PRELIMINARY DESCRIPTION OF THE CELL 31 



all of the functions delegated, as it were, to different cells (contractility, 

 motility, mechanical support, the reception and conduction of stimuli, 

 secretion, and excretion), as well as those general functions common to 

 all cells (nutrition and reproduction by division), may in the protozoa 

 and protophyta be carried on within the limits of a single cell. Such a 

 cell as, for example, the body of a Paramcecium (Fig. 7), exhibits a cor- 

 responding regional differentiation in structure, certain functions being 

 localized in definitely developed organs. Differentiation is therefore 

 something which, fundamentally, does not require multicellular struc- 

 ture for its expression; in fact the most important single step ever taken 

 in differentiation was that which set apart nucleus and cytoplasm, giving 

 the type of organic unit common to all subsequently evolved organisms. 

 It is further evident, however, that the evolution of the higher organisms 

 has unquestionably been very largely conditioned by the multicellular 

 state, and has involved a progressive division of labor in a very real sense. 

 The many functions of a single cell have become distributed among a 

 number of cells in such a way that there has been produced a harmonious 

 whole which is efficient, adaptable, and progressive to a degree not other- 

 wise attainable. 



Bibliography 2 



See Bibliography I A for general works on the cell and for reviews of early cell 

 literature. For the latter see especially the works of Boveri, Flemming, Koernicke, 

 Mark, Meves, Riickert, Waldeyer, Whitman, and Zimmermann. Other works 

 referred to in Chapter II: 

 HANSTEIN, J. 1880. Das Protoplasma als Trager der pflanzlichen und thierischen 



Lebensverrichtungen. Heidelberg. 



HARPER, R. A. 1919. The structure of protoplasm. Am. Jour. Bot. 6 : 273-300. 

 MOORE, B. 1912. The Origin and Nature of Life. N. Y. and London. 



