CRITICISM OF VERTEBRAL THEORY 155 



The chorda sheath is the common matrix of the vertebrae 

 and of a large part of the skull. The basilar plate and the 

 trabeculae, which are developed from the chorda sheath, give 

 origin to three bones, which might possibly be considered 

 equivalent to vertebral centra the basioccipital, the basi- 

 sphenoid, and the Riechbcin (ethmoid). The Rieclibcin 

 develops from the fused ends of the trabeculae. The pre- 

 sphenoid might also be considered as a vertebral body, but it 

 develops independently of the basilar plate and trabeculae. 



Now of these bones, the basioccipital is in every way 

 equivalent to a vertebral centrum, for it develops in the 

 basilar plate round the notochord. With the exoccipitals, 

 which arise just like neural arches, it forms a true vertebra. 

 The supraoccipital is an accessory bone developed in relation 

 to bigger brains. The basisphenoid appears in the basilar 

 plate, but in front of the notochord, nor does it arise in 

 exactly the same way as the centrum of a vertebra. The 

 basisphenoid with the alisphenoids, which develop independ- 

 ently in the side walls of the brain, may, however, still be 

 considered as forming a vertebra, though the resemblance is 

 not so great as in the case of the occipital ring. The pre- 

 sphenoid, being long and pointed, is very unlike a vertebral 

 body. The orbitosphenoids develop separately from it. The 

 ethmoid also differs from a vertebra, for it surrounds not the 

 whole nervous axis as the two hinder " vertebrae " do, but 

 only two prolongations of it, the olfactory lobes. In its 

 development and final form it shows no particular resem- 

 blance to a vertebra. Its body, the pars perpendicularis 

 (mesethmoid) shows no similarity with a vertebral centrum. 

 Completing the three hinder cranial " vertebras " and roofing 

 in the brain are the supraoccipital, the parietals and the 

 frontals. The premaxillaries, vomer, and nasals do not 

 belong to the cranial scheme ; they are covering bones 

 connected with the ethmoid. So, too, the ear-capsule is 

 not part of the cranial vertebrae, but is rather to be com- 

 pared to the intercalary bones in the vertebral column of 

 certain fish. Summing up as regards the cranial vertebrae 

 Rathke writes, "We find that the four different groups of 

 bones, consisting of the basioccipital with its intercalary 

 (the supraoccipital), the basisphenoid with its intercalates 



