CELLS AND NUCLEI 3 



include those with the most complex development within the cell. 

 This range of cellular evolution is matched by a range of form of the 

 nucleus and of its methods of division greater than that found in all 

 the higher animals and plants (v. Ch. II), as is indeed appropriate 

 in what must necessarily be the oldest group of organisms. In 

 many of the lower organisms (Myxomycetes, Phycomycetes, 

 some Chlorophyceae) division of the nucleus is not accom- 

 panied by division of the cell, that is of the whole organism, 

 which in consequence comes to contain many nuclei ; it is 

 multinucleate although we may still take it to be unicellular. Else- 

 where, division of the cell into two compartments takes place at the 

 same time as division of the nucleus. The daughter cells may be 

 separated by non-living secretions of the cell with a connective or 

 cementing function, or by differentiated parts of their own cytoplasm. 

 The organism developing in this way is said to be midticellidar . It 

 reproduces by the separation of either single cells or groups of cells 

 from the main body. Both unicellular and multicellular organisms 

 may pass through a multinucleate stage of development, e.g., at 

 spore formation in many Protista, in the germ tubes of the Basidio- 

 mycetes and in the pollen tubes of flowering plants. 



Apart from these special conditions the term "cell " is a convenient 

 designation for a separate body of protoplasm containing a single 

 nucleus. 



(iii) Differentiation. Certain cells may reproduce themselves 

 by mitosis without change indefinitely under constant conditions. 

 This is true of many Protista, as well as of many young cells of the 

 higher animals in tissue culture. Change of conditions leads to a 

 change in appearance or behaviour affecting the whole Ufe of the 

 organism, such as encystation in the Protista. But even under con- 

 stant conditions the products of cell division are usually dissimilar 

 from the parent cell. In the unicellular Protista in which a series 

 of different forms occur in regular or irregular sequence, making 

 up the " life cycle," the daughter cells are usually like one another 

 but unlike the parent cell. This may be spoken of as differentiation- 

 in-time. In multicellular organisms one of the daughter cells is 

 usually unlike either the parent or its sister ; in this way the 

 organism comes to consist of many cells of different forms and 



1—2 



