n6 



GENERAL PRINCIPLES OF ZOOLOGY. 



Here also occur more rarely bundles of fibrils or nerve- 



\ 



B 



FIG. 



B 



FIG. 53. 



FIG. 52. 



FIG. 51. Nerve-fibrils. (From Hatschek.) 



FTGS. 52 and 53. Single-contoured (A ) and doubled-contoured (B) nerve-fibres ; left without 

 and the right with, nuclei and Schwann's sheath. (From Hatschek.) 



fibres; nerve-fibres surrounded by a thin medullary layer 

 have been demonstrated in the annelids and arthropods. 



SUMMARY. 



Cells. i. The most important morphological element 

 of all tissues is the cell. 



2. The cell is a lump of protoplasm which is either 

 nonnucleated (Cytode) or contains one or several nuclei 

 (uninucleated, multinucleated cells). 



3. The nucleus probably determines the specific char- 

 acter of the cell, since it influences its functions; accord- 

 ingly it is also the bearer of heredity. ' 



4. Cells and nuclei increase exclusively by division or 

 budding. 



Tissues. 5- Tissues are complexes of numerous histo- 

 logically similarly differentiated cells. 



6. Histological differentiation rests in part upon the 



