STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 93 



break of continuity of structure between the hsemal arches 

 of the tail and the ribs. In the anterior part of the trunk, 

 the ribs pass outwards along the intermuscular septa till 

 they reach the epidermis. Thus the ribs are originally 

 continuous with the haemal processes. 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, which are converted into 

 the adjacent faces of contiguous vertebrae, the curved line 

 where this will be efi'ected being plainly marked out at a 

 very early stage. 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 inter- 

 vertebral cartilages are thicker in the middle line than at 

 their two ends. 



In the latest stage examined (11 centims. long) the verte- 

 bral constrictions of the notochord are rendered much less 

 conspicuous by 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 inter- 

 vertebral regions. The granular external layer of the sheath 

 in the vertebral regions is less thick than in the last stage, 

 and exhibits a faint radial striation. 



Two closely approximated cartilaginous elements now form 

 a key-stone at each neural arch above ; these are directly 

 differentiated from the ligamentum longitudinale superius, 

 into which they merge above. An osseous plate is formed 

 on the outer side of each of tliese cartilages. These plates 

 are continuous with the lateral osseous bars of the neural 

 arches, and give rise to the osseous part of tlie roof of the 

 spinal canal of the adult. Thus the greater part of the 

 neural arches is formed by membrane 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. 



The embryological part of this section is followed by a 

 comparative part treated under three headings. In the first 

 of these the vertebral column of Lepidostcus is compared 

 with that of other forms ; and it is pointed out that there 



