THE SKELETON 411 
tinuous; 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 otie 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- 
‘ated 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. 
Il. 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 we may note in advance that the 
