THE ORGANISM AND THE CELL 21 



an organ as the muscle of a frog. The flat, green expansions of curtain 

 green algae are functionally "leaves" even though they are coenocytic 

 instead of cellular like the leaves of vascular plants. 



These points are perhaps sufficient to emphasize the thought that in 

 our cytological study of animals and plants we shall miss much that is of 

 first importance if we consider cells, their components, and their products 

 only in terms convenient for the description of structure and neglect their 

 relation to the active life of the organism of which they are parts. 



Conclusions. — In this chapter we have dealt at some length with the 

 lelationship between two units or individualities present in large organ- 

 isms — the organism as a whole and the cell. Obviouslj^ there is a recip- 

 local action between them when both are present, the part affecting the 

 whole while the whole affects the part. The relative importance of the 

 two has been variously conceived. The proponents of the cell theory 

 stressed the cell as the primarj'- agent of organization, while adherents of 

 the organismal theory insist upon the primacj^ of the whole, cells when 

 present being important but subsidiarj^ parts. The former regarded 

 multicellular organisms as having arisen phylogenetically as aggregates 

 of unicellular individuals, whereas the latter hold it to be more probable 

 that single unicellular individuals became internally sul^divided as they 

 became larger. The probabilitj^ of an "evolution from Protozoa to 

 sponges and coelenterates by multiplication of nuclei in an already dif- 

 ferentiated cytoplasm" has recently been emphasized anew (Kofoid). 



This subject can be followed further in other works on biology, but it 

 has been thought well to set forth early in the present book some of the 

 basic facts of structural development insofar as the}' concern cells as units, 

 together ^\dth a suggestion of certain theoretical interpretations. It 

 should then be easier to appreciate the significance to the organism of 

 those cytological details to which we shall soon restrict our attention. 

 The prevalent multicellular state is doubtless the most efficient basis for 

 differentiation in all but very small organisms and has conditioned much 

 evolutionary progress, but we h&ve seen that the essential features of 

 development can occur without it. "The principle of protoplasmic 

 differentiation is more general and fundamental than that of cells as 

 units" (Heidenhain). 



