18 



THE ANATOMY OF VEETEBEATED ANIMALS. 



rays are now disposed in siicli a manner as to give the tail 

 an appearance of symmetiy with respect to the axis of the 

 body, and such fishes have been called homocercal. Of these 

 homocercal fish, some (as the Salmon, Fig. 6) have the 

 notochord unossified, and protected only by bony plates 

 developed at its sides. In others (as the Stickleback, 

 Perch, &c.), the sheath of the notochord becomes completely 

 ossified and united with the centiiim of the last vertebra, 

 which then appears to be prolonged into a bony urostyle. 



The Cranial System. — As has been stated, no protover- 

 tebrsB appear on the floor of the sktdl ; nor is there any 



Fig. 7. 



^<<'\ 



-..n 



Fig. 7. — The cartilaginous cranium of a Fowl at the sixth day of in- 

 cubation, viewed from below. — P, the pituitary space ; tr, the tra- 

 beculcE, uniting in front, in the bifurcated ethmovomerine plate ; Qti, 

 the quadrate cartilage ; Sc, the semicircular canals ; Co, the cochlea ; 

 A, the notochord imbedded in the basilar plate. 



cranium, nor any developmental stage of a cranium, m ' 

 which separate cartilaginous centres are known to occur inj 

 this region. 



On the contrary, when chondi-ification takes place, itl 

 extends continuously f oi-ward, on each side of the notochord, 

 and usually invests the anterior tennination of that body, 

 more or less completely, as a basilar plate. 



