NOTOCHORD AND VERTEBRAL COLUMN. 557 



The general character of the vertebral column on the comple- 

 tion of these changes is shewn in fig. 320 D. The later changes 

 are relatively unimportant. The constricted intervertebral 

 sections of the notochord rapidly disappear, while the vertebral 

 sections become partially converted into cartilage, and only 

 cease to be distinguishable at a considerably later period. 



The ossification extends from the bodies of the vertebrae into 

 the arches and into the articular surfaces, so that the whole 

 vertebrae eventually become ossified. 



The Ascalabotae (Geckos) present an exceptional type of vertebral 

 column which has many of the characters of a developmental stage in 

 other Lizards. The body of the vertebra is formed of a slightly hourglass- 

 shaped osseous tube, united with adjoining vertebras by a short inter- 

 vertebral cartilage. There is a persistent and continuous notochord which, 

 owing to the small development of the intervertebral cartilages, is narrower 

 in the vertebral than in the intervertebral regions. 



Aves. In Birds the cellular tube formed round the notochord 

 is far thicker than in the Reptilia. It is continuous in the 

 regions of the future vertebrae with neural arches, which do not 

 at first nearly enclose the spinal cord. 



On about the fifth day, in the case of the chick, it becomes 

 differentiated into vertebral regions opposite the attachments of 

 the neural arches, and intervertebral regions between them ; the 

 two sets of regions being only distinguished by their histological 

 characters. Very shortly afterwards each intervertebral region 

 becomes segmented into two parts, which respectively attach 

 themselves to the contiguous vertebral regions. A part of each 

 intervertebral region, immediately adjoining the notochord, does 

 not however undergo this division, and afterwards gives rise to 

 the ligamentum suspensorium. 



The notochord during these changes at first remains 

 indifferent, but subsequently, on about the seventh day in the 

 chick, a slight constriction of each vertebral region takes place ; 

 so that the vertebrae have temporarily, as they have also in 

 Amphibia, a biconcave form which repeats the permanent 

 condition of most fishes. By the ninth and tenth days, however, 

 this condition has completely disappeared, and in all the inter- 

 vertebral portions the notochord has become distinctly con- 

 stricted, and at the same time in each vertebral portion there 



