STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 79 1 



in the trunk, where they pass ventralwards on each side of the 

 body-cavity, immediately outside the peritoneal membrane, along 

 the lines of insertion of the intermuscular septa. These are 

 obviously the ribs of the adult, and there is no break of con- 

 tinuity of structure between the haemal processes of the tail and 

 the ribs. In the anterior part of the trunk the ribs pass out- 

 wards along the intermuscular septa till they reach the epidermis. 

 Thus the ribs are originally continuous with the haemal pro- 

 cesses. Behind the region of the ventral caudal fin the two 

 haemal processes merge into one, which is not perforated by 

 a canal. 



Each of the intervertebral rings of cartilage becomes eventually 

 divided into two parts, and converted into the adjacent faces of 

 contiguous vertebrae, the curved line where this will be effected 

 being plainly marked out. These rings are united with the 

 neural and haemal arches of the vertebrae next in front and 

 behind. As these rings are formed originally by the spreading 

 of the cartilage from the primitive neural and haemal processes, 

 the intervertebral cartilages are clearly derived from the neural 

 and haemal arches. The intervertebral cartilages are thicker in 

 the middle than at their two ends. 



In our latest stage (11 centims.), the vertebral constrictions 

 of the notochord are rendered much less conspicuous by the 

 growth of the intervertebral cartilages giving rise to marked 

 intervertebral constrictions. In the intervertebral regions the 

 membrana elastica externa has become aborted at the posterior 

 border of each vertebra, and the remaining part is considerably 

 puckered transversely. The inner sheath of the notochord is 

 puckered longitudinally in the intervertebral regions. The 

 granular external layer of the sheath in the vertebral regions is 

 less thick than in the last stage, and exhibits faint radial 

 striations. 



Two closely approximated cartilaginous elements now form 

 a key-stone to the neural arch above : these are directly differen- 

 tiated from the ligamentum longitudinale superius, into which 

 they merge above. An osseous plate is formed on the outer side 

 of each of these cartilages. These plates are continuous with 

 the lateral osseous bars of the neural arches, and also give rise 

 to the osseous roof of the spinal canal of the adult. 



