HISTORICAL SKETCH 429 



vidual unit with a clearness unapproached in any earlier treatise. This 

 was due in part to Brown's discovery of the nucleus, which enabled 

 Schwann to surpass his predecessors in distinguishing true cells from other 

 elements with which they had long been confused. With regard to the 

 general significance of cells Schwann says: 



The elementary parts of all tissues are formed of cells in an analogous, though 

 very diversified manner, so that it may be asserted that there is one universal 

 principle of development for the elementary parts of organisms, however different, 

 and that this principle is the formation of cells. . . . All organized bodies are 

 composed of essentially similar parts, namely, of cells. . . . The whole organism 

 subsists only by means of the reciprocal action of the single elementary parts. 

 [And further:] The development of the proposition that there exists one general 

 principle for the formation of all organic production, and that this principle is 

 the formation of cells, as well as the conclusions which may be drawn from this 

 proposition, may be comprised under the term Cell Theory. . . . 



It should be carefully noted that the essential point in this theory 

 was that cells, no matter how diverse they may be in appearance, are all 

 morphologically equivalent and are elementary living units whose action 

 determines the development of the organism ; the cell is the primary agent 

 of organization. It is an observable fact that most bodies are composed 

 of cells and their products, and that the life cycles of such organisms may 

 be described as cell successions, but the theory lies rather in the concep- 

 tion of the cell individual as the leader in the development of organic 

 structure and in function. With Lamarck and Mirbel, even as with 

 Wolff, cells were not very definitely individualized and were more or less 

 passive in the formation of organs in the fundamental cellular matrix; 

 theirs was a tissue theory rather than a cell theory. With Schleiden and 

 Schwann, on the other hand, cells were definite elementary organisms 

 primarily responsible for the development and activity of the body. 

 Dutrochet seems to have been the only one to approach this conception 

 previously. 



Interesting consequences followed from the extension of the cell 

 theory to the Protozoa. The protozoon was found usually to be a uni- 

 nucleate individual not greatly unlike one of the cells of a larger animal or 

 plant. It was therefore concluded that Protozoa are primitive one-celled 

 organisms that in the course of evolution somehow aggregated to form 

 cell republics, or multicellular organisms; and this was further taken 

 to mean that the protozoon was to be homologized with a single cell of 

 the multicellular body. This conception of the Protozoa and of the phy- 

 logeny of multicellular organisms has been justly criticized by many 

 biologists, who hold that if multicellular animals have evolved from 

 Protozoa it has been by developing internal cellular structure rather 

 than by colonial aggregation, and that the protozoon is therefore homol- 

 ogous with the entire multicellular individual (see Dobell, 1911a). 



