CELLS AND TISSUES 21 



build up an individual of a higher order, the multicellular organism. 

 Such an organism is thus a "cell state," or "cell republic," secondarily- 

 formed by the aggregation of a vast number of elementary individuals. 



The phylogenetic aspect of the cell theory developed when it was 

 discovered that many minute organisms are single uninucleated masses 

 of protoplasm much like the constituent cells of the multicellular forms. 

 It was concluded that such "unicellular elementary organisms" have 

 in the course of time formed loose colonies, either by direct aggregation 

 or by an acquired failure to separate after a period of multiplication; 

 and that the individual units have become increasingly interdependent 

 and knit together until individuals of a higher grade, multicellular 

 organisms, have resulted. As a consequence of this interpretation, 

 the individual protozoon has been homologized with a single cell of the 

 human body. The dominance of the cell theory, moreover, has resulted 

 in the tendency to describe everything of a biological nature so far as 

 possible in terms of cells. 



It has always been very difficult to make any plausible interpretation 

 of plasmodial or coenocytic organisms in terms of the cell theory. Such 

 organisms are multinucleate and non-septate masses of protoplasm, 

 which nevertheless build up bodies of definite form and with a consider- 

 able degree of differentiation. The nuclei are centers of action whose 

 reactions influence the cytoplasm to different distances, especially in 

 those forms in which their position is being constantly changed by 

 protoplasmic streaming. Some have taken the standpoint that in such 

 coenocytic bodies each nucleus with the portion of the cytoplasm it 

 influences "represents" a cell, while others hold that the entire multi- 

 nucleate body is one huge cell. Both of these interpretations contain 

 elements of truth, but they also appear like attempts to save a theory 

 based on a limited group of observations. 



The early dissatisfaction with the conception of the cell as the primary 

 and universal agent of organization led to the formulation of the organ- 

 ismal theory, in which the emphasis was placed on the living mass as a 

 whole rather than on the constituent cells. ^^ According to this general 

 interpretation, ontogenesis is a function primarily of the organism as a 

 whole and consists in the growth and progressive internal differentiation 

 of a single protoplasmic individual, this differentiation often, but not 

 always, involving the septation of the living mass into subordinate 

 semiindependent parts, the cells. Since the septation is rarely complete, 

 all parts remain in connection and the whole continues to act as a unit. 

 Hence development is not primarily the establishment of an association 

 of multiplying elementary units to form a new whole but rather the 



1' For a list of biologists who developed and supported this theory, see p. 436. 

 See also Whitman (1893), Sedgwick (1894), Dobell (1911a), Ritter (1919\ Ritter 

 and Bailey (1928), and Russell (1930). 



