520 



CHORD ATA. 



or nmcnus membranes, and primary bones are merely membrane 

 bones which have entered the cartilages and rejolaced them. Ac- 

 cording to this view it is conceivable that the same bone in one 

 animal may arise as a membrane bone and h\ another as a primary 

 bone, a view which is of importance in the homologies and no- 

 menclatnre of many bones. It is bnt jnst to say that this view is 

 not universally accejited. 



The cartilaginous cranium [chondrocraniitm) is most complete 

 beneath the brain. This basal portion is a direct continuation of 



Fig. 559.— Chomlriicranium of Amphiuma. arip, .Tiitorbit.il process; ap. ascending 

 process of quadrate; c, cornu trabeculfp; <\ ctlimoid plate; <■/, endolymph fora- 

 men; J, .iugular foramen; /, lamina crihrosa: m, ^leckel's cartilai-'e : ", notoeliord- 

 rK", oculomotor foramen; ncp, occipital process; "/'. optic foramen ; p. parachor- 

 dal; pii!, palatine foramen ; j)/, perilymphatic f(U'anien; <] quadrate; .•^. stapes: 

 sp, sta])cdi;il iirocess ; t, trabecula; tic, crest of trahecula ; I", VJI, \'1II. foranuna 

 for r. ril, r/// nerves. 



tlie vcrtcbra.l column, and a ])art of it (t\\v pnraclionlah) embraces 

 the anterior cn<l of the notochord, whiU^ part (the trahccida;) ex- 

 tends in front of the end of the notochord. The side walls of the 

 .sktill are increased by the cartilaginous envelopes of the two sense 

 organs, the nasal and otic capsules, around the nose and ear. Be- 

 tween tlu'S(! is a hollow for the eye -w hicli contributes nothing to 

 the skull. In only a few forms is the choudrocranium completely 



