VERTEBRATES. 



11 



THE ENDOSKELETON. 



Fig, 10.— Horizontal section of a young gar-pike on the level with 

 the notochord (nc); mt. myotomes, separated by septa or myo- 

 commata imc) attached to the ends of the hsemal arches (Aa); 

 «, corresponds to the centre of a vertebra, the anterior and 

 posterior faces of which are indicated by a dotted outline. The 

 notochord still is wider intervertebrally, iv. 



The mode of origin of the notochord has been discussed above ; its relationship to 

 the skeleton of the adult, as -well as to the other organs of the body, must now be de- 

 scribed. It is only in lower forms, mc nc 

 like Amphioxus and the Myzonts, 

 that the notochord persists in its 

 whole length in the adult. Usually 

 it is profoundly altered by the for- 

 mation of surrounding hard 2>arts. 

 On their first appearance in the em- 

 bryo of higher, as well as in the adults 

 of lower, forms, these are simply rep- 

 resented by a continuous unsegment- 

 ed investing sheath of connective 

 tissue, which has the potentiality of 

 developing into skeletal structures 

 (the skeletogenous sheath) ; but very 

 soon the segmental character becomes 

 impressed upon this, and is, no doubt, 

 traceable to the influence of the 



strains of the muscles on the sides of the sheath in locomotion. That the segmentation 

 of the sheath and the muscles have a very intimate relation to each other may be seen 

 from Fig. 10, which represents a horizontal slice 

 through the notochord of a young gar-pike. The 

 cartilaginous sheath, although continuous round 

 the notochord, exhibits a distinct segmentation ; 

 it pushes out on either side certain processes {ha), 

 the hjemal arches, which run out between the 

 muscle-segments, and are in the adult continued 

 by the ribs, although at this stage these are not 

 yet to be found in the membranous partitions 

 (the myocommata) which are attached to the ends 

 of the haemal arches, and separate the muscle- 

 segments. Opposite the haemal arches the noto- 

 chord is distinctly constricted, and this point 

 corresponds to the middle of the vertebra of the 

 adult, while in the place corresponding to the 

 interval between two contiguous vertebrae the 

 notochord is considerably thicker. Figs. 11 and 

 12 serve to illustrate more fully the relation of 

 the central skeleton to the notochord ; both rep- 

 resent frontal sections through a young gar-pike, 

 a little behind the head ; but 12 is through a verte- 

 bra, while 11 is through an intervertebral space. 

 Above the notochord is the spinal cord, giving origin to a pair of spinal nerves, and 

 separated from the surrounding muscles by an arch which, except a triangular block of 



Fig, 11.— Frontal section through intervertebral 

 space of young gar-pike in the region of the pec- 

 toral fins ; nsp, neural spine ; gp^ spinal cord ; 

 m, muscles ; v, veins ; ^, kidney ; ao, aorta ; a&, 

 air-bladder ; o, oesophagus ; c, coelom ; pc, peri- 

 cardial cavity ; ht^ heart ; pa, pectoral arch. 



