no CHORDATE ANATOMY 



organization. First there are those cell organs, such as nucleus and 

 chromatin bodies, which represent the fundamental organization of 

 protoplasm as cells. Then there are those intracellular structures such 

 as myofibrils or neurofibrils which mark the cell as belonging to a particular 

 tissue — muscular or nervous. Thirdly, there may be features which 

 identify the tissue as that of a certain organ; for example, the inter- 

 calated discs in the heart muscle of vertebrates. Finally, the individual 

 tissue element may have peculiarities which are specific for animals of a 

 certain group; for example, the striated muscle fiber of an insect differs 

 in details of structure from that of a vertebrate. Behind all of this 

 visible dilTerentiation and specificity of structure must be chemical 

 specificity. 



