92 F. M. BALFOUR AND W. N. PARKER. 



converted into cartilage^ thus forming a series of neural 

 processes^ riding on tbie membrana elastica externa, and 

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

 spinal cord. 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. Behind this point each 

 succeeding pair of haemal processes becomes larger than the 

 one in front, each process finally meeting its fellow below 

 the caudal vein, thus forming a completely closed haemal 

 arch. 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 the authors have had the oppor- 

 tunity of studying (a larva of o^ centims.), a series of well- 

 marked ve7'tedral constrictions are to be seen in the noto- 

 chord. The sheath is now much thicker in the vertebral 

 than in the intervertebral regions ; this being due to a special 

 differentiation of a superficial part of the sheath, which 

 appears more granular than the remainder, and 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, which are of course placed in the vertebral 

 regions, are now continuous with a cartilaginous tube embrac- 

 ing the intervertebral regions of the notochord, and con- 

 tinuous from one vertebra to the next. A delicate layer of 

 bone, developed in the perichondium, invests the cartila- 

 ginous 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 

 backwards. These bars, or dorsal processes, may be reckoned 

 as parts of the neural arches. Between the dorsal processes 

 of the two sides are placed median rods of cartilage, which 

 are developed separately from the true neural arches, and 

 which constitute the median spinous elements of the adult. 

 Immediately below these rods is placed the ligamentum 

 longitudinale superius. There is now the commencement, 

 not only in the tail but also in the trunk, of a separation 

 between the dorsal and ventral parts of the haemal arches 

 where the latter pass ventralwards, on each side of the body 

 cavity, along the lines of insertion of the intermuscular septa. 

 They are obviously the ribs of the adult, and there is no 



