2 28 Vertebrate Embryology 



cartilaginous, and forms a continuous cartilagi- 

 nous sheath around the notochord through- 

 out its entire length. At the sides of the 

 spinal cord there are formed paired bars of 

 cartilage, which soon fuse with the cartilagi- 

 nous sheath of the notochord, and form the 

 rudiments of the neural arches. 



Before the end of the fifth day, marked his- 

 tological changes take place in the cartilaginous 

 tube surrounding the notochord. Opposite the 

 points of attachment of the neural arches 

 the cartilage becomes more mature, while 

 in the spaces between the arches it retains 

 its embryonic character. In this way the 

 cartilaginous tube, though still an unsegmented 

 structure, is marked off into a series of ver- 

 tebral and intervertebral rings ; the vertebral 

 rings being the parts to which the neural 

 arches are attached, the intervertebral rings 

 the parts between the neural arches. 



About the end of the fifth day, each inter- 

 vertebral ring becomes divided transversely 

 into two equal parts, and each of these parts 

 attaches itself to the adjacent vertebral ring. 

 In this way, the once continuous cartilaginous 

 sheath of the notochord becomes divided into 

 a series of segments, each segment consisting 



