vi. ] GENERAL VIE IV OF INTERNAL SKELE TON. 2 1 5 



tion, man shares, speaking broadly, the characters of his class. 

 Ossification is carried further in the class of Birds, but much 

 less far in Batrachians and P'ishes. In some Fishes, indeed 

 (both of the highest and lowest forms), the entire skeleton 

 remains throughout life persistently cartilaginous, while in 

 the lowest form of all Vertebrates (the Amphioxus or Lancelet) 

 it is mainly represented by fibrous membrane only. 



2. Reviewing the form and development of the spine in 

 man, we may note certain significant facts and generaliza- 

 tions : 



(1) The backbone exhibits to us a good example of serial 

 symmetry. The successive vertebras are evidently serial re- 

 petitions of parts in some sense the same, i.e. are serial 

 homologues, or homotypes. 



(2) We find that modifications may be produced by the 

 suppression in some vertebrae of parts existing in others, as 

 e.g. of the neural arch in the coccygeal vertebrae. 



(3) We find that modifications may be produced by the 

 coalescence of parts by anchylosis, e.g. the anchylosis of the 

 sacral vertebrae to form the sacrum. 



(4) Parts bony in one portion of the spine may be repre- 

 sented by membrane only in another, as we see in the neural 

 canal of the sacrum closed in part by membrane only. 



(5) A vertebra with annexed parts (two ribs and the inter- 

 vening piece of the sternum) may completely encircle the 

 body cavity. This suggests the question whether there may 

 not be membranous representatives of ribs similarly enclosing 

 the body cavity, annexed" to paits of the spine where there 

 appear to be no bony ribs, as e.g. in man's lumbar and 

 cervical regions. 



(6) The transverse processes of some at least of the cervical 

 vertebrae arise by separate centres. This suggests the ques- 

 tion as to whether their nature may not be essentially dif- 

 ferent from that of the transverse processes of the dorsal 

 .vertebrae. 



(7) We have seen that a vertebra may include (besides a 

 centrum and neural arch with its processes) double transverse 

 processes, vertebral and sternal ribs, and a sternal segment ; 

 the whole forming an external or (parietal) ventral arch, while 

 beneath the centrum may be developed a second, deeper, 

 more internal hypapophysial arch. 



(8) The varying conditions presented by the two vertebrae 

 next the skull in different animals above Fishes, suggest the 

 question whether we can reduce to a common type that 



