. 



SKELETON. 151 



As will be recalled, the notochord extends forward as far 

 as the infundibulum, and its' anterior end is concerned in the 

 formation of the chondrocranium. On either side of this struc- 

 ture there develops a horizontal cartilaginous plate, the para- 

 chordal cartilage, which grows out laterally until it unites with 

 a cartilaginous box, the otic capsule, which forms around the 

 sac-like inner ear (p. 71). From this union of parachordals and 

 otic capsules, there is formed a trough which encloses the me- 

 dulla oblongata below and on either side. Later, in the typical 

 conditions, the cartilage gradually extends upwards and inwards 



FIG. 159. Early chondrocranium of Amblystoma, before the formation of the 

 otic capsules. ap, ascending process of quadrate ; 'bq t body of quadrate ; dp, 

 descending process of quadrate; m, Meckel's cartilage; n, notochord; oc, of, 

 foramina for oculomotor and optic nerves; /, parachordals; /, trabecula; trc, tra- 

 becular crest. From Winslow. 



from the dorsal surface of the otic capsules forming a plate 

 the synotic tectum which roofs in this region of the brain 

 above. To this region there is added (amphibia) a vertebra or 

 vertebral complex, developed like those of the vertebral col- 

 umn, which becomes finally united to the parachordals and otic 

 capsules, and closes in the cranium behind. Comparative mor- 

 phology would also lead us to regard the parachordals as formed 

 of coalesced vertebral centra ; but in their history, so far as made 

 out, they of themselves afford not the slightest clew as to the 

 number of elements fused together in this region. 



The trabeculae cranii are a pair of cartilaginous rods which 



