7QO STRUCTURE 'AND DEVELOPMENT OF LEPIDOSTEUS. 



soon become converted into cartilage, thus forming a series of 

 neural processes riding on the membrana elastica externa, and 

 extending about two-thirds of the way up the sides of the neural 

 canal. The haemal processes arise simultaneously with, and in 

 the same manner as, the neural. They are small in the trunk, 

 but at the front end of the anal fin they suddenly enlarge and 

 extend ventralwards. Each succeeding pair of hsemal arches 

 becomes larger than the one in front, each arch finally meeting its 

 fellow below the caudal vein, thus forming a completely closed 

 haemal canal. These arches are moreover produced into long 

 spines supporting the fin-rays of the caudal fin, which thus 

 differs from the other unpaired fins in being supported by parts 

 of the vertebral column, and not by separately formed skeletal 

 elements. 



In the next stage which we have had the opportunity of study- 

 ing (larva of 5^ centims.), a series of very well-marked vertebral 

 constrictions are to be seen in the notochord. The sheath is now 

 much thicker in the vertebral than in the intervertebral regions : 

 this is due to a special differentiation of a superficial part of 

 the sheath, which appears more granular than the remainder. 

 This granular part of the sheath thus forms a cylinder in each 

 vertebral region. Between it and the gelatinous tissue of the 

 notochord there remains a thin unmodified portion of the sheath, 

 which is continuous with the intervertebral parts of the sheath. 

 The neural and haemal arches are seen to be continuous with a 

 cartilaginous tube embracing the intervertebral regions of the 

 notochord, and continuous from one vertebra to the next. A 

 delicate layer of bone, developed in the perichondrium, invests 

 the cartilaginous neural arches, and this bone grows upwards 

 so as to unite above with the osseous investment of separately 

 developed bars of cartilage, which are directed obliquely back- 

 wards. These bars, or dorsal processes, may be reckoned as 

 parts of the neural arches. Between the dorsal processes of the 

 two sides is placed a median rod of cartilage, which is developed 

 separately from the true neural arches, and which constitutes 

 the median spinous element of the adult. Immediately below 

 this rod is placed the ligamentum longitudinale superius. There 

 is now a commencement of separation between the dorsal and 

 ventral parts of the haemal arches, not only in the tail, but also 



