8 ON VERTEBRATE ANIMALS 



An especial cliaracteristic of vertebrate animals consists in the 

 possession of an internal skeleton, of which the spinal column forms 

 the stem. However strange it may sound, it is nevertheless true 

 that all vertebrate animals do not possess vertebrae. The first com- 

 mencement of the skeleton is a cord situated in the back, whicli 

 is sometimes fibrous but usually gelatinous, and consists of elongated 

 cells, and is inclosed invariably by a fibrous sheath. This cord 

 {chorda dorsalis) does not undergo ossification, but during the 

 formation of the bony vertebrae is gradually superseded or included 

 by the ossified sheath. At the same time there are imperfect fishes 

 where this dorsal cord is persistent, and where no special vertebrae 

 are found. The spinal marrow lies above this cord, protected by 

 two fibrous laminae, or by cartilaginous arches. Consequently a 

 vertebral column cannot be assigned as the universally prevalent 

 type of vertebrate animals, but first of all a dorsal cord {chorda 

 dorsalis) which either persists during the whole of life, or is replaced 

 by a vertebral column ; and next, a spinal marrow, as central mass 

 of the nervous system, which is situated upon this cord or upon the 

 vertebral column. 



The basal pieces of the cranial bones, upon which tlie mass of 

 the brain rests, resemble the bodies of vertebrjB. The brain is sur- 

 rounded by very wide vertebral archest All the cranial bones 

 however do not arise from ossification of the original cartilage which 

 is to be regarded as a continuation of the vertebral column in the 

 head. This cartilage, this primordial skull, is in part covered over 

 and inclosed by bones which arise from a membranous hlastema 

 which lies upon the cartilage, and so have never been cartilaginous ^ 

 In general all fibrous and tendinous parts may become bony. Hence 

 the muscles of the legs in birds are seen to terminate in bony ten- 

 dons ; in other animals portions of bone are found in the diapliragm. 



1 Oken adopts three cranial vertebrae : Ueber die Bedeutung dcr Schddelknochen. 

 Jena, 1807, 4to; OwEN four: On the archetype and homologies of the Vertebrate 

 Skeleton. London, 1848. 



2 Attention was directed to this point by the celebrated physiological anatomist L. 

 Jacobson especially. See his memoir Om Primordial-craniet in Forhandlingar vid de 

 Skandinaviske Naturfor-sJcames tredje Mote. Stockholm, 1842, pp. 739 — 744; compare 

 also A. KOELLIKEK in Berichte von der Koniglichen Zootomischen Anstalt zu Wurzburg 

 (Leipzig, 1849), s- 35—42, where also are found anatomical notices on the labours of 

 some writers who preceded Jacoboon on this point. 



