THE HUMAN STERNUM ii 



((?) Mammals. 



My own 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 disposal by Professor Paul, 

 the first anlage of the sternum (PI. I, Fig. i) is to be seen as a dense con- 

 glomeration of mesoblastic cells, occupying 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 

 (PI. I, Figs. 2, 3, and 4). In the angle between the jaw and the thoracic 

 wall (prae-cervical sinus) there is a conglomeration in the middle line of 

 mesoblastic cellular tissue ; traceable laterally into association with the 

 shoulder girdle, and concerned with the formation of the clavicles, thinning 

 off as it passes backwards in the thoracic wall on the ventral aspect of the 

 bulging heart. 



At this stage this cellular mass is not connected with any of the 

 costal cartilages, which are extending in a ventral direction, and ending 

 distally in tapering points (PI. I, Fig. 4). There is, moreover, no indication 

 of bilateral subdivision of the mass, except at its cephalic end ; there the 

 cells are consolidated into two horns, which are concerned with the formation 

 of the clavicles and sterno-clavicular articulations, and with the anterior 

 parts of the presternum. 



(J?) In rat embryos, 10 mm. in length, an advance in development has 

 occurred (PI. I, Figs. 5-10). The sternum is still cellular. The presternum 

 is single and median (Fig. 5), and at its cephalic end the component 

 cells are more distinctly massed together on each side to produce the 



* Read at the Manchester meeting of the Anatomical Society of Great Britain and Ireland, June, 1900, 

 and published in the Journal of Anatomy and Physiology^ October, 1900. 



