CELLS AND TISSUES 9 



ing numbers of nuclei (e.g., Cladophora; Fig. 4). In many cases, however, 

 they are so intimately related that they are like one process, and the 

 result is a regularly uninucleate condition of the cells. Here growth may 

 appear to be a matter of cell multiplication, but a consideration of proto- 

 plasmic masses not showing such a correlation between nuclear division 

 and cytoplasmic septation suggests that the regularly uninucleate cellular 

 condition is the result of a special refinement in mode of growth and 

 differentiation. The great importance of this refinement in connection 

 with the evolution of organisms is indicated by its prevalence. In certain 

 organisms the number and arrangement of the cells are constant in a 

 given organ or even in the whole body.^ 



By morphallaxis is meant the development of a particular form other- 

 wise than through synthesis or enlargement directly. The specific form 

 of the body is due in part to the fact that growth at certain periods is not 

 equal in all directions, but in addition to this the rearrangement of mate- 

 rials already present may play a prominent role. A striking illustration 

 of morphallaxis is afforded by small pieces of Hydra which under proper 

 conditions will, without further growth, remold themselves into complete, 

 though dwarfed, individuals. 



As the young organism grows, it differentiates. Differentiation, in 

 the words of Conklin, is "transformation from a more general and 

 homogeneous to a more special and heterogeneous condition." It 

 involves the development of unlike functions and structures, and the 

 localization of these in different regions of the organism. Physiological 

 division of labor and morphological division of substance constitute one 

 inseparable process, and this process in its last analysis is the result of 

 physical and chemical changes in protoplasm caused by the combined 

 action of intrinsic and extrinsic factors. The next section will be devoted 

 to the subject of differentiation. 



One of the most remarkable features of a living organism is the 

 perfect correlation which is normally maintained between the activities 

 of its many diverse organs; without such correlation development could 

 never occur at all. Differentiation and integration are, in fact, two 

 aspects of one thing, namely, organization (Conklin). It is to be empha- 

 sized that correlation is not something which comes into being as the 

 differentiation of functionally distinct regions occurs: the organism 

 behaves as a consistent, correlated whole from the beginning of its devel- 

 opment onward. The correlation existing between the activities of the 

 parts of the cell with which development begins gradually becomes the 

 more extensive correlation of the highly differentiated mature organism. 

 Correlation is retained from the first, although new means for its main- 

 tenance are elaborated. What these means are will be pointed out below. 



^ See the review by Van Cleave (1932). For a review of the subject of cell form, 

 see Gray (1931). 



