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II. Ossification and Development of the Bodies of the Vertebra. 



A. General remarks on the part which the chorda takes in the 

 formation of the vertebra. 



1st. In all cases where the chorda ossifies, it is only its second layer, 

 or the sheath proper, which undergoes changes. At the same time 

 the elastica externa disappears totally, or is at least dissolved in such 

 a manner that its remnants are scarcely distinguishable, whilst the 

 elastica interna and the chorda proper generally remain unaltered. 

 In one case only, viz. in Scymnus lichia, ossification is to be seen 

 even in the gelatinous substance of the chorda. 



2nd. The ossification of the sheath of the chorda has been 

 observed as yet only in the Plagiostomes and in certain genera of the 

 osseous fishes ; but it very probably will be found in all osseous 

 fishes. On the contrary, it is absent in all higher Vertebrata accord- 

 ing to my observations, even amongst the Batrachia. 



B. Changes of the sheath of the chorda during ossification. 

 1 . Vertebral column. 



1st. In the Plagiostomes the sheath of the chorda in the first 

 place assumes a greater hardness in certain parts, these parts being 

 transformed into nbro-cartilage or real cartilage, whilst the inter- 

 vening parts retain their primitive softness. In this manner the first 

 indications appear of the vertebral bodies and intervertebral liga- 

 ments, the interior parts of which are formed by the chorda itself 

 and the elastica interna. The histological changes going on during 

 this formation of the vertebral bodies, viz. the transformation of the 

 primitive plasm- cells of the sheath into cartilage-cells, and the deve- 

 lopment of the homogeneous interstitial substance of the cartilage 

 out of the fibrous substance of the sheath, speak strongly in favour 

 of the view that both kinds of cells arid intervening substances are 

 closely allied, whatever may have been the development of the ele- 

 ments of the primitive sheath. 



In the Leptocephali the sheath of the chorda ossifies without 

 having been transformed into cartilage ; and the same seems to hold 

 good for the other osseous fishes. 



2nd. "Whilst this transformation of certain parts of the sheath of 

 the chorda into cartilaginous vertebral bodies is going on, there are 

 also formed in the interior of each of these bodies peculiar vertical dis- 



