[159] Morgan, T. H., _Regeneration_, New York, 1901.
[160] Bardeen, C. R., _Am. Jour. Physiol._, 1901, v., 1; _Arch. f. Entwcklngsmech._, 1903, xvi., 1.
As a final possible example of the influence of internal secretion or substances contained in the blood may be mentioned the following curious observation of Przibram.[161] In a crustacean, _Alpheus_, the two chelæ (pincers) are not equal in size and form, one being very much larger than the other. Przibram found that when he cut off the larger pincer in such crustaceans the remaining pincer assumes in the next moulting the size and shape of the removed large pincer; while in place of the removed pincer one of the small type is produced. Hence a reversal of the two pincers is thus brought about. If later on the large pincer is again cut off the process is repeated and the original dissymmetry is restored. Przibram was able to show that the nervous system has no connection with this phenomenon.
[161] Przibram, H., _Arch. f. Entwcklngsmech._, 1901, xi., 329.
The elements which have entered into the discussion thus far are, first, the flow of substances in preformed channels; second, the existence of general or specific substances required for the growing or regenerating organ. A third element is to be added; namely the "suction" effect upon these substances of a developing organ. Thus we see that if one or a few of the notches in a leaf of _Bryophyllum_ grow out the other notches of the leaf are inhibited from growing. There is enough material present in the leaf for all the notches to grow into shoots as is proved by the fact that all will grow out if they are isolated from each other. This was explained on the assumption that the notches of a whole which happen to develop first, create a flow of these substances from the rest of the leaf to themselves and thus prevent any getting to the other notches. We stated that this is supported by the fact that the few notches growing out in an undivided leaf grow more rapidly than the many shoots growing from each notch of a divided leaf. But why should a growing shoot or a growing point in general produce such a suction? I think this may be possible on the assumption that the consumption of these substances by the growing organs causes a low osmotic pressure of these substances in the growing region and this fall of osmotic potential will act as a cause for the further flow. This brings about the apparent "suction" effect of the growing elements upon the flow of substances.