On Relative Growth in Asellus aqimticus.–II.

SUMMARY.1  The growth of the thoracic appendages of Asellus relative to body‐length shows simple allometry, negative in anterior, and positive in posterior limbs. In both sexes there are two phases of growth, the first corresponding to a short prefunctional period. In the female there is, in addition, a change to a lower r.g.r. about the time of puberty. Sex‐differences are throughout emphasized by the larger maximum body‐size in the male. 2  There is a gradient along the body in g.r. of the series of thoracic limbs relative to body‐length. In the female the gradient is simple with a growth‐centre in thoracic 8 and a progressive decline forwards to thoracic 2. In the‐male the specialized thoracic 5 limb breaks up the gradient into an anterior and a posterior “series growth‐centre”. There is a “heterosexual vestige” of this in the female. The posterior s.g.c. is in thoracic 6 in early stages and later moves back to thoracic 8. The anterior s.g.c. is probably in thoracic 4, moving later to thoracic 3, and is therefore independent of the anterior centre for body‐width (Paper I. of this series, 1937). 3  Within each thoracic limb there is simple allometry between the individual segments and total limb‐length. There is little indication of a change in r.g.r. at the end of the prefunctional period. Again, sex‐differences (in a typical limb) are limited to growth of total limb‐length relative to body‐length. In these and other respects growth within the limb differs from growth of total limb‐length/body‐length. Differences between the various limbs of the thoracic series, however, do involve differences in segmental proportions at any particular total limb‐length. 4  The growth‐gradient along the limb involves two “limb growth‐centres,” a proximal and a distal. The latter is the more intense, especially in posterior limbs. The former is in the basipreischium, the latter in the carpus in anterior limbs, but in the propus in posterior limbs. The gradient has essentially the same form in the specialized limbs, thoracic 2, 5, but with quantitative differences and with sex‐differences. The distal l.g.c. possibly moves proximally during growth. 5  The “limb segment‐length profile” and the “series segment‐length profile” provide convenient means of studying the limb and series growth ‐gradients. The limb segment‐length profile bears a resemblance to the actual profile of the limb, perhaps not entirely fortuitous. There are “typical” and “peculiar” segments in the limb as well as typical and specialized limbs in the series. 6  These two profiles, at right angles to each other, may be combined in * segment‐length contour‐map, which suggests close interrelationship in growth between all limbs, typical and specialized, and their constituent segments. The graded effects of stimulation and inhibition by centres of growth and of inhibition are apparently not limited to any one limb but affect segments of neighbouring limbs as though the whole series of limbs formed a single structure with complete continuity. The c.n.s., a structure with complete continuity, may conceivably provide the basis for this. 7  In regeneration a thoracic limb does not “climb its ontogenetic tree”; the proximal segments are at first longer, the distal shorter, than in a normal limb of the same total length. Approximation to normal proportions is achieved on completion of regeneration. This suggests a relatively slow diffusion distally of an essential regeneration‐factor. 8  The rate of regeneration is roughly proportional to the size of the limb removed. It is relatively more rapid in the male. Regenerative power neither decreases (? apart from the second regeneration) nor increases with repeated regeneration, under constant conditions of amputation, in an individual which has virtually reached maximum body‐size, but the rate of regeneration shows minor variations which appear quite fortuitous and without correlation in different pairs of regenerates. 9  There is no positive evidence that a regenerating limb affects the growth of any other limb except its partner, whether regenerating or not. When both limbs of a pair are regenerating the right is longer than the left more often than the reverse, and this may be statistically significant. Regeneration of the distal segments, only, of a limb causes temporary hypertrophy of segments immediately proximal to them. 10  Antenna 1 shows positive allometry relative to body‐length, antenna 2 approximate isometry so that its great length is probably due to the initial size of the rudiment in the embryo. The curves of r.g. suggest that growth may be by segment‐number only in antenna 1, but in antenna 2 by segment‐length also. The rami of the uropods grow with positive allometry relative to body‐length, the value of a being approximately the same as that for thoracic 8 limb. 11  Relative growth between various internal organs of Asellus in general, shows simple allometry, which usually falls into two phases, the transition being around puberty. In some organs there are earlier changes in growth‐rate. The data, combined with those on thoracic limbs and those of Paper I., point to three important periods when g.r. may change; the natal, the juvenile, and the pubertal periods. 12  The cross‐sectional area of the hind‐gut (“rectum”) grows isometrically with that of the body, whereas that of the midgut diverticula (“liver”) grows more slowly (α=0–8). Possible functional differences between liver and rectum are considered. Zenker's cells have a higher r.g.r. than any other internal organ studied. 13  After embryonic stage 4 growth of the internal organs, liver and rectum, is entirely by cell‐size. There was not sufficient evidence to show whether or no this transition from histo‐ to auxano‐differentiation corresponded to a radical change in growth, as is possible in the thoracic limbs, where it also marks the onset of the functional period. 14  Phases of growth of “metamorphic” rapidity occur in regeneration of thoracic limbs, and probably in normal growth of some internal organs, where they occur at the same organ size in all individuals. 15  The frequent form of the curve of r.g. as a curve rhythmically undulating about a mean straight line of simple allometry is considered in relation to the sigmoid curve of absolute growth, and an adaptational cause postulated for both. Many of the features of r.g. in Asellus may possibly be adaptive. There is evidence for a specific upper limit to growth in AseUus. The frequency of an a‐value of 1–2 (or 0–8) is noteworthy. 16  Brooks's Law of Arthropod‐growth applies in Asellus only where growth is maximal. 17  There is possibly a tendency for ecdysis to be synchronous among the individuals in a small vessel. 18  During early regeneration the large spine‐setae of the limbs develop partially invaginated within the limb; on this criterion the terminal claw of the dactyl is a typical spine and not a true segment of the limb.