DEVELOPMENT OF THE STERNUM 91 



Kravetz^ has studied the development of the sternum in the 

 pig, using embryos which measured from 24 to 50 mm. in length. 

 In the 24 mm. embryo he describes the sternal anlage as consist- 

 ing of two bands of condensed mesenchymal cells which are fused 

 with each other at their anterior extremities, while caudally they 

 diverge and extend as far as the level of the ventral extremity 

 of the seventh rib on each side. He denies that there is 

 any true continuity of tissue between the ventral extremities 

 of the ribs, which are in the early stage of chondrification, and 

 the sternal anlage; indeed, the first rib, he states, does not reach 

 the sternal anlage at all. Later on the sternal bands become dif- 

 ferentiated into the cartilaginous sternal bars, chondrification 

 proceeding faster in their more cranial portions; and only after 

 this differentiation "tritt eine engere Verbindung der Leisten 

 mit der Ventralenden der Rippen ein." This stage, he thinks, 

 was incorrectly regarded by Ruge as primary. 



Finally, Charlotte Mueller^ in the course of an extensive study 

 of the development of the thoracic walls in man has made very 

 careful observations upon the development of the sternum. In 

 an embryo of 13 mm. she could find no evidence of the sternum, 

 but in one 17 mm. long the anlage was present in the form of two 

 widely separated bands of precartilaginous tissue — the term 

 'precartilaginous' being used in the chronological and not the 

 histological sense. These bands were connected with each other 

 at their cranial ends by a bridge of less closely aggregated mesen- 

 chymal cells. Caudally they diverged and were in direct con- 

 tinuity with the ventral extremities of the first seven ribs, the 

 tissue of the ribs shading off gradually into that of the sternal 

 anlage without any demarcation. In an embryo 15 mm. long 

 she found much the same condition as in the preceding case, 

 except that the bands were not so widely separated, their cranial 

 ends being almost in contact with each other; so that this embryo 



* Kravetz, L. P. 1905. Entwicklungsgeschichte des Sternums unci des Epis- 

 ternalapparates der Sauegethiere. Bull, de la Soc. Imper. des Natural. deMos- 

 cou. Annee 1905, nos. 1-2. 



* Mueller, Charlotte. 1906. Zur Entwicklung des menschlichen Brustkorbes. 

 Morph. Jahrb. Bd. 35. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 12, NO. 1 



