138 THE CAUSES OF SPECIFIC SHAPE 



the same point has been reached as when a particular reaction has been traced 

 to an external agency, for all stimuli, whether of internal or external origin, 

 act by modifying the vital activity, and it is immaterial whence a stimulus 

 is derived if its character and mode of application are the same. The sole 

 difference is that the interactions within the organism are much more 

 complicated and that the properties of each cell are subject to progressive 

 and periodic modification during its development. Moreover a cell in a 

 tissue does not float in a homogeneous medium, but has its various regions 

 exposed to dissimilar physiological, physical, chemical, and mechanical 

 surroundings. 



PART I 



THE DIFFERENTIATION OF CELLS AND OF ORGANS 

 SECTION 40. General. 



Development is determined and controlled firstly by the specific 

 properties of the organism, and secondly by the formal and directive 

 action of the external conditions. These influences may either be of 

 internal origin (correlative) and derived from other cells or organs of the 

 same plant, or they may be due to the external conditions. In unicellular 

 organisms the only correlative influences to be considered are those exerted 

 by the different organs of the cell upon each other. Here also the external 

 conditions, and those arising within the cell itself as growth progresses, 

 exercise a directive influence upon the progress of development. This is 

 also the case when all the segments preserve their embryonic character, as 

 in asomatophytes, whereas in somatophytes cells of similar origin subse- 

 quently undergo dissimilar differentiation. Even here the problem ultimately 

 returns to the individual cell, for the dissimilar differentiation of the cells in 

 tissues is determined by the varying properties of the segment-cells, and by 

 directive factors external to them. 



The more intimately the properties of the cell are known, the more 

 readily we shall be able to decide how far and by what conditions a 

 particular line of development is produced by the restriction of certain 

 potential powers of the cell and the utilization of others. We may then 

 also decide as to the respective parts played by changes in the properties 

 of the cells concerned, and by variations in the extracellular directive factors, 

 in producing dissimilar differentiation. A careful study of the reactions to 

 changes in the external conditions is, therefore, of vital importance as an aid 

 to the comprehension of the internal factors concerned in development We 

 presume that the internal factors are similar in character when the same 

 response is given in different cases to the same change in the external 

 conditions, but when different responses are given we can safely assume 

 that the internal dispositions diverge widely in the plants concerned. 



