74 COMPARATIVE ANATOMY 



IV. SKULL. 



General Part. 



The question as to the primary origin of the skull in the 

 Craniata has always taken a foremost place amongst the morpho- 

 logical problems relating to the structure of Vertebrates ; and 

 the first point which requires elucidation relates to the nature of 

 the head whether it is a structure sui generis, or whether its 

 parts are due to modifications and further developments of parts 

 present in the trunk. 



Until past the middle of the present century the theory which 

 held the field was the " vertebral theory " of Goethe and Oken, 

 according to which the skull consisted of a number of modified 

 vertebra3 (" cranial vertebrae "). On this theory, therefore, the 

 skull was regarded as a special modification of the anterior part 

 of the vertebral column, and a large number of facts were brought 

 forward in support of it : even when morphological science had 

 made further considerable advances, there still seemed to be a 

 certain amount of justification for this view. 



The arguments in support of the vertebral theory of the skull 

 may be briefly stated as follows. As in the vertebral column, a 

 cartilaginous and a bony stage may be distinguished in the skull, 

 ontogenetically as well as phylogenetically. There is thus an 

 important correspondence between these two parts of the cranio- 

 spinal axis, and this is further emphasised by the fact that the 

 notochord always extends for a certain distance into the base of 

 the skull, so that the latter is developed on the same skeletogenous 

 basis as, and in direct continuation of, the vertebral axis. More- 

 over, the cranial cavity, enclosing the brain, may evidently be 

 considered as a continuation of the neural canal. 



For a long time it was not recognised that as this theory 

 depended on giving an exact account merely of the skeletogenous 

 elements taking part in the formation of the skull, it could 

 not possibly lead to a true interpretation of the origin of the 

 vertebrate head. Any such attempt meant " putting the cart 

 before the horse," by looking upon the last acquisition of the head 

 its skeleton as the leading point for future researches. 



Although it gradually became evident that, except occasion- 

 ally in the hinder (occipital) region, no trace of segmentation 

 of the cartilaginous elements can be recognised in the head 

 of any existing Craniata, it still seemed to be an open question 

 whether such a segmentation may not have occurred in early 

 phylogenetic stages and have gradually become suppressed owing 

 to deep-seated physiological and morphological modifications. 1 



1 It is still by no means clear whether or not the sense-capsules were primi- 

 tively independent of the rest of the axial part of the skull, and it is quite 

 conceivable that the part of the latter anterior to the vagus foramen consisted 

 originally of independent skeletal portions which only secondarily became 

 connected with one another. 



