Individuality in Organisms

' Child, "Physiological Isolation of Parts and Fission in Planaria,'" Archivfiir Enpwickelungsmechanik, XXX (Festband fiir Roux), 11. Teil, basal structures develop on the lower side of the piece in contact with the underlying surface, and gradually the piece is transformed into a new small individual (Fig. 74). In most cases the old outline of the piece is still preserved by a thin layer of hardened slime secreted by the piece while in alcohol. This is indicated by the dotted line in Fig. 74. Susceptibility determinations show

Figs. 71-74. — Experimental establishment of a new major axis in a piece of Corymorpha: Fig. 71, the piece after section; Fig. 72, after reduction in alcohol; Fig. 73, appearance of new hydranth on upper side after return to water; Fig. 74, fully developed new individual; dotted lines indicate old outline of piece preserved by slime. that in alcohol the original axial gradient disappears, and that when the pieces are returned to water a new gradient arises in the direction in which the new axis develops. Since the pieces adhere to the surface soon after being placed in alcohol, it is possible to keep them in the same position throughout the experiment and so to be certain of the original direction of the major axis and gradient, even though they become hemispherical

or nearly spherical in form. In most cases, however, there is no difficulty as regards this point, because the longest diameter of the pieces coincides in direction with the original axis. A comparison of the direction of the new axis wliich arises after return to water with that of the original axis shows that the former is at right angles with the latter. The new hydranth develops without relation to either of the cut ends from the uppermost region of the piece a,s it lies in the aquarium, and this region was originally its lateral surface. In these cases the alcohol not only inhibits the increase in metabolic rate in relation to the terminal cut surfaces, which determines the development of hydranths at the two ends, but decreases the rate throughout the piece. In this way it obliterates the original gradient and dominance to such a degree that when the metabolic rate rises again on return to water the original axial relations do not reappear, but a new gradient and a new dominance arise in relation to the external conditions to which the piece is subjected, and the axis of the new individual coincides in direction with the new gradient. In all cases, so far as my experiments go, the new hydranth arises from the uppermost part of the piece, no matter what region of the piece in its original condition this part represents.

When short pieces, which have already produced biaxial hydranths (Fig. 75), are used for this experiment, the changes are very similar to those described for longer pieces. In alcohol the tentacles and the apical regions of the two hydranths die and disintegrate, but the more basal portions gradually lose their hydranth structure and the pieces become small rounded masses in which no structure is externally visible (Fig. 76). After return to water a new hydranth arises, as in the longer pieces, on the uppermost part (Fig. 77), which represents one side of the basal region of the previously existing hydranths, and the piece undergoes transformation into a new small individual (Fig. 78). In this case the two opposed metabolic gradients which were present at the beginning of the experiment were completely obliterated and a new single gradient arises at right angles, or, if the pieces are not kei)t

Figs. 75-7S. — Experimental establishment of a new major axis in a piece of Corymorpha which has already formed a biaxial structure: Fig. 75, the biaxial hydranths developed from the piece; Fig. 76, the same piece after reduction in alcohol; Fig. 77, appearance of new hydranth after return to watef ; Fig. 78, fully developed new individual. in the same position throughout, in any relation to the original gradients as determined by the external conditions.

My experiments along this line were interrupted and no opportunity to continue them has as yet arisen. I beheve, however, that the new metabolic gradient in these pieces is primarily determined by the diilerencc in oxygen supply between the free upper surface and the surface in contact, the region of highest rate representing the region of greatest oxygen supply; but further experiment is necessary to determine positively whetlier this or some other factor in the environmental conditions is the essential one. The important point is that a new metabolic gradient, major axis, or polarity is in these cases determined by external conditions, and that morphogenesis occurs with reference to this gradient.

In the case of a sea-anemone, Harenactis (Fig. 79), obliteration of the original gradient is accomplished in a somewhat different way.^ The bodies of these animals are tubular, with partial longitudinal partitions, the mesenteries. When the rather bulky mesenteries are not removed, pieces cut from the body close by gradual contraction at each end, the wounds heal, and a new disk and tentacles develop at the apical, and a new ''foot" at the basal end. If, however, rather short pieces are taken (a, h, Fig. 79) and the mesenteries are largely cut away from the interior of the body, the pieces close up and heal as indicated in the longitudinal section (Fig. 80), because there is no mass of internal tissue to prevent the two ends meeting when the piece contracts. In such pieces the apical cut surface of the body wall unites with the basal about the whole circumference, and the result is a ring or doughnut-shaped structure which makes an attempt to orient its body as it does in nature by revolving about a circular axis like a vortex ring until the region of union of the two ends lies on its upper or outer surface.

At this region of union more or less new tissue arises, particularly if the cut surfaces are irregular and do not ' Child, "Factors of Form Regulation in Harenactis attenuata, I, II, III," Jour, of Expcr. ZooL, VI, VII, 1909; "Further Experiments on Adventitious Reproduction and Polarity in Harenactis," Biol. Bull., XX, Figs. 79-83. — Reconstitution in "rings" from sea-anemone, Harenactis attemiata: Fig. 79, longitudinal sectional outline of animal, indicating regions, a, b, from which pieces are taken; Fig. 80, diagrammatic longitudinal section through a ''ring," showing method of closure by union of apical and basal cut surfaces of body wall; I-'igs. 81, 82, tentacle groups arising from the region of union of cut surfaces; Fig. S3, a perfect animal developed on a ring.

unite smoothly, and from this new tissue all gradations from single tentacles, through groups of tentacles of various sorts up to complete small anemones (Figs. 81-83) arise. The various tentacle groups in Figs. 81 and 82 and the individual in Fig. 83 are made up of cells which are descended from both apical and basal ends of the piece and a more or less definite new individuation occurs in these cells. There can be little doubt that in these cases the origin of these various degrees of individuation is associated with the growth of new tissue at the line of union between the cut surfaces. The metabolic rate in this tissue is higher than in the other regions of the piece, and if it is enough higher the new tissue becomes independent and produces a new apical region, or some part of it, according to conditions. Wherever, about the circumference, growth of new tissue is most rapid and extensive, there the new individual is most likely to arise. Often it is possible to determine beforehand the region of the circumference where such tentacle groups or individuals shall arise, by making the outline of one or both cut surfaces irregular at some point or making a number of small cuts near together in them. In such 'regions there is more growth of new tissue and a new gradient and new individual are more likely to arise.

As regards the minor axes, it is of great interest to note the wide range of variations which occurs. Many bilaterally as well as radially symmetrical and asymmetrical forms appear among the tentacle-groups, and it is evident that the symmetry of the groups is in many cases related to the line of union and not to any preexisting symmetry of the parent animal. In these rings we see new individuals being localized and develo[)in^' where it is impossible to conceive of any internal localizing and determining factors other than quantitative metabolic conditions.

In the case of Planaria I have been able to increase the frequency of biaxial heads (see Fig. 48, p. 99) in very short pieces by partially narcotizing the animals before cutting and keeping the pieces in a dilute solution of a narcotic, e.g., chloretone, for a day or two before allowing them to develop. Under such conditions the metaboHc rate in the pieces is of course decreased, and so dominance in the direction of the original gradient ii still further decreased. Consequently, when the pieces are returned to water and allowed to develop, the conditions are even more favorable for the establishment of the reversed gradient at the basal end, and biaxial structures develop in a larger percentage of cases than when the pieces are not narcotized. The effect of the narcotic is simply to aid in decreasing the dominance of the original apical region of the piece and so to increase the probability of the establishment of an effective reversed gradient and dominance at the basal end. This experiment has not as yet been attempted with Tubularia, but will no doubt be successful with proper technique.

That the range of dominance undergoes extension during development is evident from many facts. In the young Planaria, for example, a second zooid arises at the posterior end of the body when the animal is less than five millimeters in length, i.e., the range of dominance at this stage of development is only three or four millimeters.^ In the adult animal, however, the range of dominance as indicated by the length of the first zooid may be ten or twelve millimeters or even more under certain conditions. Evidently with advancing differentiation of the nervous system the conductivity has increased, and so the transmission-decrement has become less and the range of transmission greater.

In Stenostomum also the more advanced the development of a zooid, the greater the distance from its head-region at which the head-region of a new zooid is determined, as will appear by reference to Fig. 29 (p. 81). Other animal forms which undergo agamic reproduction show similar relations, and it is also probable that the increasing capacity for co-operation and control of parts with advancing development, so far as it depends on the nervous system, results to some extent from the increase in efficiency of transmission, though various other factors may also be concerned.

In plants also similar relations appear. In the differentiated part of the plant stem the range of dominance of a bud or a growing tip over others is very nmch greater than in the embryonic region of the growing tip, but their later development is inhibited by the growing tip as a whole, even though further growth has greatly increased the distance between them. The dominance of the growing tip as a whole has a much greater range in the differentiated parts of the plant than the dominance of its apical region over much nearer parts in the

» Child, "Studies on the Dynamics of Moiphogenesis. Ill," Jour, of Ex per. ZooL, XI, igii. embryonic or slightly differentiated tissue of the growing tip itself. In the higher animals the extension of dominance is evidently very much greater than in the lower forms. In the medullated nerve fibers of the higher vcrtcl^rates the transmission-decrement is so slight that some authors have denied its existence. Various lines of experiment have indicated, however, that a transmission-decrement does exist even in vertebrate nerves (see pp. 173-75). Tashiro has shown that a gradient in carbon-dioxide production exists in nerve fibers, and I have observed a distinct susceptibility gradient in certain nerves. The nerve is essentially a specialized protoplasm which conducts with less decrement and therefore to greater distances than other kinds of protoplasm, and the central nervous system arises in those regions of the body where the transmitted changes primarily originate.

The extension of dominance during the development of the higher animals is so great that the range of domi nance is undoubtedly very much greater than the size of the individual. In these forms individual size is limited, not by the range of dominance, but by the decrease in metabolic rate which accompanies the progressive differentiation, and so limits growth. Onl\- in early stages of development, or in the lower organisms, where nerves are either absent or not very good conductors, does the size of the individual ecjual tlu' range of dominance.

The course of development in the single plan! individual suggests the dominance of the growing tip of the stem, but physiological isolation of parts and reproduction of new individuals afford the only means of demonstrating experimentally the existence of dominance and its varying range. From among the accumulated data concerning what the botanists commonly call correlation, a few simple, well-known experiments Figs. 84, 85. — Diagrammatic outlines of leguminous seedlings, illustrating effect of removal of growing tip: Fig. 84, uninjured seedling; Fig. 85, development of shoots from axils of cotyledons after removal of stem-tip.

show how readily physiological isolation and reproduction may be brought about in plants. The young seedling of a leguminous plant (pea, bean) possesses the general form indicated diagram - matically in Fig. 84. The further normal development of the stem consists primarily in its elongation and the development of leaves by the activity of the growing tip at its apical end, but if this growing tip is removed a new growing tip, or in some cases more than one, arises from the axillary region of each cotyledon, as indicated in Fig. 85. These axillary shoots very rarely appear when the original growing tip is present and active, but their development results regularly from its removal. If both of the shoots grow at about the same rate they may both continue to develop and so give rise to two

stems, each of the same character as the single stem in normal plants, but if one grows more rapidly the growth of the other is usually soon inhibited and only the one continues to develop. If, instead of removing the primary growing tip, we inhibit its metabolic activity in any way without killing it or injuring it otherwise, the result is the same as if it were removed. Inclosure of the primary growing tip in plaster of paris or in an atmosphere of hydrogen accomplishes this result without injury, for it is capable of resuming growth after removal of the plaster or return to air. If the primary tip is inhibited in this way until the axillary shoots have appeared and is then allowed to resume its activity, the growth of the axillary shoots is in turn inhibited and the primary stem continues its development, unless the axillary shoots have attained a length two or three times as great as that of the main stem before the inhibition of the primary tip is removed. In that case the further growth of the primary tip may be almost entirely inhibited by the axillary shoots, and it may even die, while they, or one of them, as the case may be, continue development. Many modifications of the experiment are possible at different stages of development and in different plants. In stems with lateral buds, such as the willow, if the apical growing tip is removed the uj^jx-r- most lateral bud or buds will develop and their develoj) ment inhibits the development of those lower down, if we remove them or prevent their development b>' inclosing them in plaster, the buds next below will develop, and so on.

In many plants removal or inhibition of all tlii growing stemtips present results in the tormation «)! so-called '' adventitious" buds, which may arise from differentiated cells, as in the case of the begonia (Figs. 38, 39), and may be scattered irregularly over various parts of the plant according to the conditions of the experiment. Often the presence of a single one of the original buds is sufficient to inhibit the formation of these adventitious buds. The appearance of adventitious buds on plants in nature is usually due to the weakening of existing growing tips through advancing age or injury of some sort.

Such adventitious buds very often arise in large numbers simultaneously without any regular arrangement with reference to each other. The absence of definite space relations in such cases is undoubtedly due to the fact that they arise simultaneously, or nearly so. Various cells here and there which happen to have a slightly higher metabolic rate than others begin to develop into new buds at about the same time; consequently none of the buds is dominant over the others. If, however, one of the adventitious buds gets a start beyond the others in any way, it inhibits the further development and may even bring about the death of others within a certain distance of it. Moreover, where a gradient is present in the part on which the buds appear, so that one or more buds appear first in a certain region — the region of highest metabolic rate in the part — they inhibit the growth of others within a certain distance or throughout the part.

In various conifers the dominance of the growing tip of the main stem appears in a somewhat different form. In these trees, as long as the growing tip of the main stem is present and active, lateral branches arise radially around the main stem and grow outward from the trunk, and the branches of the second order arise in most cases more or less bilaterally on them. Removal of the main growing tip is followed by the bending upward of one or more of the uppermost lateral branches, further growth in the vertical direction, and radial instead of bilateral outgrowth of new branches. Here one or more of the lateral branches nearest the upper end of the stem react to the absence of the main growmg tip by changing direction and form of growth to that characteristic of tinoriginal tip. If this branch is removed, branches farther down the trunk react in the same way.

According to most authorities, dominance of one part over another is effective only or chiefly in one direction along the stem, namely, from the apical end downward. Buds or growing tips at or nearer the apical end are capable of inhibiting buds farther down the stem, but the latter are not capable or are less capable of inhibiting the former. In recent experimentation,^ however, it has been demonstrated that these relations may be reversed, and that if shoots lower down are allowed to grow for a long enough time and to a large enough size, while buds higher up are inhibited by artificial means, the lower shoots sooner or later acquire the abihty to inhibit the higher ones after the removal of the artificial inhibition. This is what might be expected if inhibition depends on the relations of metaboHc gradients. Under ordinary conditions the upper levels of the stem represent higher levels in the gradient and therefore inhibit or obliterate gradients

'W. Mogk, " Untersuchungen iiber Korrelationen von Knospcn und Sprossen," Archiv fur Entwickelungsjnechanik, XXXMII, 1914. lower down more reiidily than these with their lower rate are able to reverse the whole established protoplasmic gradient higher up. If, however, a new gradient at a lower level becomes established while the dominant region above is inhibited, it is conceivable that it may in time, by its gradual extension in the stem, obliterate more or less completely, or perhaps reverse, the original gradient and so dominate regions higher up, at least to some extent. This is apparently the case in the seedling mentioned above (p. 153) when the axillary shoots are allowed to grow long enough while the main growing shoot is inhibited. Under such conditions they are apparently able to inhibit what was originally the dominant region of the whole plant.

It is often possible to isolate a part of the plant from the dominance of the gro^ving tip merely by cutting the vascular bundles connecting the two parts. The development of buds on the leaves of certain plants may be induced by severing the chief vein or veins of the leaf, other tissues remaining intact. In such cases buds appear peripheral to the cut, usually near the veins, but in some plants on the leaf margins. The inhibiting influence is not confined to the growing tips of stems, for it has been shown that a leaf plays a part in inhibiting the growth of the bud in its axial. Removal of the leaf or inhibition of its activity may bring about outgrowth of the bud, if the inhibition from other sources is not too complete. In certain cases it has been shown that one part of a leaf may inhibit other parts. In Cyclamen persicum, for example, the young seedling (Fig. 86) possesses at first only a single leaf, one of the cotyledons. Removal or inhibition

by inclosure in plaster of the distal part of the blade of this leaf before its growth is completed is followed by the development of a new leaf surface from each side of the basal portion, as in Fig. 87. Wlien the whole bhidc of the leaf is cut off or inhibited, the margms of the petiole just below the level of the cut give rise to a separate new leaf on each side (Fig. 88). Here the basal portion of the leaf and the distal region of the petiole margin

Figs. 86-88. — Dominance and physiological isolation in leaf of Cyclamen persicimi: Fig. 86, intact seedling (from Hildebrand); Fig. 87, development of new leaf blade from each side of leaf base after removal of more apical portion; Fig. 88, development of new leaf from each side of petiole margin after removal of whole leaf (from Goebel). evidently possess the capacity to develop as a leaf, but are prevented from doing so as long as the original leaf or its distal portion is present or active.

Attention has been called to the fact that roots, wherever they appear on the plant, are apparently subordinate, specialized individuals and originate in definite relations to parts which represent regions or levels physiologically less remote than the root-tij) from a stemtip or bud (see pp. 104, 105). Alost plants with roots possess, however, not a single root, but a root system which is a composite individual, each root representing a single constituent individual. In such a root system relations of dominance and subordination similar to those in stem systems exist. The formation of each new root represents a reproduction and the establislnnent of a new root individual. In plants possessing a single main root with lateral roots arising from it (Fig. 84) this relation appears very clearly. As the main root grows in length directly downward, lateral roots arise

Figs. 89-91. — Effects of removal or inhibition of main root-tip on direction of growth of lateral roots (from Bruck). successively at a certain distance from its growing tip and grow obliquely downward or almost horizontally. Experiments with seedhngs show that if the growing tip of the main root is cut off, new lateral roots arise in larger numbers or nearer the end of the main root, and one or more of these nearest the cut end grows more nearly in the vertical direction downward than when the main growing tip is present (Figs. 89, 90), the behavior differing somewhat according to the level of the cut. Apparently in these seedlings the lateral roots which

have already developed do not change their direction of growth when the chief growing tip is cut off; only those which develop after the operation react, but they or some of them develop as main instead of lateral roots and later themselves give rise to lateral roots. If the outgrowth of new roots near the cut surface is inhibited after the removal of the main growing tip by inclosing this region of the main root in plaster, roots which arise above the inhibited region may react by growing more directly downward, provided they are not too far away from the cut surface (Fig. 91). The lateral roots which react in this way to the absence of the main growing tip resemble more or less closely the main root in their later development. When the growing tips of all roots are cut off, adventitious roots arise, usually in large numbers and without any definite order, on the parts remaining. Evidently the relation between the constituent parts of the root system is a relation of dominance and subordination like that in the stem system.