THE CELL DOCTRINE 97 



layers added to it by the protoplasm within, and being 

 pushed out as the tube grows in length. Here there 

 is no cell structure, and such plants are called non- 

 cellular. Also there are parts of animals, and phases 

 in the development of animals, where there is no clear 

 cell structure, but for instance a network of cytoplasm 

 with a nucleus at each " node " of the network, i.e. 

 at each thickening of the cytoplasm where several 

 branches of the network meet. This kind of structure 

 does not however differ essentially so far as its working 

 is concerned from ordinary cell structure. We have 

 already seen (p. 66) that the nucleus is essential to the 

 carrying out of the vital functions of the protoplasm ; 

 and in these cases of non-cellular or incomplete cellular 

 structure we must suppose that each nucleus governs, 

 so to speak, a certain sphere of cytoplasm around 

 it, though this sphere is not sharply separated, as it 

 is in ordinary cellular structure, from the neighbouring 

 spheres or units. Such a working unit of nucleus and 

 cytoplasm has been called an energid. An ordinary 

 living cell represents one such energid, but in non- 

 cellular living structures the energids are not separated 

 by clear boundaries. 



The second qualification of the cell doctrine that 

 has to be made relates to the unity or individuality of 

 the multicellular organism. To some degree in all such 

 organisms, and very notably in the higher animals 

 with a central nervous system, the activities of each 

 cell are modified by its connexion with the other cells 

 of the body, so that the organism works as a whole. In 

 this sense, as we saw in the last chapter, the organism 

 as a whole has functions which are something distinct 

 from the functions of the individual cells or energids 

 of which it is composed, though these are built up as 

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