60 



COMPARATIVE ANATOMY 



which ossify separately and correspond to fused ribs, as in 

 Amphibia and Amniota. During further development, however, 

 a number of other (secondary sacral) vertebrae (thoracic, lumbar, 

 and caudal), with their rudimentary ribs, become fused with the 

 two primary ones (Fig. 48), so that the entire number of vertebra? 

 in the sacrum may be as many as twenty-three. In Archaeopteryx 



the sacrum was much shorter than in 

 existing Birds, and fewer vertebrae were 

 united with it. 



In existing Birds the actual caudal 

 region always exhibits a more or less 

 rudimentary character, and in its posterior 

 portion the vertebras usually fuse together 

 to form a flattened bone, the pygostyle, 

 which supports the tail quills (Fig. 132). 

 In the Ratitae there is never more than 

 an insignificant pygostyle (Struthio), and 

 all the caudal vertebrae may remain dis- 

 tinct. That the latter is the more original 

 condition in Birds is shown by a study 

 of their development as well as by the 

 condition of the tail in Archaeopteryx, in 

 which it was supported by numerous 

 elongated free vertebras (Fig. 49). It 

 must, however, be borne in mind that 

 the pygostyle may be made up of from 

 six to ten fused vertebrae, and in the 

 sacrum even a greater number may be 

 included, so that as many as twenty or 



FIG. 48. PELVIS OF OWL 

 (Strix bubo). Ventral 

 view. 



II, ilium ; 7s, ischium ; P, 



of ribs ; W, position of 

 the primary sacral verte- 

 brae : between R and 77, 

 and behind W, are seen 

 the secondary sacral ver- 

 tebrae, fused with the pri- 

 mary ( W) ; f foramen 



pubisj /?, T Jast tAyo pairs more caudal vertebrae may be represented. 



Mammals. The notochord here per- 

 sists longer intervertebrally than intra- 

 vertebrally, but it disappears entirely by 

 the time the adult condition is reached. 

 A jelly-like pulpy mass, the nucleus pul- 



between ilium and pubis. posus, persists, however, throughout life 

 in the centre of the fibro-cartilaginous 



menisci which are developed between the centra. The whole 

 vertebral column is preformed in cartilage, and the arches 

 develop in continuity with the centra but become ossified from 

 separate centres, as do also the various processes ; these separate 

 ossifications are no longer recognisable in the adult. The 

 presence of bony discs or epipliyses on the flattened ends of the 

 centra which unite with the latter comparatively late, is very 

 characteristic of Mammals ; they are, however, absent or only 

 imperfectly developed in Monotremes and in existing Sirenians. 



True articulations between the centra are usually only formed 

 on the atlas and anterior face of the axis : well-developed articular 



