tails outside the line of symmetry of the middle piece, gradually assume a normal position (Figs. 31, 32). The writer is inclined to believe that this is due to mechanical conditions. The head _a e c_ of such an oblique piece is asymmetrical, the one side _a e_ being less stretched than the other _e c_. The higher tension of the piece _e c_ will have the effect of bringing _e_ nearer _c_, since we know that acid formation and hence energy production increases in proportion to surface, _i. e._, it must be the greater the more it is stretched. The reverse is true for the tail _d f b_, and the effect here will be that _f_ will be pulled nearer _d_. In this way purely mechanical conditions are responsible for the fact that the soft tissues of the animal are gradually restored to their true orientation. [159] Morgan, T. H., _Regeneration_, New York, 1901. [160] Bardeen, C. R., _Am. Jour. Physiol._, 1901, v., 1; _Arch. f. Entwcklngsmech._, 1903, xvi., 1. [Illustration: FIG. 28] [Illustration: FIG. 29] [Illustration: FIG. 30] [Illustration: FIG. 31] [Illustration: FIG. 32] 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.