SKULL 75 



The original segmentation of the head i.e., the segmentation of 

 the mesoderm into somites may have more or less closely resembled 

 that seen in Amphioxus ; but it must be borne in mind that there 

 is no direct connection between the Acrania and Craniata, and that 

 there must have been a whole series of intermediate forms. As a 

 matter of fact, only vestiges of the primary metamerism of the 

 head have persisted, and are more or less plainly indicated onto- 

 genetically by the ganglia, nerves, gill-arches, and myomeres. It 

 is nevertheless certain that the structural plan of the head, like 

 that of the trunk, is based on a condition of metamerism, although 

 it is doubtful how many primary segments are included, and 

 whether segmentation is not limited to the post-auditory region 

 (chordal or " spinal " portion) of the skull, and does not concern 

 the more anterior (prechordal or " prespinal ") portion. 



In any case, however, the metameric character is much more 

 plainly seen in the post-auditory (occipital) region than in the 

 more anterior part of the head, in which the primary relations are 

 no longer recognisable owing to parts having become reduced, 

 displaced, fused, lost, or functionally changed in connection with 

 the modifications resulting from the development of the brain, 

 skull, the olfactory, optic, and auditory organs, and the oral muscles. 

 A reduction, fusion, or loss of cephalic myotomes has also occurred 

 in the post-auditory parts, the occipital region being of a very varied 

 and fluctuating nature, and it may even include spinal elements. 

 It is therefore evidently impossible in this place to give more than 

 the briefest sketch of the problem under consideration before 

 making a detailed study of the parts composing the head. 



The portion of the skull which is situated along the main axis 

 in continuation of the vertebral column and which encloses the 

 brain, is known as the brain-case or cranium (neurocranium), and 

 is primarily composed of cartilage. A series of cartilaginous 

 arches arise in serial order on the ventral side of the brain- case ; 

 these encircle the anterior part of the alimentary tract like hoops, 

 incomplete dorsally, and are distinguished as the visceral portion 

 of the skull {splanchnocranium}. This bears an important relation 

 to branchial respiration, as between each consecutive pair of 

 arches a passage (gill-cleft) lined by endoderm, is present, 

 communicating between the pharynx and the exterior, and 

 through which the water passes in branchiate forms : the fore- 

 most visceral arch, which bounds the aperture of the mouth, 

 becomes modified to form the skeleton of the jaws. The arches, 

 therefore, serve primarily as gill-supports. Ossification may occur 

 in connection with the cranial and visceral portions later. 



Before the cartilaginous skeleton begins to be formed in the 

 embryo, the greater part of the head consists of a mesodermic 

 formative tissue, which gives rise to a membranous capsule around 

 the brain and in which the rudiments of the individual cerebral 

 nerves can be plainly distinguished. The paired olfactory, optic, 

 and auditory organs also appear at a very early stage ; and these, 



