GENERAL BIS TO LOOT. 



97 



sheath and sheath of Schwann are usually absent in invertebrates 

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

 observed in arthropods and annehds. On the other hand the trno 

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



FiG. 56. — Ganglion ceUs of an actinian. 



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

 cell in which the aiferent and efEerent fibrillaj are united in a 

 lattice-like manner. 



SUMMARY OF HISTOLOGIOAL FACTS. 



Cells 1. The most important morphological element of all 



tissues is the cell. 



2. The cell is a mass of protojjlasm 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. 



G. Histological differentiation rests in part ujion the fact that 

 the cells take on a definite form and arrangement, in jiartupon the 

 formation of plasmic i^roducts, which determine the character of 

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



