2 74 RE GENERA TION 



cut-surface of the long half and the more posterior cut-surface of the 

 shorter half. At least this happens if the piece is not split too far 

 posteriorly, i.e. through the region of the pharynx. If this is done, 

 a new head may develop from the posterior cut-surface. In another 

 way the development of the more posterior head can be brought 

 about. If the shorter side-piece is kept from fusing with the longer 

 side-piece in the middle line, it will invariably produce a new head 

 (Fig. 31, C). The lack of development of the posterior head, when 

 the two cross-cut surfaces are united by a connecting part of new 

 material, can, it seems to me, be best explained by the influence of 

 the developing anterior head, or of the new side on the posterior new 

 tissue, and this influence can, I think, be better appreciated if we 

 suppose some sort of tension to be the influence at work. 



Another example may be cited that shows even more clearly that 

 the internal factor regulating the growth in the new part is probably 

 some sort of tension. I refer to the development of the tail of 

 fundulus from an oblique cut, or of the bilobed tail of stenopus from 

 a cross cut. The assumption of the typical form that leads to the 

 holding in check of the growth in certain regions, as compared with 

 others, can be best understood, I think, as due to some sort of ten- 

 sion established in the different parts, that regulates the growth in 

 those regions. 



It is evident that whatever factor will serve to explain the preceding 

 cases must also be expected to apply to the development of the whole 

 embryo from parts of the egg or blastula, if the position that I have 

 taken is correct, namely, that these phenomena belong to the same 

 general group. Does the tension hypothesis make clearer the devel- 

 opment of a whole embryo from a part of an egg ? This means, can 

 we think of the readjustment that takes place as due to the establish- 

 ment of a characteristic equilibrium that expresses itself in the 

 tensions of the different regions ? There is, so far as I can see, no 

 difficulty in supposing that the organization is at bottom a system of 

 this kind ; indeed, it seems to me that from this point of view we can 

 get a better appreciation of the organization and of the series of 

 changes that take place in it during development. The example that 

 Driesch has chosen as a typical one of vitalistic action, namely, the 

 proportionate development of a part of the archenteron of the half- 

 embryo, seems to me to be likewise a case to which we can apply the 

 tension view. 



In these, as well as in all other cases, we must think of the ten- 

 sions as existing, not only in one direction, but in the three dimensions 

 of space, and of all combinations of these. The material in which 

 the tensions exist must be thought of as labile, so that a change in 

 one region involves a rearrangement in many cases of the entire sys- 



