A Statistical Study of Regeneration in Two Species of Crustacea
Authors: Agar W. E. 
Published: April 2021

ABSTRACT
The material for this study consists of over 1900 camera lucida tracings of regenerated antennae of the two Cladoceran genera, Simocephalus and Daphnia, in all of which measurements of the regenerated parts were made. In about a thousand more examination was confined to the number of setae regenerated. The standard operation consisted of the amputation of the dorsal branch of the right antenna (in some experiments, of both antennae) through the second segment (Daphnia) or third (Simocephalus), thus removing four setae in both genera, together with two and a half segments of the antennar axis in Daphnia, and one and a half in Simocephalus. The missing segments are never regenerated, but a variable number of setae are produced in place of the four removed by the operation. The setae have a fairly complicated structure. The degree of regeneration was chiefly measured by the number and length of setae regenerated. The effect of varying the segment of amputation, or level of amputation within a segment, is described. The vigour of regeneration shows no decline with age (at any rate, from late embryo to first adult instar). The regeneration of one antenna is not affected, either favourably or unfavourably, by the simultaneous regeneration of the other. No “improvement with practice” takes place. The regeneration of one antenna is either unaffected, or adversely affected, by the previous regeneration of the other. Within the wide limits tested, the state of nutrition of the animal has no influence on regeneration. The general internal condition of the animal, within the limits of variation dealt with, has very little influence on regeneration, as shown by the absence or low degree of correlation between the regeneration of right and left antennae of the same animal. The general complex of external conditions exerts a significant influence on regeneration. The main cause of variation in degree of regeneration is to be looked for in very numerous small factors localised in the regeneration blastem. The combined action of all these factors determines the regeneration potential. The value of this varies approximately according to the normal probability distribution, and expresses itself primarily in the number of setae produced. The left-hand end of the distribution of seta number resulting from the distribution of potential is cut off owing to the fact that the potential must rise beyond a certain threshold value before any setae can be produced. The distribution is condensed at the righthand end owing to a tendency for the formation of new setae to cease when four have been formed. Owing to the failure to regenerate the missing segments of the antennar axis, the formation of new setae after amputation near the base of the antenna only very imperfectly restores the original condition. When the amputation is performed through the apical segment, however, the regeneration of the setae removed by the operation practically restores the antenna to its original state. It is suggested that this is connected with the fact that after amputation near the base of the antenna, regeneration is highly variable and indeterminate, new setae being added to the regenerate throughout life; whereas after operation through the apical segment regeneration generally consists in the formation of the proper number of setae and then stops.