206 MORPHOLOGICAL DEVELOPMENT. 



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 it 

 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. 

 Still more significantly, we have an explanation of the fact 

 that the base of the skull, answering to the front end of the 

 notochord, never betrays any sign of segmentation. This, 

 which is absolutely at variance with the hypothesis of the 

 transcendental anatomists, is in complete harmony with the 

 foregoing hypothesis. For if, as we have seen, the segmenta- 

 tion consequent on mechanical actions and reactions must 

 progress from without inwards, affecting last of all the axis ; 

 and if, as we have seen, the region of the head is so circum- 

 stanced that the causes of segmentation act but feebly even 

 on its periphery ; then, it is to be expected that its axis 

 will not be segmented at all : that portion of the primitive 

 notochord which is included in the head, having to un- 

 dergo no lateral bondings, may ossify without division into 

 segments. 



Of other incidental evidences supplied by comparative 

 morphology, let me next refer to the supernumerary bones, 

 which the theory of Goethe and Oken as elaborated by Prof. 

 Owen, has to get rid of by gratuitous suppositions. In many 

 fishes, for example, there are what have been called iuter- 

 neural spines and inter-hasmal spines. These cannot by any 

 ingenuity be affiliated upon the archetypal vertebra, and they 

 are therefore arbitrarily rejected as bones belonging to the 

 exo-skeleton ; though in shape and texture they are similar 

 to the spines between which they are placed. On the hy- 

 pothesis of evolution, however, these additional bones are 

 accounted for as arising under actions like those that gave 

 origin to the bones adjacent to them. And similarly Tyitb 



