Individuality in Organisms

Most investigators have regarded the minimal size of pieces undergoing reconstitution as something absolute and have failed entirely to note that it differs with the physiological condition of the animal, the region of the body, and the various external conditions which affect metabolic rate. To determine the smallest piece of animal capable of reconstitution under given conditions is merely to determine one special case out of an indefinite number of possible cases.

The experimental evidence demonstrates, first, the essential independence of the apical region in both plants and animals, and, secondly, determination and control by this apical region of the developmental processes at other levels of the major axis of the individual. The reconstitution of pieces into new individuals is fundamentally the same process as embryonic development, and the same relation of dominance and subordination exists in both. The different results of reconstitution in pieces of different size, from different levels, in different physiological conditions, under different environmental conditions, etc., depend primarily upon relations of dominance and subordination,

determined by the relations of metabolic rate in different parts. In the higher animals other factors, such as the stability of the differentiated cellular substratum, may contribute to limit reconstitutional capacity. If the conception of physiological dominance which is presented in chap, ii is correct, the existence of a transmission-decrement in the impulses, stimuli, or excitations which are the effective agents in dominance must determine a certain range of dominance and therefore a physiological size limit or limit of length for each axis, which cannot be exceeded without physiological isolation of the part that lies beyond the range of dominance. Moreover, the limit of dominance in a given case must vary with the metabolic rate in the dominant region and the conductivity along the path of transmission. Its effectiveness upon a subordinate part ma\' also depend upon the receptivity of the part to the transmitted excitations, and this may be determined by local conditions to which the part is subjected. If the characteristic gradients are present or arise in a physiologically isolated part, such a part may become a new complete individual, if it is not so highly specialized or differentiated as to be incapable of reacting to the altered conditions by dedifferentiation and redevelopment. Some of the evidence bearing upon these aspects of the problem of dominance is considered in this chapter,

The dimensions and distance relations of parts along an axis can be altered by altering the metabolic rate in the dominant region or throughout the organism and so increasing or decreasing the length of the gradient. Figs. 60-62. — Different lengths of hydranth primordium in reconstitution of pieces of Tuhularia: Fig. 60, length at medium metabolic rate; a, b, c, d, the four regions of the primordium; Fig. 61, length at high metabolic rate; Fig. 62, length at low metabolic rate.

In Tuhularia the reconstitutional development of a hydranth is a transformation inside the perisarc of the terminal region of the piece into a hydranth without outgrowth of new tissue from the cut surface. In the early stages of this process (Figs. 60-62) the two rows of tentacles arise as two series of longitudinal ridges (Fig. 60, h, d), usually distinguishable from other parts by accumulations of red pigment. Various facts, some of which have been mentioned above (pp. 79, 96--99), show that the parts of the hydranth are determined from the apical end in the basal direction. The point of present interest in this process is the lenirth of stem concerned in the formation of the new hydranth and the length of each of the four distinguishable regions, a, b, c, d, of the developing hydranth. In pieces ol like physiological condition kept under the same external environment these lengths show a high degree of constancy, but they can readily be altered by altering the metaboHc rate in the pieces. Fig. 60 shows the length and proportions of the early stage of a hydranth developing with a medium metabolic rate, Fig. 61, with a high rate, and Fig. 62, with a very low rate. Evidently the higher the metabolic rate the greater the distance from the end of the stem and from each other at which the two rows of tentacles arise. The relative lengths of the different parts also change with metabolic rate, that of the region a increasing and that of the region d decreasing with increasing metabolic rate, and vice versa.^

The position, size, and time of appearance of hydranths and the relation of hydranths to other parts in the reconstitution of isolated pieces of Tubular in and related forms have been repeatedly investigated, but, although the facts are very definite, the various authors ^ Child, "An Analysis of Form Regulation in Tubularia. II, Differences in Proportion in the Primordia," Archiv fur Enhvickelungsmechanik, XXIII, 1907. In this paper I showed that such dilTcrcnccs in proportion appeared in hydranths from different levels and ends of the stem, but it is now known that these differences in level really represent differences in metabolic rate.

have failed to reach any very satisfactory general interpretation of them. Driesch, who has used Tubularia to a large extent as experimental material, even maintains that they cannot be interpreted on a physico-chemical basis. As a matter of fact, however, not only do the facts fall readily into line with the dynamic conception of the individual which I have outlined, but many of them constitute valuable evidence for that conception. I have found that previously existing metabolic gradients in the stem of Tubularia are rapidly obliterated and new gradients readily arise when metabolic conditions change. This is due to the fact that the protoplasmic substratum is not very stable, and, except in the hydranth, there is little structural differentiation in relation to the metabolic gradient. Wherever the stem of Tubularia is cut across, and even in many cases where section is not complete, a metabolic gradient arises in connection with the stimulation of the wound and the open end exposed to sea-water and the oxygen contained in it. The region of highest rate in this gradient is at the cut end, and the gradient extends a greater or less distance from the cut, according to the physiological condition of the stem and the direction and metabolic rate of the pre-existing gradient in the region concerned. If the metabolic gradient resulting from stimulation at the cut end is in the same direction as the pre-existing gradient, then of course there is merely an augmentation of the gradient, but if two gradients are in opposite directions, as they are at the basal end of a piece, they tend to neutralize, obliterate, or inhibit each other, and the one which has the higher metabolic rate sooner or later obliterates the other. The evi-

dence indicates that when such a gradient is sufficiently marked, that is to say^ when the metaboHc rate in its apical region is sufficiently high, and when the inhibiting or obliterating influence of a gradient in the oi)posite direction is not too great, a hydranth develops. The formation of a stolon, on the other hand, apparently represents a gradient which is partially inhibited or obliterated, or, in other words, partially dominated by a gradient in the opposite direction, but in addition to this relation a relatively high metabolic rate in the piece or individual as a whole is also apparently necessary for stolon-formation. The stem represents the lower levels of a simple uninhibited gradient, and its formation always occurs under the dominance of a hydranth or other region of higher metabolic rate.

It is also important for an understanding of the facts to note that in general the metabolic rate of these animals decreases when they are transferred from natural to laboratory conditions, and the hydranths which develop in the laboratory possess a lower metabolic rate than those in nature; consequently the range of dominance is less and physiological isolation occurs at shorter distances from the dominant regions than in animals in nature. Moreover, the development of a new hydranth at the cut end of a piece of stem is, I believe, a process essentially similar to the development of a head on a piece of Planaria (pp. 105-14). The new hydranth region is independent of other parts and becomes dominant over them, but during the early stages of its development this dominance is less complete, because the changes in the protoplasm of the stem in accordance with the new metaboHc conditions require some lime;

therefore removal of the original hydranth favors physiological isolation of basal regions of the piece. In Corymorpha the metabolic relations and the relations of the various parts of the body to the metabolic gradients are essentially the same as in Tuhidaria, and the demonstration of the metabolic gradients by means of the susceptibility method in Corymorpha, where most of the stem is naked, is not open to the objection which might be raised in the case of Tubularia, where all parts of the stem except the cut end are covered by the horny perisarc, viz., that the reagent penetrates the tissues only or chiefly from the cut end and so produces a death gradient which is merely a gradient of penetration and does not represent metabolic conditions.

Some of the facts and their interpretations in terms of metabolic gradients and physiological dominance are briefly as follows:' In pieces of Tubularia stem eight or ten millimeters or more in length and with a cut surface at each end reconstitution usually results first in the development of a hydranth at the apical end of the piece and later of a second smaller hydranth at the basal end (Fig. 63). Occasionally pieces from vigorous animals which evidently possess a high metabolic rate produce an apical hydranth and a stolon at the basal end (Fig. 64), but before it attains any great

' I have described and discussed these facts in the following papers: Child, "An Analysis of Form Regulation in Tubularia. I," Archiv fur Entunckelungsmechanik, XXIII, 1907; IV and V, ibid., XXIV, 1907; "Die physiologische Isolation von Teilen des Organismus," Vortrdge und Aufsdtze iiber Entunckelungsmechanik, H, XI, 191 1, 96-119. The discovery since these papers were written of the existence of metabolic gradients and their relation to physiological dominance affords a definite basis for most of the earlier conclusions and interpretations.

length this stolon gives rise to a hydranth at its tip. This is a process of reproduction like that occurrincr i,^ nature (Fig. 43, p. 90), and differs from it only in tliat the distance of the second hydranth from the first is less in the pieces than in the animal under natural conditions. This difference indicates that, as might be expected, the range of dominance of the apical region is less in the experimental piece than in the whole animal in nature.

In most pieces, however, the dominance of the apical region is insufficient to inhibit the establishment of a well-marked new gradient in relation to the cut basal end of the piece, and so the formation of a hydranth usually occurs at this end also, as in Fig. 63. The development of this hydranth is usually delayed, as compared with that of the apical hydranth, because the establishment of the new gradient is more or less retarded by the gradient already existing in the original direction, and the shorter the piece the greater the delay, because in shdrter pieces the dominance of the apical region is more complete, or, in other words, the gradient from the apical region is more marked at the basal end and therefore inhibits or retards to a greater

Figs. 6;^, 64. — Reconstitution of longer pieces of Tuhularia: Fig. 63, usual result of reconstitution with hydranth at basal end; Fig. 64, reconstitution with stolon at basal end. extent than in longer pieces the estabhshment of a new gradient in the opposite direction. In pieces more than eight or ten millimeters long, however, the local conditions at the basal end usually determine the result sooner or later, and the new gradient is estabUshed and a hydranth develops here.

In pieces between eight or ten and two or three millimeters in length neither hydranth nor any other outgrowth arises at the basal end in most cases. In these shorter pieces the dominance of the apical region is sufficient to inhibit the new gradient at the basal end to a sufficient degree to prevent hydranth formation, and the general metabolic rate in these as in most other experimental pieces is not high enough for stolonformation to occur.

In the very short pieces described in chap, iv (pp. 96-99) the difference in metabolic rate between the two ends of the piece dependent upon the original gradient is so slight that in many cases the local conditions at the two ends become the determining factors, and hydranths begin to form simultaneously or nearly so at both ends, the portion of each hydranth formed depending on the length of the piece. If the original gradient in the piece is sufficient to determine the more rapid reaction at the apical end this becomes dominant and a single, instead of a double, structure arises.

These are the chief facts of reconstitution in Tubularia under ordinary conditions and their interpretation in terms of metabolic gradients and dominance. It is possible, however, to obtain more positive evidence in support of these interpretations by controlling and altering the experimental conditions. By diluting the sea-water to a certain extent the metabolic rate in pieces is increased, and under these conditions pieces which in normal sea-water produce only hydranths at their basal as well as apical ends produce in a lar^^e percentage of the cases stolons which later develop hydranths at thrir tips.' The hydranths in such pieces are longer and larger than in normal sea-water.

When a piece is cut with a fully developed active hydranth at its apical end, no hydranth appears at the basal end until the metaboKc rate of the apical hydrantli decreases or its death occurs, which in Tubularia is usually within a few days at most. In Corymiorpha relations are similar. Evidently, then, a full-grown, active, apical hydranth inhibits the development of a basal hydranth in a piece, but a hydranth beginning to develop at the apical end is usually only able to retard to some extent the development of the basal hydranth. The dominance of the full-grown hydranth is more effective than that of the early stages of hydranth development.

Various investigators have observed that when the development of the hydranth at the apical end of a piece is inhibited by inclosing this end in paraffin or sticking it in the sand the development of the hydranth at the basal end is accelerated, and it has been found that in such cases the basal hydranth is longer and larger than when the apical hydranth is not inhibited. Evident l\- the inhibition of development at the apical end decreases dominance and the estabhshment of the new gradient, and so the development of a hydranth at the basal inM\

'Child, "An Analysis of Form Regulation in Tubularia. 1.' Archiv fur Entwickelungsmechanik, XXIII, 1907 is accelerated. The same result may be attained by compressing, sharply bending, or partially crushing the stem at some point between the two ends. In such cases the influence of the dominant apical region is prevented from reaching the basal end, which is therefore physiologically isolated and the establishment of the new gradient but little retarded. Often also the development of the basal hydranth can be accelerated by cutting partly through the stem, so that only a slender organic connection between the two ends remains. In these and various other ways the controlling influence of the apical region can be demonstrated.

Neither the inhibition of development of the basal hydranth by paraffining the basal end or sticking it in sand nor the partial crushing or bending of the stem at a certain level influences the development at the apical end except in very short pieces. In these, inhibition of either end may accelerate the development of the other, and a single instead of a double structure may result. These experiments show that in the longer pieces dominance extends chiefly in the direction of the original gradient, and we find correspondingly that the new gradient which arises at the basal end does not extend very far from that end. If, however, inhibition of the apical end be continued for a longer time, the gradient at the basal end extends farther from that end.

The length of the hydranths formed in very short pieces is often, though not always, less than in longer pieces, particularly in pieces from the more basal regions of the stem. Driesch has made much of this point as an indication that an adaptation of the length of the hydranth to the length of the piece takes place in order that a stem as well as a hydranth may be formed. According to Driesch this adaptation is not determined physico-chemically, but by the princi]:)le which he calls entelechy and which as he believes controls development. Unfortunately for Driesch's view this "adaptation" does not occur in all cases, and is very incomplete, for, as I have pointed out (pp. 96-99),' these short pieces often give rise to hydranths or apical jjarts of hydranths without stems or basal parts. The experimental evidence indicates that the shorter hydranths in short pieces are merely hydranths which are partialisinhibited by other regions of the piece, just as the head of Planaria may be partially inhibited by other regions of the piece. As in Planaria, short pieces, particularly those from the more basal regions of the body, are more stimulated by section, and their metabolic rate is therefore higher throughout than that of longer or more apical pieces. Under these conditions the gradient arising at the cut end is much less effective in detennining the development of a new structure, the hydranth, than it is when the general metabolic rate is lower. Figuratively speaking the new gradient is partially obliterated by the general high metabolic rate in the piece. Consequently its length is less and the length of the hydranth determined by it is correspondingly less than in longer pieces, and development is also retarded. A piece of given length may produce a single short hydranth and stem, or a longer hydranth without stem, or biaxial hydranths, or apical portions, and all these differences in behavior are determined by simple differences in the gradient relations.

I See also Child, "An Analysis of Form Regulation in rubuhiria. Regulation in Short Pieces," Archiv fur Entunckduugsmcchaiiik, XX I \". In Planaria also the positions and space relations of parts along an axis and the range of dominance can be altered and controlled by means of conditions which alter metabolic rate/ At ordinary room temperatures in well-aerated water the isolated postpharyngeal region of Planaria (Fig. 65) forms a new individual Uke that in Fig. 66. The new mouth and pharynx form near the middle of the piece at a certain distance from the new head, and the region in front of the pharynx undergoes the internal changes which make it over into the prepharyngeal region of the new individual. If, however, the rate of metabolism in such a piece is decreased by means of dilute narcotics, by the presence of carbon dioxide and metabolic products in the water, or by other means, the head develops slowly, is small and usually abnormal, and the lower the metabolic rate during development the nearer to the head the mouth and pharynx arise and the less the length of the new pharyngeal region. Fig. 67 shows the effect of a slight decrease, Fig. 68 of a greater, and Fig. 69 of a still greater decrease in metabolic rate during reconstitution. The length of the region undergoing reconstitutional change is less in Fig. 67 than in Fig. 66, still less in Fig. 68, and in Fig. 69 practically no changes occur below the level of the very rudimentary head.

Reconstitution of similar pieces with a very high metabolic rate (at high temperature) results in forms like Fig. 70, in which the pharynx and mouth arise at a ' Child, "Physiological Isolation of Parts and Fission in Planaria," Archiv fur Entwickelungsmechanik, XXX (Festband fiir Roux), II. Teil, 1910; "Studies on the Dynamics of Morphogenesis, etc. Ill," Jour, of ExPer. ZooL, XI, iqii. greater distance from the head and the prepharyngeal region is longer than in Fig. 66.

Figs, 65-70. — Space relations of parts in reconstitution of Planaria dorotocephala under different metabolic conditions: Fig. 65, outline indicating level of section; Fig. 66, reconstitution under standard laboratory conditions; Figs, 67-69, different ranges of dominance and space relations of new parts in reconstitution with low metabolic rate in different concentrations of naroctics; Fig. 70, reconstitution with high metabolic rate at high temperature.

The metabolic gradient associated with the new head shows a corresponding decrease and increase in length in such pieces. The influence of the new headregion extends to a greater or less distance according as its metabolic rate is high or low, and the position of the various organs is altered correspondingly, or, as in the extreme case of Fig. 69, no new organs are formed except the head. When the metabolic rate is high, as in Figs. 66 and 70, dominance extends nearly or quite to the basal end of the piece, though short zooids may be present as more or less distinct gradients (see pp. 92-94) at the basal end. Before section most of this region of the body consisted of one or more zooids, but the development of a head nearer to these zooids than the original head has brought about the obliteration of the gradients which represented them, except perhaps in the extreme basal region, and after reconstitution a single gradient extends over at least most of the length of the piece. When the metabolic rate is lower, as in Figs. 67 and 68, a short individual develops from the apical region of the piece, but most of the broader portion is not physiologically a part of this individual. This is very evident in the behavior of these forms, for, when creeping about, they are unable to control and co-ordinate this region to any great extent, and simply drag it about like a dead mass. As long as they remain in the narcotic they are not active enough to undergo fission, but if they are returned to water, fission may occur after a few days, although the range of dominance gradually extends, and more and more of the length of the piece comes under the control of the head.

The different types of head in Planaria (see pp. 106- 14) represent, as I have pointed out, different degrees of inhibition of head-formation, and, even after development is completed, possess different metaboHc rates, as susceptibility determinations show. The metal joHc rate is highest in the normal head, slightly lower in the teratophthalmic, and still lower in the teratomoq^hic and anophthalmic forms. In connection with these differences in the heads it is of interest to note that when the different forms are fed and grow, the length which they attain before fission varies in general with the form and metabolic rate of the head. Under ordinary conditions normal animals usually become twelve or fifteen millimeters long before undergoing fission, teratophthalmic forms usually slightly less, teratomorphic forms from eight to ten millimeters, anophthalmic, from sLx to eight or less, according to the degree of development of the head-region, while headless forms rarely become more than five or six millimeters long before dividing and often divide at a length of only three or four millimeters. These differences indicate very clearly the difference in range of dominance associated with the differences in metabolic rate in the dominant region.

There are many ways of inducing advance in development of the basal zooids and the occurrence of fission in Planaria, of which the simplest is the removal of the head of the anunal. This decreases the degree and range of dominance to such an extent that fission almost invariably occurs within a few days. By removal of new heads as fast as they develop fission may be induced even in anunals much shorter than those which usually undergo fission.' These and various other methods all serve merely to increase the degree of physiological isolation of the basal region by decreasing the degree and range of dominance.

In the case of the hydroid Corymorpha (see pp. 92, 132) the original gradient can readily be obliterated and the estabhshment of new gradients determined by experimental conditions. Reconstitution in pieces four or five millimeters or more in length from the naked region of the stem in sea-water under the usual laboratory conditions is like that in most of the longer pieces of Tuhularia stem (see Fig. 63, p. 133). A hydranth develops at the apical end of the piece, and later a second smaller hydranth appears at the basal end. The metabolic conditions are also similar to those in Tubularia, and reconstitution can be altered and controlled in much the same way in both forms. If, however, such pieces of Corymorpha are placed after cutting in 2-2^ per cent alcohol in sea-water the cut ends heal, but hydranths do not develop. In the course of a few days the pieces become shorter and more rounded, decrease in size, and lose the characteristic structure of the Corymorpha stem. The changes in shape are indicated in Figs. 71 and 72. On removal to water after several days in alcohol a new hydranth begins to develop on the upper side of the piece (Fig. 73), then a stem arises below it, and