GENERAL HISTOLOGY. . 97 



sheath and sheath of Schwann are usually absent in invertebrates 

 even in the peripheral nerves. A thin myelin layer has been rarely 

 observed in arthropods and annelids. On the other hand the true 

 conducting elements, the nerve fibrillae, have been seen in inverte- 



FIG. 56. Ganglion cells of an actinian. 



brate nerve-fibres, and these have been followed into the ganglion 

 cell in which the afferent and efferent fibrillae are united in a 

 lattice-like manner. 



SUMMARY OF HISTOLOGICAL FACTS. 



Cells. 1. The most important morphological element of all 

 tissues is the cell. 



2. The cell is a mass of protoplasm which contains one or 

 several nuclei (uninucleated, multinucleated cells). 



3. The nucleus probably determines the specific character of 

 the cell, since it influences its functions; accordingly it is also the 

 bearer of heredity. 



4. Cells and nuclei increase exclusively by division or budding. 

 Tissues. 5. Tissues are complexes of numerous similar his- 



tologically differentiated cells. 



6. Histological differentiation rests in part upon the fact that 

 the cells take on a definite form and arrangement, in part upon the 

 formation of plasmic products, which determine the character of 

 the tissue (muscle-fibres, connective-tissue fibrils). 



