PROTOPLASM, LIFE AND DEATH 181 



the other cells by something resembling a system of 

 electrical wires and batteries these are the nerve-cells 

 (Fig- 37 D), with their fine, thread-like branches, the 

 nerve-fibres, which are long enough to permeate every 

 part of the body and place it in connection with the 

 nerve-cells in the great centres called brain, spinal cord, 

 and ganglia. 



At one time it was thought that the cells in the 

 tissues of plants and animals could originate de novo by 

 a sort of precipitation of liquid matter. But it is now 

 known that every cell has originated by the division of a 

 pre-existing cell into two, the nucleus of the mother cell 

 first dividing and then the rest of the cell. " Every cell 

 originates by the fission of a preceding cell " is the law, 

 and to that is added, " Every individual organism, plant 

 or animal, itself originates from a single cell, the fertilised 

 germ-cell." These are two laws of fundamental impor- 

 tance in the study of living things. They are true of 

 man as well as of the smallest worm ; of the biggest tree 

 as well as of the most insignificant moss or water-weed. 

 When the fertilised egg-cell divides, and its progeny keep 

 on dividing and growing in bulk by the conversion of 

 nutriment into protoplasm, the dividing cells do not neces- 

 sarily become entirely nipped off from one another. In 

 large tracts of cells (or tissues) we often find that the 

 neighbouring cells are connected to one another by ex- 

 cessively fine filaments of protoplasm. Only twenty years 

 ago it was supposed, whilst the neighbouring cells were 

 thus connected as a rule in animals, as well as being often 

 connected to the finest nerve-filaments, yet that in plants 

 the firm, box-like cases which surround the protoplasm 

 and when seen dried and empty by Robert Hook led him 

 to introduce the word " cell " to describe them form 

 completely shut cases, so that the living protoplasm of 

 each plant-cell is entirely cut off from its neighbour. 



