CHAPTER IV 

 ORGANISMS OF TISSUES 



VERY little observation is needed to show that a Paramecium 

 is a complicated mechanism, despite the fact that it is only a 

 single cell. The various parts of the cell are differentiated for 

 the performance of different functions, whereas in Metazoa the 

 same functions are performed by aggregates of cells and tissues. 

 Such cells and tissues, like the parts of the protozoan cell, are 

 specialized for different functions. Fundamentally alike in 

 their physiological activities, there is, nevertheless, a vast dif- 

 ference between organisms of one cell and organisms of many 

 cells. With the aggregate of cells there is a great possibility of 

 differentiation which is absent in unicellular forms, and with this 

 differentiation the possibility of structural complications is 

 vastly increased. 



Could we collect and hold together all the progeny of a fertil- 

 ized Paramecium cell into a harmoniously working whole, the 

 result would be an organism composed of tissues, or in this case, 

 of a single tissue, for all the cells would be fundamentally alike 

 and performing the same functions. While this condition does 

 not exist for Paramecium, there are protozoa in which the 

 progeny after division remain connected, forming aggregates 

 of many cells. These protozoan aggregates, colonies, are des- 

 ignated according to their method of formation, as gregaloid, 

 sphaeroid, arboroid and catenoid. In all of these, except the 

 first, the colony results from the incomplete division of the 

 cells and their products. Gregaloid colonies differ in that 

 they are formed by the coalescence of adult cells into a loose 

 group, as in the case of Microgromia socialis. A catenoid colony 

 is formed by the attachment, end to end or side by side, of sister 

 cells resulting from division. An arboroid colony is formed by 

 the daughter cells of one organism remaining attached to the 



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