7Q2 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



Thus the greater part of the neural arches is formed of mem- 

 brane bone. The haemal arches are invested by a thick layer of 

 bone, and there is also a continuous osseous investment round 

 the vertebral portions of the notochord. The intervertebral 

 cartilages become penetrated by branched processes of bone. 



Comparison of the vertebral column of Lcpidosteus with that of 



other forms. 



The peculiar form of the articulatory faces of the vertebrae of 

 Lcpidostens caused L. Agassiz (No. 2) to compare them with the 

 vertebrae of Reptiles, and subsequent anatomists have suggested 

 that they more nearly resemble the vertebrae of some Urodelous 

 Amphibia than those of any other form. 



If, however, Gotte's account of the formation of the am- 

 phibian vertebrae is correct, there are serious objections to a 

 comparison between the vertebrae of Lepidosteus and Amphibia 

 on developmental grounds. The essential point of similarity 

 supposed to exist between them consists in the fact that in both 

 there is a great development of intervertebral cartilage which 

 constricts the notochord intervertebrally, and forms the articular 

 faces of contiguous vertebrae. 



In Lepidosteus this cartilage is, as we have seen, derived from 

 the bases of the arches ; but in Amphibia it is held by Gotte to 

 be formed by a special thickening of a cellular sheath round the 

 notochord which is probably homologous with the cartilaginous 

 sheath of the notochord of Elasmobranchii, and therefore with 

 part of the notochordal sheath placed within the membrana 

 elastica externa. 



If the above statements with reference to the origin of the 

 intervertebral cartilage in the two types are true, it is clear that 

 no homology can exist between structures so differently de- 

 veloped. Provisionally, therefore, we- must look elsewhere 

 than in Lepidosteus for the origin of the amphibian type of 

 vertebrae. 



The researches which we have recorded demonstrate, how- 

 ever, in a very conclusive manner that the vertebrae of Lepi- 

 dosteus have very close affinities with those of Teleostei. 



