IK VEBTEBRATA. 519 



111 order to allow for bending where complete centra are present vari- 

 ous conditions occur, (a) Opisthocale vertebra have a socket on the 

 hinder surface which receives the convex anterior end of the succeeding 

 centrum, forming a ball-and-socket joint. (6) ProrMoas vertebra; have 

 these relations reversed, the socket being in front, (c) The vertebra may 

 articulate with a 'saddle joint' (birds), (d) Between two successive 

 vertebrae an elastic intervertebral ligament may occur (mammals). The 

 neurapophyses may bear, in addition to the transverse processes, anterior 

 and posterior articulating processes (zygapophyses) connecting the sepa- 

 rate vertebrfe. 



The skull, the anterior continuation of the axial skeleton, 

 occurs in all vertebrates; it appears before the vertebra?, for it is 

 found in the cyclostonies, which lack these. It surrounds the brain 

 as the vertebraa do the spinal cord: and, like them, its first stages 

 are formed in the skeletogenous layer surrounding the anterior end 

 of the notochord. It is so related to the surrounding parts tiiat 

 it may in general be said to be equivalent or homodynamous with 

 the vertebra, although we cannot agree with Oken aiul Goethe, the 

 founders of the vertebrate theory of the skull, that it has arisen 

 by the fusion of vertebra. ( )n the other hand skull and vertebraa 

 are parts arising in the common basis of the skeletogenous layer, 

 but which have developed in different directions. 



Three stages are recognized in the development of the skull: 

 the membranous, the cartilaginous cranium, and the bony skull. 

 The first, which consists of connective tissue, occurs only in the 

 early embryonic stages, scarcely a trace of it persisting in the 

 adults. It is early replaced by the cartilaginous skull, which may 

 persist unaltered throughout life in the lower fishes (elasnio- 

 branchs, sturgeon). In most vertebrates, however, ossification sets 

 in, embracing a part (fishes, amphibians) or the -whole of the carti- 

 lage (birds, mammals), converting it in the latter case into a bony 

 capsule. In the bony skull two kinds of bone, primary and sec- 

 ondary, are recognized, these varying in their origin. The pri- 

 mary or cartilage liones develop from the cartilage itself, either in 

 its interior {eiifochondroxfose'<) or in its envelojiing perichondium 

 (ectochondrosfo.ses). The secondary or membrane bones are, in 

 their origin, foreign to the axial skeleton and arise from the ossifi- 

 cations in the skin (scales) or in the mouth (teeth), already re- 

 ferred to (p. 515 ). They sink into the deeper portions and apply 

 themselves to the axial skeleton, especially to those parts where, 

 from lack of cartilage, no primary bones can be formed {paroslones). 

 Still it is not settled how far these distinctions may be carried. 

 According to (J-egenbaur all ossifications arose primarily in the skin 



