THE AXIAL GRADIENTS IN HYDROZOA. III. EXPERIMENTS ON THE GRADIENT OF TUBULARIA

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Abstract
1. This experimental work was undertaken as a reply to a paper published by Banus ('18). 2. The existence of a metabolic gradient in the stem of Tubularia is demonstrated in this paper in four different ways. (a) Differential susceptibility of apical and basal regions of the stem to ether and cyanide. Apical regions are more susceptible. (b) Differential capacity of apical and basal regions to reduce potassium permanganate. The apical end of the organism has the greatest reducing power. (c) Difference iń electrical potential along the stem. Apical regions are electronegative (galvanometrically) to basal levels within the limits of the individual. (At a certain distance from the original hydranth of Tubularia a new individual is arising and the apical end of this is likewise electronegative to regions anterior to its level.) Since in general electronegativity is associated in protoplasm with increased oxidative metabolism, this difference in electrical potential along the stem of Tubularia is evidence that distal levels have a higher metabolic rate than proximal levels. (d) Difference in the rate of regeneration of apical and basal pieces. Work upon this point constitutes the bulk of the paper and is summarized under the subsequent heads. 3. Apical halves of the stem of Tubularia regenerate oral hydranths markedly faster than basal halves. In cutting such pieces it is essential to discard the original hydranth and the first millimeter or two of the stem; the remaining stem is then cut into two equal halves. The difference between such halves has been demonstrated by: (a) Mass experiments in which all of the apical halves have been placed in one dish, the basal in another. In such cases, the number of apical pieces which have regenerated oral hydranths is nearly always in excess, rarely equal to, and never less than the number of basal pieces which have regenerated. (b) Individual experiments, in which the number of hours required for the emergence of the oral hydranth on each piece was recorded. The apical pieces regenerated oral hydranths first in 91 per cent. of the cases (122 pairs of pieces observed). 4. Apical pieces regenerate on the whole more rapidly than basal pieces, even when the latter are twice as long as the apical pieces. Such apical pieces must not however be less than 5 mm. in length. In pieces over 10 mm. in length, length has very little effect upon the time of regeneration; in pieces less than 10 mm. in length, the longer pieces regenerate faster than shorter ones having their distal ends at the same anterior level but this effect of length is not sufficient to overcome the influence of level except in very short pieces (under 5 mm.). 5. The difference in rate of regeneration of apical and basal pieces which exists under normal condition can be somewhat reduced by using stems bearing branches and cutting the apical piece above the branch and the basal piece below the branch. Since the first branch marks the limit of the Tubularia individual, the apical pieces above such branches are really the basal regions of the principal Tubularia individual, and the basal pieces below the branch are near the apical end of the second individual. In consequence of these relations, the difference between the time of regeneration of such apical and basal pieces is less than is the case when pieces are cut from corresponding levels of stems without branches. 6. The difference in rate of regeneration of apical and basal pieces of the stem of Tubularia can be reversed by putting both sets of pieces for a certain time after cutting into appropriate concentrations of depressing agents like cyanide and ether. Under such circumstances the basal pieces regenerate in advance of the apical ones on the whole. This is due to the fact that depressing agents affect most strongly those regions having a higher rate of chemical activity. Since apical pieces have a higher metabolic rate than basal pieces, they are more affected by the same concentration of depressing agent and hence their regeneration is more retarded. In such cases the basal pieces regenerate the more rapidly. That this explanation is correct is further evidenced by the fact that the action of depressing agents is greatly influenced by temperature. At lowered temperatures a higher concentration of the agent must be employed to obtain the same effect produced at higher temperatures by lower concentrations. 7. These results are in accord with those obtained by a number of previous investigators and are directly opposed to the results presented by Banus. Banus claims that there is no difference on the average between the time of regeneration of oral hydranths on apical and basal pieces of the stem of Tubularia. Personal communication with Banus has elicited the fact that his usual method of cutting the apical pieces was erroneous. He cut them in such a way that the distal end of the apical piece was taken just below the base of the hydranth. In such cases, as shown in this paper, the distal ends of the apical pieces die away and the regeneration and time of emergence of the oral hydranth on these pieces is greatly delayed. It is believed that Banus's results are invalidated by such a method of procedure. 8. The results presented in this paper together with those of others quoted in the paper show that the rate of regeneration of pieces of Tubularia depends, when other factors are equal, upon the level which those pieces occupied in the intact stem; it is more rapid the nearer the pieces lie to the original distal end of the stem. A metabolic gradient exists in the stem of Tubularie which is the primary cause of these regional differences in rata of regeneration.

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