22 PROFESSOR PATERSON. 



laterally into association with the shoulder girdle, and con- 

 cerned 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 direc- 

 tion, and ending distally in tapering points (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 stern o-clavicular articulations, and with the an- 

 terior parts of the pre-sternum. 



(b) In rat embryos, 10 mm. in length, an advance in develop- 

 ment has occurred (figs. 5-8). The sternum is still cellular. The 

 pre-sternum is single and median (fig. 9), and at its cephalic 

 end the component cells are more distinctly massed together 

 on each side to produce the sterno-clavicular joints, and the 

 inner ends of the clavicles. The cellular meso-sternum (figs. 

 10-12) consists of two definitely separate strands of mesoblastic 

 cells, which are separated in the middle line by a layer of 

 sparser and more loosely arranged cells. The cellular sternum 

 is now joined by the first six costal cartilages (the seventh 

 is still separate on each side), but there is an obvious distinc- 

 tion in the character of the cells composing sternum and costal 

 cartilages (figs. 11, 12). The eighth costal cartilages are not in 

 any way related to the sternum, and there is no indication 

 of a meta-sternum. 



In rat embryos, 13 mm. in length, the process of develop- 

 ment has perceptibly advanced. Though the sternum is still 

 cellular, and not yet converted into cartilage, the median 

 separation between the two halves of the meso-sternum has 

 disappeared, and the meso-sternum consists of a single median 

 band of cellular tissue (fig. 14). The meta-sternum (fig. 15) 

 has appeared ; cellular in character, and tapering off in the 

 angle between the chest wall and the diaphragm. The clavicle 

 in its outer part is undergoing ossification, and its inner end is 

 cartilaginous. It is clearly demarcated from the pre-sternum, 

 although no joint cavity has yet appeared in the sterno-clavicular 

 articulation (fig. 13). All tlie seven costal cartilages are now 



