HISTOLOGY 163 



Unstriated muscle fibers appear much the same whether they are in the 

 wall of a stomach or of a lung. 



Nevertheless tissues and cells may exhibit characteristics which mark 

 them as belonging to a particular organ or animal. The nerve cells of a 

 spinal gangUon differ from the motor nerve cells in the spinal cord of the 

 same animal. The striated muscle of the vertebrate heart differs from 

 that of the body wall. Further, vertebrate cardiac muscle differs from 

 cardiac muscle of a lobster. Epidermal tissue of a fish differs from that of 

 a reptile. Anyone familiar with the nervous tissues of annelids could 

 easily distinguish gangHonic tissue of an earthworm from that of a leech. 



It follows, therefore, that the individual tissue cell may, in its visible 

 structure, exhibit characteristics reflecting as many as four grades of 

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

 chromatin bodies, which are common to all cells and 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 intercalated 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. 



Beneath all of this structural differentiation there is chemical speci- 

 ficity. Although it does not necessarily always manifest itself in visible 

 structural differentiation, there must exist from top to bottom of this series 

 of levels of protoplasmic organization a chemical specificity corresponding 

 to a specific genetic group of animals — or, indeed, corresponding even to 

 the individual animal. 



