Prof. Kollikcr on the Chorda dorsalis of Fishes. 325' 



different processes, — one in which the crural cartilages themselves 

 play a part in this formation, and a second, where only the periosteal 

 layer between them is concerned. 



2nd. Where the crural cartilages take a part, they form, in the 

 first placCj by their coalescence an outer cartilaginous layer around 

 the body of the vertebra, which took its origin from the chorda, and 

 which we shall henceforth call the chordal vertebral body. 



3rd. This outer cartilaginous layer ossifies in many cases ; and 

 this ossification may take place in two places only, viz. on the right 

 and left side of the vertebral body, as in Ilejitanchus, or in four 

 places, in which case a superior point of ossification at the floor of 

 tlie neural canal, and an inferior one at the roof of the haemal canal, 

 are added to tlie two lateral ones {Acanthias, Scymnus). 



4th. These external ossifications of chondriform bone may retain 

 their primitive form of plates, and may then be called the lateral, 

 superior, and inferior osseous plates ; or they acquire by additional 

 growth, at the expense of the outer cartilaginous layer, the form of 

 wedge-shaped or cuneiform bodies, and may be named the lateral, 

 superior, and inferior wedges {Zapfen, Keile, Germ.). 



ijth. In both cases these external ossifications comport themselves 

 in two different ways with regard to the chordal vertebral Ijody, 

 inasmuch as in some cases both coalesce at their ends (Scymnus, 

 Acanthias), whilst in others they remain separated (Ilejitanchus). 



Gth. In some peculiar cases (Squali, possessing a nictitating eye- 

 lid, viz. Mustelus, Carcharias, Galeus, Sj/hyrna) the cartilaginous 

 arches remain separated, and then the intermediate periosteal layer 

 j)erforms the part of an osteogenic stratum. Tbe osseous parts pro- 

 duced in this way lie at the same places as the bony plates mentioned 

 inider -1 and i) ; they always possess the form of wedges, and coalesce 

 with the chordal vertebral body, in some cases only at their ends, hi 

 others in their whole length. Although these ossifications are not 

 developed from cartilage and have a very peculiar structure (they 

 consist of a calcified fibro-cartilage with jieculiar ossified strong fibres 

 running straight through their whole thickness), it is clear enough 

 that they exactly correspond to the above-mentioned j)lates and 

 wedges of other Plagiostomes formed out of the coalesced crural 

 cartilages. 



From certain modes of transformation of the sheath of the chorda, 

 combined v.ith certain changes of the outer ossifying layer, the 

 following tvpes in the composition of the vertebral bodies may be 

 established. 



Tyi»k I. — The vertebral body takes its origin entirely from tlie 

 •proper sheath of the chorda. 



A. Sheath of the chorda thick. 



1st. Vertebral bodies soft (fibro-cartilaginous), incomj)letely 

 separated from each other, and only distinguished by the interior 

 septa of the chorda. Jlexanchus. 



2nd. Vertebral bodies jiartly cartilaginous, with annular ossifica- 



