CHAPTER 6 

 FROM ONE CELL TO MANY CELLS 



Knowledge of the structure, function and multiplication of single 

 cells should pave the way for an understanding of the more intricate 

 structure, function, and interrelations of the complex animals or metazoa. 

 As a step toward such an understanding it will be useful to reflect upon 

 some of the consequences of the differences between the complex and 

 the simple. 



Insight into the nature of multicellular organisms would be furnished 

 by some certain knowledge of how they became multicellular. It seems 

 clear that living things have not always existed in the highly complicated 

 form that many of them now show. There must have been an origin of 

 complex beings from simpler ones. This conclusion is often couched in 

 the statement that multicellular organisms must have arisen from 

 unicellular ones, but it would be somewhat safer, as we shall see, not to 

 imply that cells were involved in the change. Some biologists hold that 

 a step comparable to a change from one cell to many was made before 

 these living things had arrived at a genuinely cellular constitution. But 

 whatever the origin of multicellular organisms was, if we knew that origin 

 we should have an important clue to some of their other characteristics. 



Relation of Parts to the Whole. — Two schools of thought have arisen 

 concerning the relation between multicellular animals and the cells of 

 which they are composed. One school has held that the whole is the 

 sum of its parts; hence that many-celled organisms are what their cells 

 make them. If cells of a certain structure and certain capacities are 

 assumed, any body composed of them will have the combined structures 

 and ca])acities of those cells. The other school has regarded the whole 

 as superior to its parts. A living thing is a whole first of all; its parts are 

 secondary. Animals and plants are not determined by the cells com- 

 posing them. Instead, they impress upon their cells certain properties 

 because the parts of the given whole must have those properties. Tlio 

 former view is the more easily understood of the two, though probably 

 only because in the physical and industrial world about us we see many 

 examples of construction of wholes, such as buildings, out of units, such 

 as bricks, whose properties are predetermined and do not change, just 

 as bricks do not change when they are set in a wall. We are not accus- 



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