Chap. VIII.] THE SKELETON. 165 



In the young rabbit there are cartilaginous intervals in each 

 centrum near the ends ; these separate the main mass (diaphysis) 

 of the centrum from its end plates (epiphyses) which are charac- 

 teristic of mammalian vertebrae. 



It will be remembered that the first indication of the axial 

 skeleton in the body is the notochord which underlies the 

 neural axis. This becomes invested by a cartilaginous sheath, 

 which comes into relation with a number of cartilaginous 

 arches derived from the mesoblastic investment of the spinal 

 cord. As growth proceeds, the notochord is encroached upon 

 by its cartilaginous investment, which becomes differentiated 

 into vertebral and intervertebral regions. In the vertebral 

 region the notochord is finally obliterated ; but in the in- 

 tervertebral . region it persists throughout life as part of the 

 nucleus pulposus in the axis of the intervertebral ligament. 

 Before the differentiation of the vertebrae, there is split off 

 from the upper and outer part of each somite a muscle-plate 

 (see p. 120), from which is formed the voluntary muscle of that 

 part of the trunk. It is to be noted, however, that the ver- 

 tebral segmentation takes place in such a way that the centrum 

 of the newly-formed vertebra does not correspond in position to 

 the middle of the muscle-plate, but to the line separating two 

 muscle-plates; the muscles formed from which, therefore, act 

 upon two adjoining vertebras. 



The odontoid process of the axis ossifies from a distinct centre, 

 and is at- first a distinct bone. It is indeed a portion of the 

 centrum of the atlas, which has lost its primitive connection 

 and become conjoined to the axis. The so-called transverse 

 processes of some (cervical, sacral) vertebrae are to be regarded 

 as ankylosed ribs. 



(2.) In the Fowl there are sixteen cervical vertebrae, of which 

 the last is ankylosed to the first three thoracic. The fourth 

 thoracic is free ; but the fifth or last coalesces with a number 

 of succeeding vertebrae to form the compound sacrum. In 

 this sacral mass there are ankylosed : one (the fifth) thoracic ; 

 three lumbar; five (ribless) sacral; six (with ankylosed ribs) uro- 

 sacral; fifteen in all. Following on the sacrum are six free caudal ; 



