SKELETON 55 



or penetrates the former. So it follows that the plates labelled i 

 intercalaria of most authors) are derived from the cranial half 

 lero tomes. 



The vertebrae are outlined at an early stage of the embryo and 

 their number is not subsequently increased. Consequently increase 

 in length of the vetrebral column can occur only by growth of the 

 vertebrae themselves. When first formed each centrum encircles 

 the notochord and prevents its increase in diameter at this point, 

 while between the centra it can expand. As a result the notochord 

 soon resembles a string of beads (is moniliform) with intervertebral 

 enlargements. Then, as additions are made to the centra to in- 

 crease their length, the new parts must form around the intervertebral 

 enlargements and in this way the ends of the centra become cup- 

 shaped and the amphicoelous condition (fig. 49, /) is produced. 

 In some urodeles this stage is followed by the deposition of cartilage 

 in the cups (fig. 49, //) producing intervertebral constrictions of 

 the cord. As this progresses absorption of the cartilage begins 

 between the ends of the vertebrae (ic) and continues in such a way 

 that the result is a ball of cartilage attached to the hinder vertebra 

 and a corresponding cup in the one in front; in other words, an 

 opisthocoelous condition. The proccclous condition (fig. 43, B) 

 is the result of a similar process, except that the socket is in front, 

 the ball behind. 



Regions of Vertebral Coluirm. — Several regions may be dif- 

 ferentiated in the vertebral column, these being the most numerous 

 in the higher groups of vertebrates. These are (i) the cervical, in 

 the neck, with great reduction or even absence of ribs; (2) the thoracic, 

 following the cervical, with distinct ribs; (3) lumbar, without ribs; 

 (4) sacral, including one or more vertebrae with which the pelvis 

 is connected; (5) caudal, the tail, behind the sacrum. Sometimes 

 the ribs extend back to the sacrum so that thoracic and lumbar 

 (annot be distinguished, all being then grouped as dorsal. Then in 

 the fishes and some higher vertebrates (snakes, whales, etc.) sacral 

 vertebrae are not differentiated, and in the fishes there is no line 

 between cervicals and dorsals, so that only trunk or abdominal, and 

 caudal regions can be distinguished, the line being drawn (fishes) 

 at the point where haemal arches are transformed into so-called ribs 

 (see p. 60). 



One or two of the anterior vertebrae are modified in the higher 

 (tetrapodous) vertebrates and have received names. The first, 



