456 



CHORDATA 



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

 mouth (teeth), already referred to (p. 451). 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. 

 It is not settled how far these distinctions are valid. According to Gegen- 

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

 and primary bones are merely membrane bones which have entered the 



FIG. 515.- Chondrocranium of Amphinma. anp, antorbital process; ap, ascending 

 process of quadrate; c, cornu trabecula?; c, ethmoid plate; ef, endolymph foramen; j, 

 jugular foramen; /, lamina cribrosa; 111, Meckcl's cartilage; n, notochord; oc, oculomotor 

 foramen; ocp, occipital process; of, optic foramen; p, parachordal; pal, palatine foramen; 

 ff, perilymphatic foramen; q, quadrate; s, stapes; sp, stapedial process; t, trabecula; trc, 

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



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

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

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

 nomenclature of many bones. It is but just to say that this view is not 

 universally accepted. 



The chondrocranlum is most complete beside and beneath the brain 

 (fig- 5 T S)- 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 trcbeculcc} extend in front of the 



