100 DESCRIPTIONS OF PREPARATIONS. 



a layer which, according to Grassi, is an elastica externa, but, according to Gotte, 

 is composed of flat nucleated cells in a single layer, with an abundant intercellular 

 substance. In this substance Gotte states that the first ossifications forming the 

 primary centrum start. The chorda grows intervertebrally, and shrinks to a 

 fibrous cord in the middle of the vertebra. Hence the amphicoelous centra. 

 The cuticula thickens and becomes fibrous intervertebrally, as does the external 

 layer, forming the ligaments which unite the centra peripherally. The neural 

 and haemal arches rest with their bases on the chordal sheaths, but do not grow 

 round the chorda as they do in the Elasmobranch. The main, i. e. secondary 

 osseous substance, is periosteal, formed from connective tissue, which developes 

 between the bases of the arches and imbeds them. 



There are four principal types of structure in the vertebrae of adult Teleos- 

 teans. (i) Concentric laminae are laid down round the primary centrum. The 

 bases of the arches ossify and fuse indistinguishable with the laminae ; but isolated 

 masses of cartilage, remnants of them, may persist here and there temporarily, 

 e. g. the Cyprinoids. (2) Delicate bony radial laminae with intervening clear 

 connective tissue, appear first of all ; finally the bony matter becomes spongy, 

 and the connective tissue is converted into a fatty medulla. The bases of the 

 arches persist as cartilage, and form the well-known cross, e.g. Pike. The majority 

 of Teleostei belong to this type. (3) The bony matter forms a delicate spongy 

 mass ; the intervening substance consists of small round cells with a clear, firm 

 matrix ; the bases of the arches may retain isolated masses of cartilage ( Cyclopterus 

 lumpus) or may ossify completely ( Chironectes sp. ?). (4) There are delicate 

 radial united by a few concentric, bony laminae. The intervening substance is 

 hyaline cartilage : Monacanthus penirilligerus, Diodon. 



The differences depend upon (i) the persistence of cartilage in the bases 

 of the arches or the total conversion of the latter into bone, and (2) either the 

 total or partial conversion of the intervening connective tissue into bone; and 

 in the latter case the persistence of the remnant as connective tissue or its 

 conversion into either medulla or cartilage. 



The neural or superior appear before the haemal or inferior arches, and 

 both structures appear earlier in the anterior than in the posterior region of the 

 column. In Cyprinoid fishes certain of the arches may ossify without previous chon- 

 drification. In the anterior region of the column the haemal arches may become 

 entirely imbedded within the centrum, e. g. in the Perch ; and in the posterior region 

 they become not only more and more ventral in position, but they bend more 

 and more towards the ventral median line. Finally they appear to unite, inclosing 

 the caudal canal. The most posterior, however, attached to the terminal vertebrae 

 fuse into solid knobs. It may be observed that in the dorsal region ribs are 

 articulated to their ends. These ribs are developed continuously with the arches 

 in Elasmobranchii, Ganodei so far as is known, and in some Teleostei, whereas 

 in others they are developed independently, but yet in close apposition to their 

 ends. In Amia and Lepidosteus the caudal canal is formed by the haemal arches 

 plus ribs which are cut off from them, but which articulate with them, as in the 

 dorsal region. And it is these ribs that unite distally, completing the canal. 

 The presumption is, that in other cases the canal is formed by haemal arches 

 with ribs remaining continuous with them. This view was strongly supported 



