THE CELL AND PROTOPLASM. 123 



must look deeper than simple cell structure for 

 their explanation of the life processes. While the 

 study of cells has thrown an immense amount of 

 light upon life, we seem hardly nearer the centre 

 of the problem than we were before the beginning 

 of the series of discoveries inaugurated by the 

 formulation of the doctrine of protoplasm. 



Fundamental Vital Activities as Located in 

 Cells. We are now in position to ask whether our 

 knowledge of cells has aided us in finding an 

 explanation of the fundamental vital actions to 

 which, as we have seen, life processes are to be 

 reduced. The four properties of irritability, con- 

 tractibility, assimilation, and reproduction, belong 

 to these vital units the cells, and it is these prop- 

 erties which we are trying to trace to their source 

 as a foundation of vital activity. 



We may first ask whether we have any facts 

 which indicate that any special parts of the cell 

 are associated with any of these fundamental ac- 

 tivities. The first fact that stands out clearly is 

 that the nucleus is connected most intimately with 

 the process of reproduction and especially with 

 heredity. This has long been believed, but has 

 now been clearly demonstrated by the experiments 

 of cutting into fragments the cell bodies of uni- 

 cellular animals. As already noticed, those pieces 

 which possess a nucleus are able to continue their 

 life and reproduce themselves, while those without 

 a nucleus are incapable of reproduction. With 

 greater force still is the fact shown by the process 

 of fertilization of the egg. The egg is very large 

 and the male reproductive cell is very small, and 

 the amount of material which the offspring de- 

 rives from its mother is very great compared with 

 that which it derives from its father. But the 



