520 



CHORD AT A. 



or mucous membranes, and primary bones are merely membrane- 

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

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

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

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

 menclature of many bones. It is but just to say that this view is 

 not universally accepted. 



The cartilaginous cranium (chondrocranium) is most complete 

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



FIG. 559. Chondrocranium of Amplduma. o?ip, antorbital process; r/p, ascending 

 process of quadrate; c, cornu trabeculee; e, ethmoid plate; ef, endolymph fora- 

 men; j, jugular foramen; I, lamina cribrosa; m. Mockers cartilage ; N, notocnorcr 

 oc, oculomotor foramen; ocp, occipital process; o/, optic foramen ; p, parachor- 

 dal; pal, palatine foramen ; p/, perilymphatic foramen; q quadrate; s, stapes; 

 sp, stapedial process ; f, trabecula; trc, crest of trabecula; V, VII, VIII, foramina 

 for V, VII, VIII nerves. 



the vertebral column, and a part of it (the paracliordals) embraces 

 the anterior end of the notochord, while part (the trdbeculce) ex- 

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

 skull are increased by the cartilaginous envelopes of the two sense 

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

 tween these is a hollow for the eye which contributes nothing to 

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



