THE UNIT OF STRUCTURE 27 



their kind in fact, carry out all the functions of life as isolated 

 cells, they conceived the idea that a visible plant or animal 

 is a community of cells, each an organism in itself. As bees 

 are units of a swarm, as men and women are units of a state, 

 cells are units which for the sake of mutual protection remain 

 associated in a multicellular body. The physiological or 

 sociological aspects of this conception we shall consider shortly ; 

 but the anatomical basis of the cell theory was laid without a 

 sufficient testing of the facts upon which it rests ; or, rather, 

 one ought to say that, although the axiom, enunciated by 

 Virchow when he applied the cell theory to tumours and other 

 morbid growths, Omnis cellula a celluld, holds good, the applica- 

 tions of the theory which certain of its later exponents have 

 made are not necessary sequents. 



Every plant, every animal, commences its existence as a 

 single cell. An organism which is permanently unicellular 

 divides. Each of the separate cells into which it divides is a 

 new individual. Higher plants set aside certain cells as 

 ovules, which in due course, after conjugation with pollen 

 grains, grow into plants. In the same way the ova of animals, 

 by repeated cell division, reproduce the species. The individual 

 commences as a single cell. Its complicated body, composed 

 of various organs and various tissues, is formed by the multi- 

 plication of cells. Each of the innumerable cells of which it 

 is composed has the structure, and may therefore be presumed 

 capable of performing all the various functions,- of a uni- 

 cellular organism. But it does not follow that the cells retain 

 their individuality. Even unicellular plants (e.g., the extra- 

 ordinary vinegar and tan fungi, myxomycetes) may for a time 

 merge their individuality in a common mass formed by the 

 aggregation of many cells. 



The cells of higher plants are not always, or even generally, 

 anatomically distinct. Their protoplasm, the essential part of 

 every cell, is united with the protoplasm of neighbouring cells 

 by threads which traverse the cell- walls. The cells of the 

 connective tissues of animals are united into a web, or syn- 

 cytium. This is especially noticeable during early stages of 

 growth. Nerve- cells are connected together by conducting 

 filaments (neurofibrillse). It is possible that nerve-cells and 

 the muscle-fibres which they innervate are from the beginning 



