VERTEBR.^ 567 



represents the basidorsal, while the interdorsals and inter- 

 ventrals are the intercalary pieces, the latter being small. 

 The supradorsals, generally miscalled neural spines, are extra 

 cartilages (Fig. 443). In the Stegocephalia the arcualia, in 

 various patterns, appear as separate bones. In the frog the 

 vertebral column develops from largely unsegmented cartilages, 

 but in the amniotes a fairly clear derivation from the arcualia 

 can be made out, the neural arch being formed from the basi- 

 dorsal, and the centrum mainly from the interdorsal (Fig. 444). 

 Ribs are formed in the connective tissue between thf muscle 

 blocks, and so are intersegmental and become connected with the . 

 basidorsals and basiventrals. 



The detailed form of tetrapod vertebrae has been adequately 

 shown by the descriptions of those of the frog, pigeon, and rabbit. 

 A peculiarity of amniotes is that the interdorsals of the first 

 vertebra (atlas) fuse with the centrum of the second (axis) to 

 form its odontoid process, while all that remains of the centrum 

 of the atlas is formed from the basiventrals, which, with the 

 neural arch (basidorsals) form a ring. 



THE HEAD 



If one considers the mammals alone, the definition of the head 

 seems easy, but it is by no means certain that what is called the 

 head in mammals is exactly homologous with the part of the 

 body which bears the same name in birds or reptiles. Further, 

 in amphibians and fishes where there is no neck it is not always 

 very clear where the head ends and the trunk begins. We shall 

 therefore, as is often the best way with biological terms, postpone 

 any attempt at definition until we have considered some of the 



facts. 



In the protochordates there is no head, but there is some degree 

 of ' cephalisation ', or development of the anterior end in the 

 direction of a head. In Branchiostoma, for example, the segmenta- 

 tion runs fairly uniformly to the tip of the snout, but there are 

 some specialisations in the mouth and anterior sense organs, 

 and a slight enlargement of the central nervous system. In the 

 dogfish, by contrast, there is a well-developed head with large 

 sense organs, a skeleton which has no obvious resemblance to 

 that of the trunk, and a complete absence of any visible trace of 

 segmentation. Embryological investigations in the third quarter 



