THE SHAPES OF VERTEBRATE SKELETONS. 205 



lit those pai-ts of the notochord where the strain is greatest ; 

 it is clear that this formation cannot so go on as to produce 

 a continuous mass : the perpetual flexions must prevent this. 

 If matter that will not yield at each bend, is deposited while 

 the bendings are continually taking place, the bendings will 

 maintain certain places of discontinuity in the deposit 

 places a which the whole of the stretching consequent on 

 each bend will be concentrated. And thus the tendency will 

 be to form segments of hard tissue capable of great resistance 

 to compression, with intervals filled by elastic tissue capable 

 of great resistance to extension a vertebral column. 



And now observe how the progress of ossification is just 

 such as conforms to this view. That centripetal develop- 

 ment of segments which holds of the vertebrate animal as a 

 whole, as, if caused by transverse strains, it ought to do, and 

 which holds of the vertebral column as a whole, as it ought 

 to do, holds also of the central axis. On the mechanical 

 hypothesis, the outer surface of the notochord should be the 

 first part to undergo induration, and that division into seg- 

 ments that must accompany induration. And accordingly, 

 in a vertebral column of which the axis is beginning to 

 ossify, the centrums consist of bony rings inclosing a still 

 continuous rod of cartilage. 



258. Sundry other general facts which the comparative 

 morphology of the Vertcbrata discloses, supply further con- 

 firmation. Let us take first the structure of the skull. 



On considering the arrangement of the muscular flakes, or 

 myocommata, in any ordinary fish that comes to table an 

 arrangement already sketched out in the Amphioxus it is not 

 difficult to see that that portion of the body out of which the 

 heal of the vertebrate animal becomes developed, is a por- 

 tion which cannot subject itself to bendings in the same 

 degree as the rest of the body. The muscles developed there 

 must be comparatively short, and much interfered with by 

 the pre-existing orifices. Hence the cephalic part will not 



