THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 603 



of the vertebral column, namely, between the second and the sixth 

 year after birth. For a long time the five sacral vertebrae remain 

 separated from one another by their intervertebral discs, which 

 begin to ossify in the eighteenth year ; the process has usually come 

 to an end by the twenty -fifth year. 



Behind the sacrum there follow four or five rudimentary coccygeal 

 vertebrae, which represent the caudal skeleton of Mammals and do 

 not acquire centres of ossification until very late. In the thirtieth 

 year or later they may fuse with one another, and sometimes with 

 the sacrum. 



Atlas and epistropheus (axis) now demand special mention. These 

 vertebrae acquire their peculiarities of form by an early fusion of the 

 cartilaginous body of the atlas (fig. 328a) with the epistropheus (e) 

 to form the odontoid process of the latter. The one therefore 

 contains less, the other more than a normally developed vertebra. 



That the odontoid process is the real body of 

 the atlas is recognisable even later by means of 

 two facts. First, like every other vertebral o 



body, it is traversed, as long as it remains 4 



cartilaginous, by the chorda, which at the tip Pig>888 ._ J ^ llliaeettal 

 of the process is continued into the ligamentum through the body and 



suspensorium and from this into the base of the 



cranium. Secondly, it acquires in the fifth in the cartilage two cen- 



month of development a separate centre of S&jJESi. 



ossification (fig. 328 a), which is not com- 



pletely fused with the body of the epistropheus until the seventh 



year. 



The neural arches of the atlas, which have remained independent, 

 are joined together on the ventral side of the odontoid process by a 

 tract of tissue in which an independent piece of cartilage is formed 

 (hypochordal cartilage -rod of FRORIEP) a structure which, according 

 to FRORIEP, is present in every vertebra in the case of Birds. This 

 piece of cartilage develops in the first year after birth a special centre 

 of ossification, fuses between the fifth and the sixth year with the 

 lateral halves, and constitutes the anterior [ventral] arch (KOLLIKER). 



(b) Development of the Head-Skeleton. 



From its position the skeleton of the head appears as the most 

 anterior part of the axial skeleton, but it is on the whole very unlike 

 the posterior part, the vertebral column, because it is adapted to 



