1881.] On the Structure and Development of Lepidosteus. 115 



way up the sides of the spinal cord. Haemal processes arise simul- 

 taneously witli 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 ventral wards. 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, pro- 

 duced 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 opportunity of study- 

 ing (a larva of 5 \ centims.), a series of well-marked vertebral constric- 

 tions are to be seen in the notochord. 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 em- 

 bracing the intervertebral regions of the notochord, and continuous 

 from one vertebra to the next. A delicate layer of bone, developed in 

 the perichondium, 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 

 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 inser- 

 tion of the intermuscular septa. They are obviously the ribs of the 

 adult, and there is no 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 



