THE STEENUM: ITS EAKLY DEVELOPMENT AND 

 OSSIFICATION IN MAN AND MAMMALS^ {Pre- 

 liminary Communication). By Professor Pateeson, Liver- 

 pool. (Plates II., III.) 



In the following pages the record is given of some recent obser- 

 vations upon the early development and ossification of the 

 sternum in man and certain mammals (chiefly rodents). The 

 morphological problems associated with the anatomy and develop- 

 ment of this bone are so numerous and important, that it has 

 seemed desirable to make, ^n the first place, a brief statement 

 of certain facts of development, so as to clear the way for future 

 discussion and consideration of the more general anatomical 

 questions in which the sternum is implicated. 



I. Early Development. 



The following observations upon the early development of the 

 sternum were made on rat and human embryos. 



{a) In a series of continuous sagittal sections through a human 

 embryo in the second month, prepared and placed at my dis- 

 posal by Professor Paul, the first anlage of the sternum (fig. 1) 

 is to be seen as a dense conglomeration of mesoblastic cells, 

 occupj'ing the anterior part of the thoracic wall in the middle 

 line, and tapering off posteriorly: disappearing as a separate 

 structure about the middle in length of the thoracic wall. 



There is no indication of any bilateral arrangement of this 

 cellular tissue. The inner end of the clavicle and the costal 

 cartilages are composed of cartilage. The first three costal 

 cartilages join the cellular median sternum ; the fourth and fifth 

 cartilages join those above them; the sixth and seventh have 

 free pointed ends. 



In rat embryos of 9 mm. in length, a very similar condition 



is found (figs. 2, 3, 4). In the angle between the jaw and the 



thoracic wall (preecervical sinus) there is a conglomeration 



in the middle line of mesoblastic cellular tissue ; traceable 



^ Read at the Manchester meeting of the Anatomical Society of Great Britain 

 and Ireland, 1900. 



