DIFFERENTIATION 93 



These organisms are called unicellular, because they 

 consist of one such unit, in contrast with the great 

 majority of plants and animals which are composed 

 of many such units, each with its own cytoplasm and 

 nucleus — which are in fact multicellular. 



The unicellular organism may show a considerable 

 differentiation of parts within the cell. The primary 

 differentiation into nucleus and cytoplasm is found in 

 all (except, perhaps, the bacteria, see Chapter IX), 

 but in Protococcus we find the differentiation, charac- 

 teristic of all green plant cells, of the cytoplasm into 

 the chloroplast and colourless cytoplasm. Many of 

 the higher Protozoa have quite complicated cells. For 

 instance the cell may have a firm cortical cytoplasm 

 (gel structure) which maintains the shape of the body, 

 a definite opening {mouth and gullet) through this leading 

 into the central more motile cytoplasm (sol), as well 

 as numerous exceedingly fine threads of cytoplasm 

 {cilia) which project from the surface of the body, and 

 by their rapid beating pull the animal quickly through 

 the water. Thus we have different parts of the cell, 

 differentiated and performing different functions for the 

 whole, e.g. the mouth and gullet for feeding, the 

 cilia for motion, etc. Other kinds of Protozoa have 

 skeletons of non-living substance, enclosing the 

 protoplasm which projects through openings in the 

 skeleton. 



Though this differentiation is often carried much 

 further in a unicellular organism hke a Protozoon than 

 in the individual cells of a multicellular organism, the 

 differentiation between different cells which is possible 

 in a multicellular organism is much greater, because 

 there is no limit to the size of the organism. In the 

 simpler multicellular plants (e.g. Green Algae) there is 



