INTRODUCTION 19 



economy of the whole and each cell is differentiated for the purpose 

 of its specialized function. Tissue cells, therefore, are physiologic- 

 ally unbalanced cells since they are preeminently specialized for 

 secretion, or contraction, or irritability, etc. Division of labor in 

 a physiological sense here reaches its highest expression. 



In the lower Metazoa the organ systems are less highly special- 

 ized; fewer organs are present to perform the same fundamental 

 vital activities and the tissue cells have relatively more kinds of 

 work to do for the organism as a whole. Thus the supporting and 

 covering cells of a coelenterate combine the functions of respiration, 

 irritability, muscular contraction, excretion and circulation with 

 the primary functions of an epithelium. Each of them is more 

 nearly balanced physiologically than a single cell of the higher 

 types, but it still needs the activities of other cells, and the organism 

 is again the sum-total of all its cellular parts. 



In the protozoon, finally, we find a cell which is physiologically 

 balanced; it is still a cell and at the same time a complete organism 

 performing all of the fundamental vital activities within the con- 

 fines of that single cell. Whitman, in his essay on " The Inadequacy 

 of the Cell Theory" (1S93) clearly expressed the inconsistencies in 

 the common use of the designation "cell" for this variety of struc- 

 tures, and later writers, notably Gurwitsch (1905) and Dobell 

 (1911) have followed in a similar vein. 



As organisms the Protozoa are more significant than as cells. 

 In the same way that organisms of the metazoan grade are more and 

 more highly specialized as we ascend the scale of animal forms, so 

 in the Protozoa we find intracellular specializations which lead to 

 structural complexities difficult to harmonize with the ordinary 

 conceptions of a cell. In perhaps the majority of the Protozoa the 

 fundamental vital activities are performed, as in the simpler i\.moebae 

 or simple flagellates, by the protoplasm as a whole and without other 

 visible specializations than nucleus and cell body. In other forms, 

 however, intracellular differentiations lead to intracellular division 

 of labor which in some types becomes as complicated as are many 

 of the organisms belonging to the Metazoa. Thus Diplodinium 

 ecaudatum, one of the Infusoria, according to Sharp (1914) has 

 intracellular differentiations of extraordinary complexity (Fig. 2). 

 Bars of denser chitinous substance form an internal skeleton; 

 special retractile fibers draw in a protrusible proboscis; similar 

 fibers closing a dorsal and a \'entral operculum; other fibrils, func- 

 tioning as do nerves of Metazoa, form a complicated coordinating 

 system; cell mouth, cell anus, and a fixed contractile vesicle or 

 excreting organ, are also present. All of these are differentiated 

 parts of one cell for the performance of specific functions, and all 

 perform their functions for the good of the one-celled organism which 

 measures less than ^io^ i^ch in length. Analogous, if not so com- 



