Individuality in Organisms

The evidence presented here concerns primarily the major axis, because the facts are simpler and more complete with respect to this axis. Experimental isolation of pieces with reference to the minor axes is usually complicated by the presence of the major gradient, and the order along the major axis is often such that parts necessary for continued life are absent from various regions of the minor axes. For these reasons the experimental investigation of dominance and subordination in relation to the minor axes is variously complicated and limited in different cases. Nevertheless, the fundamental similarity of the different directions of order in the individual is indicated by various lines of evidence, and there are no grounds for hesitation in extending to the minor axes general conclusions reached concerning the major axes.

Reproduction can be induced experimentally in the plants and many of the lower animals by the isolation of pieces and in various other ways. These experimental reproductions, when properly controlled and analyzed, constitute invaluable material for study of the problem of the individual, for it is often possible to increase or decrease dominance and so to extend or decrease its range, to alter the conductivity of protoplasm, to determine the elimination of old and the establishment of new metabolic gradients, and in these and other ways to control the process of individuation to some extent, and to determine the results of such control

Most plants and many of the lower animals give rise to new individuals by division, budding, and other agamic processes, and the new indi\iduals thus formed often remain organically connected and give rise to a composite individual, such as a tree among plants or a hydroid colony among animals. In such reproductions definite and orderly space or distance relations are observable, which themselves suggest the existence of a limited range of dominance. The occurrence of division

when a certain size or length is attained, or the appearance of buds at a certain distance from the chief growing tip in plants, are cases in point. In many cases experimental control and alteration of these relations throw a flood of light upon the problem of their nature. It is with material and experiments of this sort that the present chapter is largely concerned. I have shown elsewhere that the process of progressive development and differentiation in the individual is accompanied by a decrease in the metabolic rate determined by the accumulation of relatively inactive constituents in the protoplasm. These changes,

Figs, 42, 43. — Tuhiilaria: Fig. 42, a single individual; Fig. 43, asexual reproduction from tip of stolon. senescence, may end in death if they go far enough. On the other hand, any change which brings about the removal of such previously accumulated material makes possible an stitute rejuvenescence. The facts indicate that all reproductive processes bring about rejuvenescence to some degree, and it is certain that the new individuals which arise by division or budding from other individuals or from experimentally isolated pieces are to some extent physiologically younger than the parent individual from which they arose/ Rejuvenescence in such cases results from the loss of the differentiation as a part in that portion concerned in the reproductive process, and with the new individuation a new process ot senescence begins.

Among the lower animals which have served as material for the study of regeneration or regulation two forms have been used to a large extent in my own experiments and must be briefly described here. The hydroid Tuhularia in its simple unbranched form as a single individual (Fig, 42) consists of hydranth, stem, and stolon, the hydranth forming the apical end of the stem and bearing two sets of tentacles, reproductive organs between them, and a mouth at its apical end. The stem grows vertically from the surface of attachment, and the stolon adheres to the surface, forming an organ of attachment, and elongates by growth at its tip„ Stem and stolon are covered by a horny cuticle, the perisarc. The apical end of the metabolic gradient of the major axis is the apical region of the hydranth, and from this region the rate decreases basally through tht* hydranth. In the stem the metabolic rate is lower than in the hydranth, and there is a slight decrease in rate in the basal direction, but at the growing tip of the stolon there is a short, slight gradient in the opposite direction.

The primary form of asexual reproduction in Tubularia is represented in Fig. 43. When the stem and stolon together attain a certain length, which varies with the metabolic condition of the animal but under favorable conditions may be five to eight centimeters, the stolon turns away from the substratum and gives rise to a hydranth; then a stem forms and elongates below this hydranth, and a new stolon arises from the base of this stem. This process of reproduction itself suggests that the tip of the stolon is subordinate to the original hydranth until it attains a certain distance from it and then is able to produce a new hydranth, and experiments show that this is true. If the original stem elongates still further new hydranths may arise along the stolon and at the base of the stem, as these regions become physiologically isolated.

In Corymorpha, a form related to Tuhularia, the hydranth is much larger, the stem naked except near the base and reaching a length of ten to twelve centimeters, and instead of a stolon the basal end is imbedded in sand and bears delicate root-like outgrowths as holdfasts (see Figs. 74, 78, pp. 143, 145). Planaria dorotocephala (Fig. 30, p. 82), a flatworm and one of a number of species much used in experiment, is a much more highly differentiated, bilaterally symmetrical form, with distinct head and "brain" and two ventraJ nerve cords, and with definite, though rather diffuse, alimentary and excretory organs. Sexual organs appear in this form only under certain conditions. This, as well as various other species of the group, undergoes fission after it attains a certain variable size, the separation usually occurring at about the level f in Fig. 44

The separated posterior portion becomes a new animal, while the anterior portion develops a new posterior end, and fission is sooner or later repeated. There is no morphological indication of a second individual or zooid in the posterior region of the body, but one or more such individuals are indicated by the metabolic gradient of the major axis and by various other physiological differences. The apical region of this gradient is the head of the animal, and from the head the metabolic rate decreases to the level where separation occurs in fission; there a sudden rise in rate occurs, and then again a downward gradient toward the posterior end. The region where the rate rises suddenly represents the apical end of the second individual and the downward gradient following is the gradient of the major axis of this zooid. In the shorter animals only one of these zooids is present, but as the length increases the basal body region may show two, three, or more of these distinct gradients. Represented graphically the metabolic gradient in such an animal is like the curve in Fig. 45; a is the head-region,

Fig. 44. — Planaria dorotoceplujla, outline, indicating several zooids in basal region; ff, usual level of fission. the long slope the body of the anterior chief zooid, which forms most of the body of the worm, h represents the apical end of the second zooid, c that of a third, etc. These zooids are the result of successive physiological isolations of the basal region as the animal grows in length. First a single zooid is formed at the basal end, but the range of dominance is short in this undeveloped individual, and as growth proceeds its basal region soon becomes physiologically isolated, and a second zooid arises, and so on. While the degree of physiological isolation is not

Fig. 45. — Graphic representation of major axial gradients in a Planar ia with several zooids: a, head of animal; b, c, apical regions of secondary zooids. sufficient to permit the development of the new individual to proceed very far, some degree of rejuvenescence in the part does occur and its metabolic rate rises slightly, and with each successive isolation there is a further increase in rate, so that in each successive zooid the gradient is at a level somewhat higher than that of the preceding.

The act of fission in this animal consists of an indc' pendent motor reaction of the posterior zooid or group When the worm is creeping quietly, the posterior zooid or the zooid group suddenly attaches itself to the surface on which the animal is creeping, wliile the whole anterior individual endeavors to advance and the body in front of the attached region becomes greatly stretched (Fig. 46) and finally ruptures. The occurrence of fission can often be controlled experimentally in a way that shows the variable range of dominance very clearly. If an animal is very slightly stimulated, e.g., by a slight jarring of the aquarium, the posterior zooid will often attach itself, and fission will occur, while with stronger stimulation the animal is able to control this region and it does not become attached but advances with the rest of the body! Evidently when the animal is only moderately active the posterior region is physiologically isolated, but when it is intensely active the range of dominance of the anterior individual extends to this posterior region and determines its subordination in behavior. Similarly, in very old animals which have been prevented from undergoing fission by keeping

them on a layer of vaseline or other surface to which they cannot attach themselves, the tissues are often so tough that rupture does not readily occur, and the anterior indi\adual struggles more and more violently to free itself from the hindrance which is preventing its advance. In these animals such struggles often terminate in the complete subordination of the posterior zooid: it is not torn loose from its attachment, but lets go its hold and no longer reacts independently. Later, when the anterior individual has become more quiet, the same procedure may occur again. Evidently as the activity of the anterior individual increases the range of dominance increases, and, if fission does not occur at once, the posterior zooid may finally be brought under control. Moreover, one of the simplest ways of inducing fission in this species is to cut off the head of the anterior individual. Such animals creep about even in the absence of the head, but under these conditions the posterior zooid is more completely physiologically isolated and separation soon occurs if the tissues are not too tough.' Experiments to be described below will show other ways in which the existence of dominance can be demonstrated and its range varied and controlled in these and other animals and in many plants.

The apical region of the organic individual is, to a large extent, independent and is capable of developing, ' For a more extended consideration of the process of fission and the various indications of the presence of the posterior zooids see Child, "Physiological Isolation of Parts and Fission in Planaria," Archiv fiir Entwickelungsmeclmnik, XXX, II. Teil, 1910; "Studies on the Dynamics of Morphogenesis, etc., Ill," Jour, of Ex per. Zool., XI, 191 1; "Studies, etc., VI." Archiv fiir Entwickelungsmechanik, XXXV, 191.^.

at least to an advanced stage, in the complete absence of other parts of the body. This independence is very evident in Tubularia and Planaria. Pieces one or two Fig. 47. — Reconstitution of single and biaxial apical structures from short pieces of stem of Tubularia, to illustrate independence of apical region. millimeters in length cut from the stem of Tubularia usually develop into hydranths with a very short stem or partial hydranths with more or less of the basal region absent (Fig. 47). The result depends on the condition

of the animal, the length of the piece, and the level of the stem from which it is taken. The shorter the piece from a given level of the stem the more completely is its development limited to apical parts, as Fig. 47 shows. The shortest pieces give rise to nothing but the apical ends of the hydranths, with mouths and the apical row of tentacles. In no case do such pieces produce basal parts of the hydranth without apical parts. Where anything is missing it is always the more basal region, either stem or more or less of the basal hydranth region. The results in such pieces constitute, I believe, a demonstration that the apical region of the individual arises first and other regions are determined later, as far as the length of the piece permits.

The development of hydranths or apical portions of hydranths may occur at one or both ends of such short pieces as indicated in Fig. 47. This difference arises according as the original metabohc gradient in the stem is more or less marked. In such short pieces of the stem the difference in metabolic rate at the two ends of the piece is but sHght in any case. If, however, the rate at the apical end of the piece is enough higher than that at the basal end, development at the apical end proceeds more rapidly than at the basal end, the apical end is dominant, and the piece produces a single hydranth or part. But if the gradient is very slight in the piece the two ends react at the same rate, and since the presence of the wound at each end brings about an increase in metabolic rate at each end, equal or nearly equal gradients in opposite directions arise and hydranths or apical parts arise at both ends with their axes opposed. Often, even in such cases, the original gradient appears

in the smaller size or more incomplete condition of the structure formed at the basal end of the piece. In Fig. 47 one case near the bottom of the figure is shown in which one end is a hydranth with both sets of tentacles, the other a partial hydranth w^ith only the apical set and the reproductive organs.' In Planaria the development of short pieces is essentially similar. Short pieces from various levels of the body may undergo complete transformation into single or double heads without the formation of other

Fig. 48. — Reconstitution of single and biaxial apical structures from short pieces of Planaria, to illustrate independence of apical region. parts of the body or with more or less of the anterior bodyregion (Fig. 48). When a single head arises, it is at the anterior end of the piece. The conditions determining the development of these heads are the same as those in Tuhularia. As in Tubularia also, the original gradient may appear to some extent in the more rapid and more complete development, larger size, and dominance in motor activity of the head at the original anterior end of the piece, as in the case at the right of Fig. 48.

^ For more extended consideration see Child, "Analysis of Form- Regulation in Tuhularia. V, Regulation in Short Pieces," Archiv fiir Entwickelungsmechanik, XXIV, 1907; "Die physiologische Isolation von Teilen des Organismus," Vortrage und Aufsiitze iihcr Eiitwickduugsmechanik, H, XI, 191 1, 101-19. Further references are given in these papers. These double apical regions and heads have been observed by many investigators in various animals and have commonly been called axial heteromorphoses, because the apical structure at the basal end of the piece was regarded as something which was out of place and abnormal. This, however, is not actually the case, for the development of these double or biaxial structures is, as I have shown, subject to exactly the same laws as the development of the usual single individual, only in these pieces the conditions are such that the original gradient is almost absent, and the increased activity at the basal end may establish a new gradient in the reverse direction, although some indication of the original gradient may remain in the smaller size or less complete development of the part at the basal end. In these short pieces, in fact, the original polarity is almost obliterated and the establishment of a new reversed polarity in relation to the basal cut end is possible. At each end the relation between the metabolic gradient and the development of an apical structure is exactly the same as in any other case of development. The apical region arises at the apical end of the gradient and the development of other parts follows as far as the gradient extends from each end, or in the case of single structures as far as the length of the piece permits. By means of the susceptibility method I have been able to demonstrate these relations between the metabolic gradients and the single or double development of such pieces.

The development of biaxial or multiple apical structures from pieces has been observed in various other animals, and, while their relations to the metabolic gradients have not been determined, their character and the conditions of their development indicate that whenever an apical structure arises it represents the a[)ical region of a metabolic gradient. In the plants also conditions are apparently similar. The apical region of a plant individual may arise independently of other parts, and if it becomes structurally connected with them later the connection develops progressively from the new apical region toward other parts and not in the opposite direction. The formati(jn of buds on the leaves of begonia and in wound callus, described above (pp. 83-86), are cases in point, and many other similar cases might be cited. The developmental gradients in such cases indicate that the new apical structure or part represents the apical region of a metabolic gradient.

These conclusions concerning the independence of the apical region and its relation to the metabolic gradient, which are based upon experimental demonstration for certain cases and highly convincing evidence for others, are in full agreement with the facts of embryonic development. There also, so far as experimental evidence has been obtained, the apical region of the individual is the apical region of a metabolic gradient, and precedence of the apical region in development and the developmental gradients in the direction of the major iixis indicate that this relation is general. I belie\e we are justified in concluding that in this respect development of the organic individual is always and everywhere the same. Further evidence in support of this conclusion will be presented in the following pages.

The existence of a relation of dominance and subordination along the major axis is shown by the fact that, while the apical region is independent, other levels of the body can develop only in organic connection with more apical levels or with the apical region itself. In Tubularia and related forms stolons arise only in relation to stems or hydranths and stems, stem regions appear only in relation to higher levels in the gradient, etc. Stolons may grow out from stems in the absence of hydranths, and under certain conditions when the metabolic gradient is slight stolons may even arise at both ends of a piece of stem, but no case has ever been observed of the development of a stolon independently of other more apical levels.

This relation is also very evident in Planaria.^ The reconstitutional development of pieces from the middle and posterior regions of the anterior individual, such as a and b in Fig. 49, ranges according to the physiological condition of the animal and with experimental conditions from a normal complete animal like Fig. 50 through various intermediate forms, of which the anophthalmic is shown in Fig. 51, to headless forms, like Figs. 52 and 53. The headless forms produce all parts of the body basal to the level which they represent, but never give rise to any part characteristic of more apical levels. The reason why they do not produce heads will appear in the following section. Thus, headless

' Child, "Studies on the Dynamics of Morphogenesis, etc., I," Jour, of Ex per. ZooL, X, 1911; 11, ibid., Xl, 191 1; "Experimental Control of Morphogenesis in the Regulation of Plavaria," Biol. Bull., XX, igii. forms from the level h of Fig. 49 give rise to new tails and to all parts below their own level (Fig. 53). but never produce a mouth and pharynx, while headless forms from the level a or any level apical to it give rise to mouth and pharynx as well as to postpharyngeal regions (Fig. 52), but never to regions representing more apical levels than themselves.

If, however, a head of any sort, even a rudimentary, anophthalmic head, like that of Fig. 51, with no eyes and small, very incompletely developed, cephalic ganglia, arises on a piece from the level h, then the regions of the piece adjoining the new head give rise to the parts representing all levels between the head and the level which the piece h occupied in the original Figs. 49~53- — Phuiar'ui dorotocephala: Fig. 49, outline indicating regions a and h from wliich pieces arc taken; Figs. 50-53, dilTercnt results of reconstitution, depending on presence or absence of a new head -region.

the development of parts apical to the original level of the piece takes place only in relation to the development of a new apical end, while the development of parts basal to the original level of the piece is determined by the piece itself, even in the absence of a head. All the facts indicate that the same relation exists in other animals. It has already been pointed out (pp. 83-86) that when new growing tips arise in wound callus or from differentiated cells of plants, growth and differentiation proceed from these, not toward them. Plant stems, lateral branches, and leaves are subordinate parts or individuals of the plant and develop only under the dominance of growing tips. The root of the higher plant is likewise a subordinate individual. It possesses a growing tip and between this growing tip and other parts of the root indi\ddual the same relations of dominance and subordination exists as between the stem-tip and other levels of the stem, but the root as a whole develops only in subordination to S(mie part of the plant, a stem-tip, a stem, a branch, a bud, a leaf, or some part of a root system already present. The same is true for the root-like structures, the rhizoids of the lower plants. The roots and rhizoids of the plant have apparently much the same relation to the organism as a whole as do the stolons of Ttibularia and related fonns. They are individuals, each with an axial gradient and a dominant region of their own, but they are specialized individuals, and arise from the basal region of the major axis of the individual which controls their formation, whether it is a single bud or branch, a leaf, or the whole stem of a composite plant individual. It is probable that these subordinate individuals really represent

partially inhibited gradients (see pp. 178-81). Certain external conditions, such as moisture and darkness, favor the development of roots, but do not determine their origin. It is commonly stated by botanists that roots may arise on any or almost any part of a plant where external conditions permit their development or where the need for them exists. This is true in a sense, because most plants are composite individuals, and when one of the constituent individuals of the plant, such as a bud, branch, or leaf, is sufficiently isolated from an existing root system, or under certain external conditions, that individual may develop a root or roots. Physiologically or physically isolated parts of a plant may undergo transformation into stem-tips without relation to other parts and the stem-tips determine the fomiation of other parts, but even though various parts of plants may give rise to roots in the absence of stem-tips, in no case does any other isolated part of a plant undergo transformation into roots alone. Moreover, in development in nature roots and rhizoids in general arise only after the primary apical region has been determined. They are, in short, subordinate to the individual as a whole, but, like leaves and various other plant ''organs," possess a certain degree of individuation of their own. The question of the nature of the correlative influence of the root system upon other parts of the {)kint is one of considerable interest and is touched upon in cluq). \- (pp. 159-63)-

In the case of Planaria dorotocephala it has been possible to analyze the process of reconstitution to some extent and so to control it experimentally in various ways, and my experiments have led to certain conclusions concerning the nature of reconstitution. A part of the evidence on which these conclusions are based has already appeared in various papers,' but some of it is still unpublished. Here only some of the more important points and the conclusions are briefly presented. The results of the reconstitution of pieces in Planaria dorotocephala differ widely in different cases. I have found it convenient to distinguish five different forms: the normal (Fig. 50, p. 103), an individual in all respects like the type of the species; teratophthalmic (Fig. 54, A, B), in which the eyes show various degrees of fusion, inequality, or other departures from the usual condition, but the head as a whole shows the usual form; teratomorphic (Fig. 55, A, B), usually with a single eye in the median line and the cephalic sensory lobes more or less approximated or completely fused at the front of the head instead of in a lateral position; anophthalmic (Fig. 56, A, B), with an outgrowth more or less like a head and containing a small ganglionic mass, sometimes with cephahc lobes fused at the front, but without eyes; headless (Figs. 52, 83, p. 103), in which the cut end merely heals without outgrowth of new tissue. Different degrees of development of the cephalic ganglia

' Child, 'Studies on the Dynamics of Morphogenesis, etc., I," Jour, of Ex per. ZooL, X, 1911; II, ibid., XI, 1911; IV, ibid., XIII, 1912; VII, ibid., XVI, 1914; VIII, ibid., XVII, 1914. See also Child, "Experimental Control of Morphogenesis in the Regulation of Planar^'a" Biol. Bull., XX, 191 1 ; "Certain Dynamic Factors in Experimental Reproduction and Their Significance for the Problems of Reproduction and Development," Archiv fUr Entimckelungsmechanik, XXXV.