STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 789 



primitive vertebral constriction of the notochord ; (2) the nearly 

 completed constriction of the notochord in the intervertebral 

 regions ; (3) the complete ossification of the vertebral portions 

 of the bodies of the vertebrae, the terminal so-called intervertebral 

 portions alone remaining cartilaginous ; (4) the complete ossifi- 

 cation of the basal portions of the haemal and neural processes 

 included within the bodies of the vertebrae, so that in the case 

 of the neural arch all trace of the fact that the greater part 

 was originally not formed in cartilage had become lost. The 

 cartilage of the dorsal spinous processes was, however, still 

 persistent. 



The only points which remain obscure in the later history 

 of the vertebral column are the history of the notochord and of 

 its sheath. We do not know how far these are either simply 

 absorbed or partially or wholly ossified. 



Gotte in his memoir on the formation of the vertebral bodies 

 of the Teleostei attempts to prove (i) that the so-called mem- 

 brana elastica externa of the Teleostei is not a homogeneous 

 elastica, but is formed of cells, and (2) that in the vertebral regions 

 ossification first occurs in it. 



In Lepidosteus we have met with no indication that the mem- 

 brana elastica externa is composed of cells ; though it is fair to 

 Gotte to state that we have not examined such isolated portions 

 of it as he states are necessary in order to make out its structure. 

 But further than this we have satisfied ourselves that during 

 the earlier stage of ossification this membrane is not ossified, 

 and indeed in part becomes absorbed in proximity to the inter- 

 vertebral cartilages ; and Gegenbaur met with no ossification of 

 this membrane in the later stage described by him. 



Summary of the development of the vertebral column and ribs, 



A mesoblastic investment is early formed round the noto- 

 chord, which is produced into two dorsal and two ventral ridges, 

 the former uniting above the neural canal. Around the cuticular 

 sheath of the notochord an elastic membrane, the membrana 

 elastica externa, is next developed. The neural ridges become 

 enlarged at each inter-muscular septum, and these enlargements 



