THE SKELETON 411 



tinuoiis; cartilage centers then arise separately in different parts 

 for each of the bones: similarly for the hind-limbs and pelvic 

 girdle, etc. Separate centers of ossification may likewise appear 

 in a continuous embryonic cartilage, as for instance, in the base 

 of the skull or in the cartilaginous coraco-scapula, or ischio- 

 ilium. Such centers may become separate bones or they may 

 subsequently fuse together. In the latter case, they may repre- 

 sent bones that were phylogenetically perfectly distinct elements, 

 as for instance, the prootic, epiotic, and opisthotic centers in 

 the cartilaginous otic capsule; or they may be of purely func- 

 tional significance, as for instance, the separate ossifications in 

 the sternum of birds, or the epiphysial and diaphysial ossifica- 

 tions of the long bones of mammals. It is usually possible on 

 the basis of comparative anatomy to distinguish these two cate- 

 gories of ossification centers. 



Phylogenetic reduction of the skeleton is also usually indi- 

 cated in some manner in the embryonic history. Where elements 

 have completely disappeared in the phylogenic history, as for 

 instance, the missing digits of birds, they often appear as mem- 

 brane formations in the embryo, which then fade out without 

 reaching the stage of cartilage; if the latter stage is reached the 

 element usually fuses with some other and is therefore not really 

 missing, e.g., elements of the carpus and tarsus of birds (though 

 not all). But the ontogenetic reduction may go so far that 

 the missing elements are never distinguishable at any stage of 

 the embryonic history; thus, though the missing digits of birds 

 are indicated in the membranous stage, their component phalanges 

 are not indicated at all. 



II. The Vertebral Column 



The primordia of the vertebral column are the notochord 

 and sclerotomes. The former Is the primitive axial support of 

 the body, both ontogenetically and phylogenetically. In both 

 components, notochord and sclerotomes, we may recognize a 

 cephalic and trunk portion. The notochord, as we have seen, 

 extends far into the head, and the sclerotomes of the first four 

 somites contribute to the formation of the occipital portion of 

 the skull. The cephalic parts are dealt with in the development 

 of the skull. The history of the notochord and sclerotomes will 

 be considered together, but w^e may note in advance that the 



