This was the most extreme position of the cell doctrine.  The cell was the unit of living action, and the higher animal or plant simply a colony of such units.  An animal was simply an association together for mutual advantage of independent units, just as a city is an association of independent individuals.  The organization of the animals was simply the result of the combination of many independent units.  There was no activity of the organism as a whole, but only of its independent parts.  Cell life was superior to organized life.  Just as, in a city, the city government is a name given to the combined action of the individuals, so are the actions of organisms simply the combined action of their individual cells.

Against such an extreme position there has been in recent years a decided reaction, and to-day it is becoming more and more evident that such a position cannot be maintained.  In the first place, it is becoming evident that the cell substance is not to be entirely obliterated by the importance of the nucleus.  That the nucleus is a most important vital centre is clear enough, but it is equally clear that nucleus and cell substance must be together to constitute the life substance.  The complicated structure of the cell substance, the decided activity shown by its fibres in the process of cell division, clearly enough indicate that it is a part of the cell which can not be neglected in the study of the life substance.  Again the discovery of the centrosome as a distinct morphological element has still further added to the complexity of the life substance, and proved that neither nucleus nor cell substance can be regarded as the cell or as constituting life.  It is true that we may not yet know the source of this centrosome.  We do not know whether it is handed down from generation to generation like the nucleus, or whether it can be made anew out of the cell substance in the life of an ordinary cell.  But this is not material to its recognition as an organ of importance in the cell activity.  Thus the cell proves itself not to; be a bit of nuclear matter surrounded by secondary parts, but a community of several perhaps equally important interrelated members.

Another series of observations weakened the cell doctrine in an entirely different direction.  It had been assumed that the body of the multicellular animal or plant was made of independent units.  Microscopists of a few years ago began to suggest that the cells are in reality not separated from each other, but are all connected by protoplasmic fibres.  In quite a number of different kinds of tissue it has been determined that fine threads of protoplasmic material lead from one cell to another in such a way that the cells are in vital connection.  The claim has been made that there is thus a protoplasmic connection between all the cells of the body of the animal, and that thus the animal or plant, instead of consisting of a large number of separate independent cells, consists of one great mass of living matter which is aggregated into little centres, each commonly holding a nucleus.  Such a conclusion is not yet demonstrated, nor is its significance very clear should it prove to be a fact; but it is plain that such suggestions quite decidedly modify the conception of the body as a community of independent cells.