4o6 



CIIORDATA 



axial skeleton and arise from tlie ossifications in the skin (scales) or in the 

 mouth (teeth), alreaih" referred to (p. 451). They sink into the deeper 

 portions and apply ihemsehes to the axial skeleton, especially to those 

 parts where, from lack of cartilage, no primary bones can be formed. 

 It is not settled how far these distinctions are \'alid. According to Gegen- 

 baur all ossification arose jirimarily in the skin or mucous membranes, 

 and primary bones are merely memlirane bones which have entered the 



Fir.. 515. — Chondrocranium of A }>i plihiijia . mip, aiiUivliilal process; i7/>, asccnrlina; 

 process of C|uadrate; c, cornu trabccula-; c, clhmoiil plate; <-/, cndolymph foramen; ^ 

 jugular foramen; /, lamina cribrosa; m, Meckel's cartilage; n, notochord; dc, oculomolor 

 foramen; ocp, occipital process; of, oplic foramen; ;i, parachordal; pal. palatine foramen; 

 pf, perilymphatic foramen; q, quadrate; 5, stapes; s/>, stapedial process; (, trabecula; Ire, 

 crest of trabecula; V, VII, VIII, foramina for V, VII, VIII nerves. 



cartilages and replaced them. Accordingly it is conceivable tliat the same 

 bone in one animal may arise as a membrane bone and in another as a 

 primary lione, a point of importance in deciding the homologies and 

 nomeirclature of many bones. It is but just to say that tills view is not 

 universally acceptetl. 



The diondrocraniiim is most complete beside and lieneath the brain 

 (fig. 515). This basal portion is a direct continuation of the vertebral 

 column, and a part of it (the parachordals) embraces the anterior end 

 of the notochord, while parts (the irabcciihc) extend in front of the 



