ix THE VERTEBRATE HEAD 499 



evolution of a head we may take it that the principal factors 

 involved are probably the following : 



(1) The habit of active movement in a direction corresponding 

 with the prolongation of the axis of the body, 



(2) The concentration of organs of special sense towards the end 

 of the body which is in front during movement, 



(3) The concentration of nerve centres to form a brain in 

 proximity to these organs of sense. 



In the case of the Vertebrate the brain has reached a comparatively 

 large size and in correlation with this the protecting skeleton has 

 become highly developed and has lost the flexibility which is 

 characteristic of it in the trunk. Further in the Vertebrate the 

 walls of the buccal cavity and pharynx have become highly 

 specialized, particularly in the matter of their skeleton, in relation 

 to the functions of ingestion and mastication of the food on the one 

 hand, and of respiration on the other. 



Each of these various factors involves structural change, not 

 affecting merely one organ but causing modification of the whole 

 complex arrangements of the head-region. Thus associated with the 

 loss of flexibility we find (1) loss of segmentation of the skeleton, 

 (2) disappearance or great modification of the myotomes, (3) 

 corresponding changes in the nerves supplying these myotomes and 

 (4) disappearance of the coelomic cavities. 



The full appreciation of the importance of this feature of the 

 Vertebrata makes it, in the present writer's opinion, impossible to 

 doubt that the possession of a definite head is a feature that has 

 come down from the unknown ancestral form from which the 

 Vertebrate stock has evolved. If this be correct it follows that 

 the relatively feeble differentiation of the head end of the body seen 

 in Amphioxus is to be regarded as a secondary condition, correlated 

 with the peculiar mode of life of this animal, and devoid of phylo- 

 genetic significance. 



It has already been pointed out that organs of great complexity 

 in the adult tend to be laid down at an early stage of individual 

 development, time being thus obtained for the development of their 

 complex detail. It is perhaps in direct relation to this principle 

 that the highly complex head -region of the Vertebrate, which 

 comes to assume control over most of the activities of the in- 

 dividual, develops particularly early in ontogeny — the various 

 developmental processes making themselves as a rule first appar- 

 ent in the head region and spreading thence tailwards along the 

 trunk. This fact is of practical importance to the embryologist 

 for in the case of segmentally repeated organs it enables him to 

 find a series of developmental stages within the body of a single 

 embryo. 



Though this precocious cephalization is a marked feature of 

 Vertebrate ontogeny it never goes within this phylum to the length 

 it does amongst certain Invertebrates where the larval stage 



