376 Autoreferate.

The rate of regeneration increased directly with the level of the injury, as the level of injm-y moved eephalad from the tip of the tail. The removal of 20 mm. of tail constituted a fatal injury to tadpoles whose tail length was 26 ram.

Other factors being the same, the amount regenerated varied in the same direction as the temperature, between 66 ~ and 84 ~ F.

There is a temperature limit below which tadpoles will live but do not regenerate the tail. This limit was between 57 ~ and 66 ~ F.

There are four distinct periods of regeneration: (1) the first of slow regen- eration during the first few days after the operation; (2) the second of rapid regeneration; (3) a third in which the regeneration is much slower than the second; (4) a last period in which the processes of regeneration gradually cease.

The time elapsing between the operation and the cessation of regeneration, that is, the period of regeneration, varies with the level of injury. The higher the injury the longer this period; though the period is relatively longer for the series operated upon at the lower levels.

0REN E. FROZEE, T h e Effect of E lec t r i ca l St imulat ion upon the Rate

of Regene ra t i on in Rana pipiens and Amblystoma fiffersonianum.

An acceleration of the rate of regeneration was secured, first, when a small current density and a low voltage were used; and, second, when a small current density and a comparatively high voltage were used provided that the stimulation was of short duration. A decrease in rate was secured when any considerable degree of current density, or a high voltage applied for more than a very short time were used.

CHARLES ZELENY, T h e Effect of Success ive Remova l upon the Ra te

of Regenera t ion .

The effect of successive regenerations was studied with special reference to the determination of the pure effect of successive removal ofter elimination of the accessory factors. In the best controlled series it was found that successive removal does not cause a decrease in rate of replacement but either no change or an increase in rate.

In one group of experiments the effect of successive removal as a whole was studied. In the larval salamander, Amblystoma jeffersonianum, the exper- iment was well controlled and showed in each set an advantage in favor of the later regenerations. In Cassiopea the number of individuals was insufficient to make the result absolutely certain, but each of the later regenerations was distinctly in advance of the first. In the crayfish, Cambarus barrel,i, and the gulf-weed crab, Portunus sayi, there was no change with successive removals. Both animals furnish a special difficulty in the shape of the molting habit, but this was controlled as perfectly as possible. In the case of the gulf-weed shrimp, Pal~mon tenuicornis an important factor, the age factor, was not elim- inated, and while in single individuals there was a decrease in rate of regener- ation with successive injury, this may have been due to normal age decrease. The number of individuals also was insufficient.

The local effect was studied in the chelze of the Wood's Hall shrimp, Palcemonetes vulgaris, and in the margin of the disk in Cassiopea xamachana.

Autoreferate. 377

The chehe of Palremonetes showed a slight advantage of the first regeneration over the second, but since it amounted to only 1.4 per cent of the length re- generated it may not be significant. The margin of the disk in Cassiopea showed an advantage of the second regeneration over the first, but not as pronounced a one as in the cases in which the whole constitutional effect was in question.

Two suggestions are offered as regards the significance of the tendency to an increase in rate of regeneration with successive removal. In so far as it is a local phenomenon it may be explained by the supposed relative abundance of undifferentiated cells in the neighborhood of the cut surface in an organ whose regeneration is in progress or just completed. An unusual abundance of each cells has not been definitely demonstrated but is probable. It does not however explain the cases in which the third and fourth regenerations are more rapid than the second.

The point of most general interest in the results concerns their bearing on the question of the source of regeneration and growth potential. The ex- periments prove that the a v a i l a b l e supply of the necessary materials for regeneration if it changes at all with successive removal increases rather than decreases. Either there must be a multiplication of such material ofter each removal or the remainder is put into more available form.

CHARLES ZELENY, The Rela t ion b e t w e e n D e g r e e of I n j u r y and Ra te

of Regenera t ion . Addi t iona l Obse rva t ions and Genera l Discussion.

The additional experiments include the tail and legs of larvae of the salamander, Amblystoma opacum, the tail of tadpoles of the frog, Rana clamitans, and the chelze of the shrimp~ Pal~monetes vulgaris, of two species of crabs and of two species of crayfish. Taken together with previous ones they give the following general statement a high degree of probability.

The rate of regeneration of an organ increases with increase in additional injury to the individual up to an optimum degree beyond which further injury leads to a decrease in rate. The position of the optimum is different in the different organs of an individual and in corresponding organs of different species. It is also changend by a change in the level of removal in the organ under observation.

CHARLES ZELENY~ Some Expe r imen t s on the Effect of Age upon the

Rate of Regenera t ion .

Experiments were performed on Cassiopea xamacha~a, Ophioglyp]~a lacertosa, Amblystoma jeffersonianum, Portunus sayi, Cambarus propinquus, Cambarus bar- to~i, Palremon tenuicornis and Palczmonetes vulgaris to determine the effect of age upon the rate of regeneration. The data obtained in most of the cases confirm the general view that the replacement of a removed organ is completed sooner in younger than in older animals. Some exceptions to the rule were however found. In a great majority of the cases also the actual r a t e o f p r o l i f e r a t i o n o f t h e new t i s s u e s in regenerating parts of growing animals increases with increase in age. This increase in a few cases is sufficient to bring about a completion of the part as soon in older as in younger animals. In the majority of the species however it is not sufficient.