THE OUGANISM AND THE CELL 



17 



Mac. 



Mic- 



above have an important bearing on the ciuestion should lie evident in 

 what follows. 



Differentiation. — ^By differentiation is meant a progressive change from 

 a generalized and uniform condition to a more specialized and hetero- 

 geneous condition in the protoplasmic system. It results in the structural 

 alteration of regions in which certain functions come to be localized, so 

 that what is at first a mass in which all portions appear and act alike 

 becomes a system of unlike but correlated parts, each performing one or 

 more special functions. In differentiation "the level of organization is 

 raised." 



Differentiation can occur in any of the proto- 

 plasmic growth patterns reviewed above. Even 

 the structure of the typical protoplast is reason- 

 ably (though not necessarily) considered to be 

 the result of an ancient differentiation in primitive 

 living svibstance, the c^^toplasm, nucleus, and 

 membranes representing specialized portions act- 

 ing in harmony. The astonishing extent to which 

 differentiation can be carried in a unicellular 

 organism is illustrated by those protozoa which 

 have clearly specialized locomotor, digestive, 

 excretory, and neuromotor regions (Fig. 9). After 

 examining such organisms as this, one can no 

 longer accept the statement that "protozoa are 

 simple organisms." They are small and unicellu- 

 lar, but considering their size and mode of life 

 they are perhaps as well differentiated as we are. 

 No other single cells are quite so intricately 

 organized. 



A growing mass of protoplasm having any one 

 of the "patterns," although it retains the primi- 

 tive cUfterentiation into cytoplasm, nucleus, and membranes, may still be 

 regarded as generalized -vvith respect to other expressions of differentiation 

 that are yet to appear within it. A mass of embryonic tissue, for 

 example, has many nuclei, memliranes, and other components, but if this 

 same type of structure and the same general functions pervade the whole 

 mass uniformly, it is said to be imdifferentiated so far as the development 

 of further specialized organs is concerned. 



That differentiation resulting in the same general form of body or in 

 organs having the same function may occur in protoplasmic systems of 

 unlike pattern is shown by the following examples. The three green 

 algae Stigeoclonium, Cladophor-a, and Vaucheria all develop branching, 

 filamentous bodies, although the first has numerous uninucleate cells and 



Fig. 9. — A protozoan 

 (Diplodinium). M, 



mouth; iV, neuromotoi- 

 apparatus; Mac, macro- 

 nucleus; Mic, micro- 

 nucleus; V.V., contractile 

 vacuole; A, anal canal; 

 C, contractile region; S, 

 skeletal plates. {After 

 R. G. Sharp.) 



