findings by a description of 

 the sternal anlage as presented by a pig 24 mm. long.^ The 

 anlage is first encountered about 12 sections anterior (cranial) 

 to the level of the ventral extremities of the first ribs as an aggre- 

 gation of mesenchymal cells lying transverse to the median plane 

 of the body. At the level of the first ribs (fig. 1) it is somewhat 

 triangular in cross section, with the apex of the triangle directed 

 ventralward. Each lateral angle of the base is connected with 

 the corresponding first rib. The rib is in an early stage of car- 

 tilage formation, but its extremity is capped by a zone of sclerog- 

 enous tissue which fades ofT into the sternal anlage without any 

 definite demarcation, that is to say, there is direct continuity 

 of tissue between the anlage of the rib and that of the sternum. 

 In the figure the ventral portion of the triangle is labeled ' median 

 anlage' because, as will appear later, it has a different origin from 

 the more lateral portions. As the series of sections is examined 

 proceeding in the posterior (caudal) direction, indications of a 

 division of the triangular mass can be seen beginning on the 

 dorsal aspect, until, just posterior to the level of the ventral ex- 

 tremities of the first ribs, two somewhat crescentic bands of mes- 

 enchymal cells united across the median plane by embryonic 

 connective tissue separate out from the mass (fig. 2). Before the 



m 



* All the embryos were fixed in Zenker's fluid, and the measurementsweremade 

 in 80 per cent alcohol. 



