THE SEC ME. VTA TIOX OF THE HEAD. 2OI 



THE SEGMENTATION OF THE HEAD. 



SINCE in the vertebrates the region of the body behind the 

 head is made up of segments repeated one after the other 

 (metamerism), there has naturally arisen the question, Is the 

 head itself similarly composed of somites ? If so, how many 

 of these somites are to be recognized ? Various attempts have 

 been made to solve these problems, but with varying results. 

 Only the merest outline of the attempted solutions can be given 

 here. 



In the trunk and tail regions the parts which are meta- 

 merically arranged are as follows : myotomes, spinal nerves, ver- 

 tebrae, nephridial tubes, and the intersegmental blood-vessels. 

 Each and all of these structures have been employed in the 

 attempt to carry the segmentation forward into the head. The 

 existence of the problem was first recognized by Oken (1807), 

 who attempted its solution upon a vertebral basis. In the 

 mammalian skull he recognized three vertebrae, the centres of 

 which were represented respectively by the basioccipital, sphe- 

 noid and ethmoid bones, while the neural arches were formed 

 by ex- and supraoccipitals, parietals, and frontals. Later stu- 

 dents recognized four vertebrae in the skull, the increase being 

 effected by recognizing basi- and presphenoid centres. 1 In 

 1869 Huxley showed that this theory was untenable, and that 

 the ' vertebrae ' of the skull could not be homologous with those 

 of the trunk, since they were, in part, composed of membrane 

 bone. He also pointed out that in those vertebrates (elasmo- 

 branchs) where one would naturally expect to find the vertebrae 

 best developed, there was a continuous unsegmented brain case. 

 His attempt at the solution of the problem was based upon the 

 nerves and gill clefts, thus transferring the question from the 



1 The interested student will find the extreme development of this ' vertebral theory of 

 the skull ' in the first volume of Owen's ' Anatomy of the Vertebrates.' 



