222 MORPHOLOGICAL DEVELOPMENT. 



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 which 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 disclosed by the com- 

 parative morphology of the Vertebrata, 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 which 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 head of the vertebrate animal becomes developed, is a 

 portion 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 

 partake in any considerable degree of the lateral undula- 

 tions ; and there will not tend to arise in it any such distinct 

 segmentation as arises elsewhere. We have here, then, an 

 explanation of the fact, that from the beginning the develop- 

 ment of the head follows a course unlike that of the spinal 

 column; and of the fact that the segmentation, so far as il 

 can be traced in the head, is most readily to be traced in the 

 occipital region and becomes lost in the region of the face. 

 For if, as we have seen, the segmentation consequent on 



