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) vertebra? (thoracic, lumbar, 

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

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

 in the sacrum may be as many as twenty-three. In Archa?opteryx 



the sacrum was much shorter than in 

 existing Birds, and fewer vertebra? 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 vertebra? usually fuse together 

 to form a flattened bone, the pyc/osty/e, 

 which supports the tail quills (Fig. 132). 

 In the Batita? there is never more than 

 an insignificant pygostyle (Struthio), and 

 all the caudal vertebra? 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 Archa?opteryx, in 

 which it was supported by numerous 

 elongated free vertebra? (Fig. 49). It 

 must, however, be borne in mind that 

 the pygostyle may be made up of from 

 six to ten fused vertebra?, and in the 

 sacrum even a greater number may be 

 included, so that as many as twenty or 

 more caudal vertebra? 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- 

 posus, persists, however, throughout life 

 in the centre of the fibre-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 epipliyscs 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 ;iiv usually only formed 

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



FIG. 48. PELVIS OF OWL 

 (Strix bubo). Ventral 

 view. 



It, ilium ; /.y, ischium ; P, 

 pubis ; H, last two pairs 

 of ribs ; W, position of 

 the primary sacral verte- 

 br;f : between ft and //, 

 and behind W, are seen 

 the secondary sacral ver- 

 tebra^, fused with the pri- 

 mary (IF); f foramen 

 between ilium and ubis. 



