430 THE BABBIT. 



1, The Vertebral Column. 



On the tenth day, the notochord, which up to this time has 

 been the sole skeletal structure present, becomes surrounded 

 by a membranous sheath ; and, during the eleventh and twelfth 

 days, a cartilaginous tube begins to form around this sheath. 



By the fourteenth day, the cartilaginous tube is definitely 

 established ; and in it a distinction, as regards histological cha- 

 racters, is apparent, from the first, between the vertebral and 

 the intervertebral regions. The tube thickens on its inner 

 surface, and so begins to encroach upon the notochord. Opposite 

 the centra of the vertebras the notochord becomes constricted, 

 and finally completely obliterated. 



Between the successive vertebrae, in the intervertebral re- 

 gions, the notochord remains of full width for a long time ; and, 

 according to Kolliker and others, it even persists throughout life, 

 as part of the nucleus pulposus in the axes of the intervertebral 

 ligaments. 



From the vertebral centra the neural arches (Fig. 163, FN) 

 grow up at the sides of the spinal cord, during the fifteenth and 

 sixteenth days ; but the completion of the neural canal clorsally 

 does not occur until a late stage. 



The first two vertebras undergo modifications similar to those 

 already described in the bird ; the centrum of the first vertebra 

 or atlas (Fig. 167) separating from the rest of the vertebra, and 

 fusing with the centrum of the second, or axis, vertebra to 

 form its odontoid process. 



The transverse and other processes of the vertebras arise as 

 outgrowths from the cartilaginous centra or from the neural 

 arches. 



2. The Ribs and Sternum. 



The ribs (Fig. 163, Ri) arise as bars of cartilage, in the con- 

 nective tissue septa between the several muscle-segments or 

 myotomes of the thorax. In the rabbit, the two or three most 

 anterior ribs are at first continuous with the vertebrae, and 

 appear as elongated transverse processes. At a later stage a 

 joint is formed between the rib and the vertebra, and in this way 

 the tubercular articulation is acquired (cf. Figs. 163 and 167). 

 The head or capitulum of the rib develops as an outgrowth 



