600 . EMBRYOLOGY. 



of the arch are fused. Each vertebra is then separable after 

 destruction of the soft parts into two pieces, into the body and the 

 arch. These are united between the third and eighth years. 



In addition to the pieces of bone just described, accessory centres of ossification 

 appear on the vertebras in subsequent years ; it is in this way that there arise 

 the epiphysial plates at the end-surfaces of the body and the small bony pieces 

 at the ends of the vertebral processes (the spinous processes and the transverse 

 processes). SCHWEGEL gives detailed information concerning the time of their 

 appearance and their fusion. 



Cartilaginous skeletal parts, which serve for the support of the 

 lateral and ventral walls of the body, the ribs and the breast bone, 

 contribute to the completion of the axial skeleton. 



The ribs are developed independently of the vertebral column, in 

 Man during the second month, by the chondrification of strips of 

 tissue in the intermuscular ligaments between the successive muscle- 

 segments. They are at first visible as small bent rods in the imme- 

 diate vicinity of the body of the vertebra, and from here they rapidly 

 extend ventrally. 



In early stages of development ribs are established from the first 

 to the last segment of the vertebral column (the coccyx in Man 

 excepted), but only in the case of the lower Vertebrates (Fishes, 

 many Amphibia, and Reptiles) are they developed into large bows 

 supporting the wall of the trunk in a uniform manner in all regions, 

 whei'eas in Mammals and in Man they exhibit in the separate regions 

 of the vertebral column different conditions. In the neck, lumbar 

 and sacral regions, they appear from the beginning in a rudimentary 

 condition only, and undergo metamorphoses to be described later. It 

 is exclusively in the thoracic region that they attain important 

 dimensions, and here at the same time they give rise to a new skeletal 

 part the breast bone, or sternum. 



The sternum, which is wanting in Fishes and Dipnoi, but is present 

 in Amphibia, Reptiles, Birds, and Mammals, is a formation derived 

 from the thoracic ribs, and is originally established, as RATHKE was the 

 first to discover, as a paired structure, which early fuses into an 

 unpaired skeletal piece. 



RUGE has followed the development of the sternum in Man in a 

 very thorough manner, and has found that in embryos 3 cm. long the 

 nrst five to seven thoracic ribs have become prolonged into the ventral 

 surface of the breast and by a broadening of their ends have united 

 at some distance from the median plane to form a cartilaginous band, 

 whereas the following ribs end free and at a greater distance from 



