CLASSIFICATION OF CELLS. 43 



processes.* Inasmuch as the cement is included in the series 

 of intercellular substances, it must be admitted that there is 

 no fundamental morphological difference between the material 

 connecting epithelial cells, endothelial cells, and the cells of 

 the connective tissues ; in all these we have to do with meta- 

 morphosed cell substance, by means of which the morphological 

 elements are united. 



Besides the mode of union by means of intercellular sub- 

 stance, we are also acquainted with a mode in which cells 

 unite through the intermediation of processes, and we have 

 already noticed that, under certain circumstances, cells may 

 become fused together, an occurrence that may take place 

 whilst they are yet living. We cannot therefore doubt that the 

 protoplasmic masses are capable of directly uniting with one 

 another. Nevertheless the microscopic proof of the direct 

 fusion of cells has not been quite satisfactorily demonstrated. 

 It is possible that the union may be established by means of 

 cement, but this, up to the present time, has not been clearly 

 shown. 



From a physiological point of view, however, we must admit 

 that fusion of the processes of nerve cells may take place ; at 

 all events, it would be in opposition to our experience respecting 

 the conduction of nervous force, were we to admit that any 

 cementing substance intervened between the individual nerve 

 cells. 



With the exception of the nerve cells, the above-mentioned 

 objection holds good for all supposed or actually proved in- 

 stances of cell union. 



CLASSIFICATION OF CELLS. Cells are usually classified in ac- 

 cordance with their physiological function. This, however, is 

 not a very satisfactory mode, since we are still ignorant of the 

 functions of many groups of cells. For example, we have no 

 precise knowledge of the functions fulfilled by the colourless 

 blood corpuscles, and it is moreover improbable that all the 

 cells distributed over the surface of a membrane, as epithelial 

 and investing cells, should possess identical functions. We find, 



* F. E. Schulze, loc. cit. 



