THE ORGANISM AND THE CELL 19 



parts even after a portion of the undivided egg is removed. This indi- 

 cates that subdivision into cells and differentiation are two processes that 

 may be variously correlated in time, differentiation beginning relativelj'' 

 late in the first instance and relatively early in the second. In Amphibia 

 it has been shown that, if a group of cells from one region of a sufficiently 

 >oung lar\'a be transplanted to another region, the structure into which it 

 develops will be appropriate to the new position rather than the original 

 one. 



It appears, then, that when differentiation of regions takes place in 

 growing protoplasm these regions need not be delimited by cell l)()undai-ies. 

 Cellular structure is accessory, and cell division is an incident of growth 

 rather than a cause of differentiation. The kinds of subsidiary units, 

 ])resent — cells, hyphae, nuclei, plastids — do, of course, share in deter- 

 mining the kinds of specialization that can occur, but which of these do 

 take place in the various regions is determined by their positions in the 

 whole system. The region behaves as it does, not l)ecause it is cells, but 

 because it is protoplasm with a certain physicochemical constitution 

 responding to conditions that are in some degree peculiar to that portion 

 of the growing mass. This is strikingly illustrated by the coenocytic alga 

 Bryopsis, which develops a regular and characteristic body form while the 

 streaming cytoplasm carries the nuclei about from region to region within 

 it. In a word, ontogenetic differentiation is to be regarded as an act of 

 the developing system as a whole, whatever its protoplasmic growth type. 



The controlling influence of the whole upon the activities of its parts in 

 multicellular tissues has been emphasized anew in recent studies of cell 

 behavior during the development of plant organs. In the transforma- 

 tion of the small ovary into the large fruit of the squash plant, it is 

 found that growth in early stages is chiefl}^ by cell multiplication, whereas 

 in later stages it is entirely by cell enlargement. Meanwhile the fruit 

 increases in size and in dry weight at constant rates, and its parts differ- 

 entiate and change in relative proportions; in other words, growth and 

 differentiation of the whole proceed uniformly whether the constituent 

 cells are multiplying rapidly, slowly, or not at all and whether they are 

 becoming progressively smaller (by divisions) or larger. Moreover, the 

 rate and the time of cessation of cell division in fruits of this kind are the 

 same regardless of cell size. In long gourds the nuclear division figures 

 when first formed lie at various angles with respect to the long axis of the 

 fruit, but at later stages they turn more nearly parallel to it as though in 

 response to some polarizing force acting along the axis; hence the new 

 partitition walls develop at right angles to this axis and the number of 

 cells along it is increased. 



Similar studies of growing grass roots likewise indicate a dependence of 

 the cell upon forces acting in the organ as a whole. In regions where the 



