210 THE FOURTH DAT. [CHAP. 



ally extends backwards. It does not commence in the 

 arches till somewhat later. For each arch there are 

 two centres of ossification, one on each side. 



The notochord persists for the greater part of foetal 

 life and even into post-fcetal life. The larger vertebral 

 portions are often the first completely to vanish. They 

 would seem in many cases at any rate (Gegenbaur) to 

 be converted into cartilage and so form an integral part 

 of the permanent vertebrae. Rudiments of the inter- 

 vertebral portions of the notochord may long be detected 

 in the ligamenta suspensoria. 



We may remind the reader that in the adult bird we find 

 between each of the vertebrse of a neck and back a cartilaginous 

 disc — -the meniscus — ^which is pierced in the centre. These discs 

 are thick at the circumference but thin oflf to a fine edge round 

 the central hole. Owing to the shape of these discs there are left 

 between each pair of vertebrae two cavities, which only commu- 

 nicate through the central aperture of the meniscus. Through 

 this central aperture there passes a band called the 'ligamen- 

 tum suspensorium,' connecting the two vertebrae. 



In the tail the menisci are replaced by bodies known as the 

 'annuli fibrosi,' which precisely resemble the similarly named 

 bodies in mammals. They differ from the menisci in being 

 attached over their whole surface to the ends of the vertebral 

 bodies, so that the cavities between the menisci and the vertebrae 

 do not exist. They are pierced however by a body corre- 

 sponding with the ligamentum suspensorium and known as the 

 ' nucleus pulposus.' 



In the intervertebral regions the chorda, soon after the com- 

 mencement of ossification, entirely disappears. The cartilage 

 around it however becomes converted (in the region of the neck) 

 into the ligamentum suspensorium, which \mites the two ver- 

 tebrce between which it is placed. 



In the tail the cartilage becomes the nucleus pulposus, which 

 corresponds exactly to the 'ligamentum suspensorium' of the 

 neck and back. 



