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A Statistical Study of Regeneration in Two Species of Crustacea by W

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A Statistical Study of Regeneration in Two Species of Crustacea by W 349 A STATISTICAL STUDY OF REGENERATION IN TWO SPECIES OF CRUSTACEA BY W. E. AGAR, Professor of Zoology, University of Melbourne. (Received 22nd April, 1930.) (With Three Text-figures.) CONTENTS. PAGE I. Introduction 349 II. Methods of culture 350 III. Growth and sex 351 IV. The material available . .351 V. Structure of the antenna 352 VI. The operation . • . 353 VII. General nature of the regeneration 353 VIII. Measuring and recording the amount of regeneration .... 355 IX. The influence of certain factors on regeneration ..... 357 (a) The segment through which amputation is performed . -357 (6) The level of amputation within the segment .... 357 W Age 358 (d) The simultaneous regeneration of the other antenna . 358 (e) The previous regeneration of the other antenna . -359 (/) Food 360 (g) General internal conditions of the animal 360 (h) General external conditions 361 X. The nature of the intrinsic factors controlling regeneration as disclosed by statistical analysis 362 XI. Discussion of results .......... 365 XII. Summary 367 I. INTRODUCTION. THIS study of regeneration in the two Cladoceran species, Simocephalus gibbosus and Daphnia carinata, grew out of a Lamarckian experiment on the possibility of improving (or otherwise altering) the power of regeneration in these animals by making them regenerate the antenna in many successive generations. Up to the present, these experiments have been completely negative in this respect, though they have furnished much data concerning the process of regeneration. The power of regeneration in a line of Simocephalus in which the antenna has been amputated a^»egenerated for seventy-four successive generations, and in a line of Daphnia for eighty generations, shows no difference from the power of regeneration of the JEB'VIliv 23 35° W. E. AGAR controls which had no ancestral experience of regeneration. This experiment in progress and will be reported in detail later. In the course of the experiment it was found necessary to analyse the factors influencing regeneration, and besides the main experiment, many subsidiary experiments directed to the elucidation of special points have been carried out. Altogether, some thousand regenerated antennae have been drawn and measured in Simocephalus, and about 950 in Daphnia, while about a thousand more have been examined and the number of setae recorded without measurement of length. One striking fact about regenerated parts in all animals is the great variation exhibited by them compared with the corresponding parts produced by embryonic development. The large mass of material which forms the basis of this investigation enables us to study this variation more closely than has been possible in previous studies of regeneration. A further great advantage possessed by this material is its genetic uniformity. All the specimens described in this paper are descended by parthenogenesis from a single female of Simocephalus or Daphnia respectively. Many experiments by myself, by Banta and others have shown that, except for occasional mutations, genetic differences do not normally arise between members of a clone. Thus what- ever may be the causes of the great variation in degree of regeneration which is exhibited in this material, we can exclude genetic diversity as one of them. II. METHODS OF CULTURE. The standard method adopted throughout these experiments (and adhered to wherever the contrary is not stated) was to rear each animal separately in a cylin- drical glass phial containing 50 c.c. of culture medium. This amount was measured wherever similarity of conditions was essential (as in the "test experiments"), but judged by the eye when this was not relevant. The culture medium used throughout the greater part of the experiment was prepared as follows. A rich culture of green Protophyta (largely flagellates) was grown in a medium prepared by boiling 3 gm. of dried fowl dung in a litre of water. The filtered fluid obtained thus, exposed to sunlight in glass jars, provides a very rich culture of Protophyta, which lasts for several months. Every few days a stock culture medium was made by adding about 100 c.c. of the thick protophyte culture (pipetted from the bottom of the jar) to 2500 c.c. of pond water, previously strained through bolting silk. In some of the experiments, an infusion of horse dung, as recommended by Banta, was added to the culture, but it was found that the protophyte culture was sufficient by itself, and less liable to fluctuation in quality. In this culture medium the animals thrive exceedingly well, and produce abundant eggs. The phials were placed in a water bath maintained at 22-5° C. by a toluol- mercury gas regulator. Statistical study of regeneration in two species of Crustacea 351 III. GROWTH AND SEX. After casting its embryonic cuticle, which happens immediately after birth, the animal moults four times (Simocephalus) or five times (Daphnia) before becoming mature—that is to say, laying its first batch of eggs. A very small percentage of animals of both genera become mature after one moult fewer than the normal. The period between the casting of the embryonic cuticle and the first ecdysis is the first instar. In the fourth (Simocephalus) or fifth (Daphnta) instar the ovaries mature. Immediately after the next moult the first batch .of eggs is laid. This constitutes the first adult instar. During the development of these eggs in the brood pouch, the second batch of eggs is maturing in the ovary, and the birth of the brood is followed in half-an-hour or so by an ecdysis and the laying of the second batch of eggs (the second adult instar) and so on. At a temperature of 22-5° C, the shortest duration of the life cycle (egg to first egg) is about seven days. Long series of measurements show that no appreciable growth takes place during the instars, but only at ecdysis. This applies to the regenerating antennae as well as to normal growth. Out of the thousands of individuals reared in this way, no sexual female occurred in the Simocephalus lines, and only three in Daphnia. A very few male broods occurred in the Daphnia lines, and a fair number in Simocephalus. These were not used for these experiments, which concern parthenogenetic females only. In confirmation of the work of Smith and of Banta, I may mention that although no single sexual Simocephalus female has occurred among the thousands produced in the main experiment (in which each individual was isolated at birth), I have never failed to obtain large numbers of these females by crowding. When a population of a hundred or so is allowed to form in a 500 c.c. jar, sexual females are always conspicuously numerous. IV. THE MATERIAL AVAILABLE. The material dealt with in this study is' comprised as follows: (1) The "Test Experiments." At intervals comparatively large numbers of animals were tested for Lamarckian inheritance against control animals with no ancestral experience of regeneration. In these tests great care was taken to keep the conditions uniform for all the members of that particular test. The animals (with insignificant exceptions) were operated on 2-4 hours after birth, reared separately in phials containing 50 c.c. of culture medium drawn from the same, constantly stirred, stock jar, the phials all standing in a water bath maintained at 22-5° C. by a thermostat. Seven such test experiments have been carried out in the Simocephalus lines, co^kising altogether 537 individuals. In Daphnia there have been five tests, cornprising 530 antennae. 23-2 352 W. E. AGAR (2) 37° Simocephalus and 360 Daphnia reared under somewhat diA-| n conditions and only utilised for the effect of external conditions (Section IX (' (3) A large number of both genera used for special experiments on the effects of various conditions on regeneration. V. STRUCTURE OF THE ANTENNA. In both genera the second antenna consists of a basal segment and two branches, the dorsal branch having four segments and the ventral three (Fig. 1). Fig. 1. Second antennae of Daphnia (above) and Simocephalus. The dorsal branch is to the right in both figures. The arrows indicate the level at which amputation was performed. The lower figure is at a higher magnification than the upper one. The dorsal branch carries four setae, and the ventral one five, in the positions shown in the figure. Each seta is jointed, approximately in the middle, into two segments, and is thickly covered or feathered with very fine hairs, which are longer in Daphnia than in Simocephalus. In the latter genus, the most dorsally situated of the terminal setae differs from all the rest in structure. The hairs are replaced by a very fine short comb, and the tip of the seta is sharply recurved to form a minute hook. It is by means of this hook that the animal suspends itself in characteristic fashion from weeds, or minute roughnesses on the glass sides of an aquarium. In Daphnia this seta is similar to the others, and therefore this genus has not the power of suspending itself by its antenna. The antennae are cylinders, the walls composed of two layers, hypodermisTITid Statistical study of regeneration in two species of Crustacea 353 Inside the cylinder are bundles of muscles, bathed in blood. These muscles run^J the tips of both branches of the antenna, and give off a twig to each seta. The setae are so jointed on to the antenna as to permit of movement through a right angle only. They can lie flat against the antenna (the point of the seta directed distally) or stand out at right angles to it. Observation of muscular twitches in freshly amputated and still living antennae show that these changes of position of the setae are carried out by muscles inserted into them.
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