VERTEBRAE OF COMMON COD. IOi 



by the late Professor Balfour in his paper on Lepidosteus, published in connection 

 with W. N. Parker (infra]. In some Elasmobranchii, e.g. Scy Ilium and some 

 Teleostei, the anterior portion of the caudal canal is apparently inclosed by haemal 

 arches, to which ribs are attached laterally. The explanation of this arrangement 

 is as follows (Balfour) : The canal in this region is secondarily acquired : it is 

 a prolongation forward of the posterior part which is normally developed, and 

 it is formed by the growth of two processes one from each haemal arch near 

 its base, towards one another which meet and fuse, the true haemal arch being 

 that portion of cartilage or bone which bears the rib. 



In some instances, e.g. Pike, the right and left neural arches are not united 

 by bone in the median line. It should be noted also that in the region of the 

 trunk at least, the ribs have free ventral ends. There is no sternum. 



Other structures to be noted are : (i) the ligamentum vertebrale superius 

 which runs above the spinal cord and connects the successive vertebrae. (2) A 

 pair of cartilages which appear in development between the ligament and the 

 spinal cord : they project posteriorly, ossify or become surrounded by bone and 

 become continuous ultimately with the arch. They are probably homologues 

 of the intercalaria neuralia of the Elasmobranch which lie between the neural 

 arches. They are present also in Ganoids. (3) The interspinal bones which 

 support the azygos fins seen in the preceding preparation (Prep. 17). These, 

 according to Gotte, may be regarded as dissociated parts of vertebral spines, but 

 this view is extremely doubtful. Dohrn has recently suggested, and Mayer has 

 strongly supported his view, that the azygos fins are derived from paired rudiments ; 

 the anal and ventral element of the caudal fins, by the coalescence behind the 

 primitive cloaca of the two ventro-lateral ridges from which the pectoral and pelvic 

 fins originate : the dorsal fins and the dorsal element of the caudal fin from two 

 similarly coalesced dorso-lateral ridges. If this is so, then the interspinal bones are 

 really equivalents of the basal cartilages in the paired fins. They are not connected 

 with the vertebral column in most Elasmobranchii. In Dipnoi they remain articulated 

 to the neural spines in the dorsal region, and to the haemal in the ventral region, 

 where in other types the anal fin is formed. They appear in the young Teleostean 

 just above the summits of the arches, but when the latter lengthen, come to lie . 

 between them. They are developed also in regions where the azygos fins are deficient; 

 but it must be borne in mind that this system of fins is primitively represented 

 by a continuous fold. (4) The set of bones known as epi-neurals, epi-centrals, epi- 

 pleurals (Fleischgraten). They are formed in the intermuscular septa (myocommata), 

 and never pass through a cartilage stage. Gotte has suggested that the series 

 which lies on the ends of the inferior arches in the region of the trunk are ribs 

 homologous with the ribs of Elasmobranchs owing to their position. But this 

 view appears to have little to support it, and the mode of formation of the bones 

 in question is against it. 



It may be noted that the ribs of all fish except Elasmobranchii lie at the 

 base of the myocommata, immediately below the peritoneum. In Elasmobranchii 

 they extend ouhvards in the fibrous septum which separates the dorso-lateral and 

 ventro-lateral muscle-masses, and therefore not below the peritoneum. Hence 

 these ribs have been regarded as not homologous with the ribs of other fish. 

 But in Lepidosteus the free ends of the ribs, the bodies of which are normally 



