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I. Chorda dorsalis. 

 A. Structure. 



The chorda dorsalis of the Plagiostomes, of Chimara, Acipenser, 

 Scaphirhynchus, Toxodon, and Lepidosiren, shows four distinct parts, 

 viz. 



1st. The outer elastic membrane, a homogeneous elastic coat, 

 which is not ^infrequently perforated with holes of different sizes, of 

 the same kind as those of the fenestrated membrane of Henle. 



2nd. The proper sheath, formed of connective tissue of fibrous 

 appearance, and generally provided with many plasm-cells. 



3rd. The inner elastic layer, a reticulated elastic membrane ; and 



4th. The gelatinous substance of the chorda itself, made up of 

 soft cartilage-cells, of different sizes and generally provided with 

 nuclei. 



Of these four layers it would seem that only the third and fourth 

 are present in the higher animals, from the Amphibia (with the ex- 

 ception of the Batrachians) upwards ; if, at least, my opinion be 

 correct, that the structureless envelope of the chorda of these animals, 

 generally called the sheath proper, corresponds to the third layer in 

 the cartilaginous fishes. On the other hand, it seems that many of 

 the osseous fishes present the same complications of structure as 

 the Plagiostomes, if it is true that the bodies of their vertebrae are 

 developed from the proper sheath of the chorda. So, for instance, 

 there exists a beautiful elastic internal layer outside of the remnants 

 of the gelatinous chorda in the genus Orthagoriscus. 



B. Form of the chorda proper. 



1st. The chorda retains in some instances its original cylindrical 

 form, and this is the case when the vertebral column shows no indi- 

 cation of vertebral bodies (Cyclostomes, Acipenser, Chimara, Lepi- 

 dosiren, Tilurus, Hyoprorus* (anterior vertebra)), as well as where 

 vertebral divisions exist (Leptocephalus, Helmichthys, Hyoprorus 

 (last vertebra)). 



2nd. In other cases the chorda is contracted in the middle region 

 of each vertebral body, which seldom happens where there is no 

 trace of ossification (Hexanchus), but is very generally the case in 



* Two genera belonging to the Leptocephalidae, described by me (see Kaup, 

 Apodal Fishes of the British Museum. London, 1856). 



