THE NEW CELL DOCTRINE II 



further we learn that alterations occur so that we can observe 

 the progressive separation of the single cells; thus, for example, 

 in epithelium there arise partition walls, Fig. 7, separating the 

 cells finally and completely from one another. In mesen- 

 chyma the connections may become interrupted by which the 

 protoplasmic masses around the single nuclei are joined 

 together, Fig. 8. In this way the cells become completely 

 isolated. When we encounter cells which have been separated 

 in this way we have to do not with a primitive but with a 

 secondary condition. 



The descriptions just given lead us to one of the chief con- 

 clusions of the new cell doctrine. We have learned that the 

 relations are much more complicated than was previously 

 assumed. 



We turn to the discussion of protoplasm, or, as we have 

 termed it before, of the cell body. It is necessary to direct at- 

 tention to the fact that in the living world we know two chief 

 types of cells; first, such cells as exist alone, the so-called uni- 

 cellular organisms. Of such cells there are very many species 

 which have been grouped into numerous genera. Each genus 

 and each species has its special peculiarities which we learn 

 chiefly through the microscope. When a cell of any of the 

 just-mentioned species is observed for a longer period few al- 

 terations in its structure can be observed. The chief changes 

 we can observe are, first, an enlargement of the cell, and sec- 

 ond, the inner alterations which are usually specially notice- 

 able in the nucleus, which lead gradually to the division of the 

 cell. The two new daughter cells remain extremely similar to 

 the original mother cell in all peculiarities. Such an organism 

 propagates itself in this manner endlessly and without essen- 

 tially changing its structure. 



Very different are the conditions in the second type of 



