NOTOCHORD AND VERTEBRAL COLUMN. 459 



luminous neural arches. The latter are placed iu the vertebral regions, 

 and in these regions ossification very early sets in, while the noto- 

 chord remains relatively unconstricted. In the intervertebral regions 

 the cartilage becomes thickened, as in Amphibia, and gradually con- 

 stricts the notochord. The cartilage in each of the intervertebral 

 regions soon becomes divided into two parts which form the articular 

 faces of two contiguous vertebra. 



The general character of the vertebral column on the completion 

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



o o 



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 Ascalabotte (Geckos) present an exceptional type of vertebral 

 column which has many of the characters of a deve'opuieutal stage in 

 other Lizards. The body of the vertebra is formed of a sli^htlv hourglass- 



* ~ v O 



shaped osseous tube, united with adjoining vertebrae 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 vertebras 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 differ- 

 entiated 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 ver- 

 tebras 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 intervertebral portions the notochord has 

 become distinctly constricted, and at the same time in each vertebral 

 portion there have also appeared two constrictions of the notochord 

 giving rise to a central and to two terminal enlargements. 



On the twelfth day the ossification of the cartilaginous centra 

 commences. 



