158 THE FOURTH DAY. [CHAP. 



not commence in the arches till somewhat later than in the 

 bodies. For each arch there are two centres of ossification, 

 one on each side. 



We may remind the reader that in the adult bird we find between each of the 

 vertebrae 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 

 off to a fine edge round the central hole. Owing to the sliape of these 

 discs there are left between each pair of vertebra; two cavities, which only 

 communicate through the central aperture of the meniscus. Through this 

 central aperture there passes a band connecting the two vertebra; which is called 

 the 'ligamentum suspensorium.' 



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 

 cease to exist. They are pierced however by a body corresponding with the 

 ligamentum suspensorium and kno«n as the 'nucleus pulposus.' 



Ill the intei vertebral regions the chorda, soon after the commencement of 

 ossification, tntirely disappears. The cartilage around it however becomes 

 converted (in the regiim of the neck) into the ligamentum suspensorium, 

 which unites the two vertebrae 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. 



Shortly after the formation of the ligamentum suspensorium the remaining 

 cartilage of the intervertebral segments is converted in the neck and back 

 into the meniscus between each two vertebra?, and in the tail into the annulus 

 fibrosus. Both are absent in the sacrum. These points together with the 

 anatomy of these parts in the ad\ilt were first made out by Jager {Wirbelkiirper- 

 (/elenlc der Vd;/el. Sitz. der k. Akad. Wien, vol. xxxill. 1859). 



In the bodies of the vertebra the notochord does not entirely disappear as 

 in the intervertebral regions, but, according to Gegenbaur, undergoes ultimately 

 a direct conversion into cartilage. The contour of the sheath becomes 

 indistinct ; the cells by the accumulation of matrix round them take on the 

 f jrm of cartilage-cells, so that at the time of the exclusion of the bird from the 

 egg the limits between the altered notochord and the cartilage of protovertebral 

 origin can only with diflBculty be made out. 



15. While the chief mass of a proto vertebra, having 

 given rise to a muscle-plate and a ganglion, is converted into 

 the body and arch of a permanent vertebra with its several 

 appurtenances, a small portion of the exterior grows down- 

 wards as the rudiment for the formation of a rib. These 

 costal growths are of course confined to the dorsal region. 

 They are seen on the sixth day as cartilaginous rods, whose 

 cells are arranged in horizontal rows. By this time they 

 are quite separate from the bodies of the vertebrse, with 

 whose arches they are in transverse section seen to alternate. 

 Thus in one section the vertebral arch will be distinctly seen 

 but no trace of the rib ; while in the next the rib will be 

 visible but the arch will be absent. 



