20 THE MAIN CURRENTS OF ZOOLOGY 



existence in the condition of a single cell, and from 

 this primary condition, the many cells arise by 

 successive divisions of the original one. These many 

 cells become separated into different groups which 

 exhibit different properties, and, by the combination 

 of cells having similar properties, the tissues arise. 

 When, finally, about 1865, all eggs, as well as their 

 fertilizing agents, or sperms, were recognized as cells, 

 we had the logical explanation of the origin of cells, 

 and, also, an insight into the nature of tissues. 



At the time of Schleiden and Schwann a cell was 

 thought of as a box-like compartment or a little space 

 surrounded by walls but, presently, this conception 

 was materially changed. The cell- wall was seen to be 

 formed by the living protoplasm within, and atten- 

 tion became directed to that substance as the essen- 

 tial part of the cell. Many animal cells were observed 

 without cell-walls and the conclusion was soon reached 

 that the cell-wall is unimportant and may be lacking. 

 Accordingly, about 1860, Max Schultze defined the 

 essential qualities of a cell as "A mass of protoplasm 

 containing a nucleus." Up to this time the cell-theory 

 was chiefly a morphological doctrine, but now com- 

 bined with the protoplasm idea, which was essentially 

 physiological, it assumed a wider significance. 



