Physiological Foundations of Behavior

It is a familiar fact that the physical isolation of parts of the body of organisms is in general followed by changes in their behavior. In the higher animals the specialization of structure and function is so great that such physically isolated parts soon die, though under properly controlled conditions, e. g., in tissue culture media, even small groups of cells may be kept alive for long periods and may grow and divide. In the simpler animals such physical isolation of parts, if not carried too far, is followed by dedifferentiation of some or all of the cells of the isolated piece and this in turn is followed by a new developmental process which gives rise either to a complete new individual (Figs. 129-131) or to the more apical or anterior portions of such an individual (Figs. 95-98). Such a process is known as regeneration, form regulation, restitution, reconstitution. It involves a change in behavior and structure from that of a part toward that of a whole organism.

Among the lower plants every cell of the body may be capable, when physically isolated, of transforming into a new growing tip and so into a new individual, but in the multiaxiate higher plants physical isolation of pieces containing buds, merely determines the outgrowth of some of the buds. In such cases there is simply the substitution of one axis for another, rather than the development of new axes. It appears to be true then that a physiologically isolated part of an organism tends in general to lose its characteristics as a part and to become or approach the conditions of a new whole individual. The capacity for such reconstitution is greatest in the simpler organ-

Fies. 129-131.—Reconstitution of pieces of Planaria dorotocephala from different levels: Fig. 129, outline of body indicating levels, (a) and (6), from which pieces are taken; Fig. 130, reconstitution of (a); Fig. 131, reconstitution of (>). In Fig. 130 only the head, in Fig. 131, the head with a short region of the body behind it, develops from the new tissue at the cut surface: in Fig. 131 almost the whole prepharyngeal, and the pharyngeal regions are formed by reorganization of the regions posterior to the cut surface (from Child, 21 a).

isms, because the greater degree and fixity of specialization in the higher forms, particularly the higher animals, limits their potentialities of change in behavior. It is very generally admitted that the changes which occur in physically isolated pieces of organisms are in some way the results of their isolation. That is to say, their original development as parts of the individual was determined by their physiological relations with other parts and primarily, as I have tried to show, by their position in a physiological gradient or gradients. When we isolate such a part physically, e. g., by section, we of course isolate it from the action of the physiological factors which have made it the particular part it is. If its specialization as such a part is not fixed, it does not persist in the absence of physiological factors which determined it and dedifferentiation occurs. The course and character of the reconstitutional process in a particular case depends on the axial relations. In some isolated pieces the old gradients persist (Figs. 124— 126), in others new gradients arise (Figs. 102-109).

Various reasons have been given repeatedly for believing that the physiological relations in a gradient must be primarily excitatory-transmissive in character, 7. e., that the high end of the gradient differs from other levels as a region of excitation differs from an unexcited or less excited region (Child, "15 ¢, ’21a; see also Chaps. XI and of excitation with a decrement unquestionably plays an important part in the origin and establishment

of the gradients. According to this conception physiological dominance is fundamentally an excitatory and transmissive relation. In the absence of specialized structural paths and in at least some nerves, the excitatory change usually undergoes a decrement in physiological effectiveness and finally a point is reached beyond which it is ineffective. Under such conditions the range of dominance must be limited by the decrement in transmission. In other words, at a certain distance from the dominant region its control becomes ineffective. The range of dominance may differ widely with different conditions: for it may be very short in early embryonic stages in which the conductivity of protoplasm for excitation is rather low, and it may increase during development until in the nerves of vertebrates it is certainly very great and is regarded by many as unlimited (Child, ’21a, pp. 228 ~228). Again, experiment has shown that the range of dominance varies with rate of metabolism in the dominant region. Decrease in rate determines decrease in range of dominance and vice versa.

Since the range of dominance is limited, particularly in the earlier developmental stages and in simpler organisms throughout life, the possibility exists that under certain conditions some portion or portions of the body may come to lie beyond the range of dominance, even though they are still in protoplasmic or cellular continuity with it. For isolation brought about in this way the term ‘““yhysiological isolation”? has been used (Child, 11 e.) In the simpler organisms, in ~ which the isolated part is capable of reacting to the altered conditions, the result of physiological isolation is essentially the same as that of physical isolation. The physiologically isolated part or region tends to lose its characteristics as a part and to develop into a new individual.

In many processes of budding, fission, ete., in both plants and animals the occurrence of physiological isolation can be demonstrated in various ways, or very clearly indicated. Such isolation is probably concerned in some way and to some extent in all processes of agamic reproduction. For example, one indication of physiological isolation of a part is the partial or complete inability of the animal to control it. Such a condition is very clearly shown in Planaria dorotocephala. After the animal attains a certain length the posterior region of the body is no longer under complete control of the anterior region under ordinary conditions, though when the animal is strongly excited this region is often brought under control. Sooner or later this posterior region reacts independently of the anterior region by attaching itself to the substratum in reaction to some slight stimulus, while, the

anterior region continues to creep forward. When such reaction occurs the body in front of the attached region may become greatly gree of development of head, being least in zooids atest in those with most developed heads. stretched (Fig. 132) and if the animal has sufficient strength, or if the degree of excitation of the anterior region does not become go great that the posterior region is brought under control, fission results. In many other animals physiological isolation results in actual development of the new individual before Separation occurs. The flat-

worm Stenostomum serves as an example (Fig. 133). Here the range of dominance and therefore the distance of the fission plane from the head in each new fission differs according to the development of the head and nervous system, 7. e., with the range of dominance in that zooid. The zooid in which nervous system is most est length before a new fisearlier the stage of denervous system, the when fission begins. Even changes or changes in berence of physiological isoonstrated by the presence dients, as indicated by KMn0O,, electric potential (See Chap. VII). In vahas been shown that there which physiological isolastrictly speaking, four cerned in physiological possible to determine and through all four of these

development of head and advanced attains the greatsion plane appears and the velopment of the head and shorter the zooid at the time where no morphological havior appear, the occurlation may often be demof new physiological grasusceptibility, reduction of or respiratory exchange rious earlier publications it are four possible ways in tion may occur, or, more factors which may be conisolation, and it has been control its occurrence factors.

Increase in size. — may result from increase organism, this increase bringing certain parts beyond the range of dominance in that individual. Budding and fission resulting from increase in length of an axis are cases ; Fia. 134.—Tubularia, showing physiological isola- : : tion of tip of stolon with consequent development In point. In Planaria of a new hydranth. mum (Fig. 133) fission commonly results from increase in length of the polar axis and consequent physiological isolation of the posterior end. In the hydroid, Tubularia, the basal end of the stem forms a holdfast and gradually grows along the substratum as a stolon (Fig. 91, p. 118), but when the length of the stem plus stolon, 7. e., the distance of the stolon tip from the dominant hydranth region, becomes sufficiently great, the tip of the stolon transforms into a new hydranth (Fig. 134). In various other species of hydroids the

formation of a new hydranth bud is possible only ata certain distance on the stem from an active hydranth and this distance varies with the activity of the hydranth. Numerous other cases of physiological isolation and fission, budding or some other form of agamic reproduction might be mentioned. Decrease in dominance. — A decrease in activity, or removal of the dominant region may bring about physiological isolation. Such isolation and the following reproduction may occur without any increase in size of the organism concerned. That physiological isolation and outgrowth of buds in plants can be experimentally induced by removal of the growing tip or by retarding its metabolism has long been known to botanists. In animals similar results can be obtained. In Planaria, for example, removal of the head favors development of the posterior zooid and fission (Child, 10 a, ’11-d). In Tubularia removal of the apical hydranth or inhibition of the development of a hydranth at the apical end of a stem or stem piece is a factor in bringing about physiological isolation at the basal end and so favors or determines development of a hydranth there (See Fig. 92, also Child, ’07 a, d). As noted in Chapter VI, Wood-Jones has described relations of dominance and subordination in the staghorn corals which are almost identical with those in multiaxial plants. In these corals, removal of the apical dominant. zooid of the branch results in physiological isolation of parts below and some of the lateral zooids undergo transformation into apical zooids of new branches.

Both botanists and zo0logists have repeatedly called attention to the occurrence of various processes of agamic reproduction under “unfavorable conditions” and such reproduction has often been regarded as an adaptation directed toward preserving the life of the ‘Species under the unfavorable conditions. Doubtless it serves this purpose in many cases, but physiologically it results from decrease or obliteration of dominance because of the fact that the dominant region is more susceptible than other parts to the injurious conditions, The individual disintegrates physiologically into two or more smaller individuals. The “unfavorable” conditions under which such reproduction occurs are in general conditions which decrease metabolism and so shorten or more or less completely obliterate the gradient because of the differential Susceptibility of different levels (see pp. 80—

Block. — Physiological isolation of subordinate parts may also be brought about by physiological block, 7. e., by blocking or obstructing through physiological change the passage of the correlative factor at some point in its course from the dominant to the subordinate region. McCallum (’05) brought about some degree of physiological isolation in the bean seedling by subjecting a short zone of the stem to vapor of an anesthetic. More recently Bellamy and I have found a zone

Fias. 135, 136.—Physiological isolation by low temperature block in scarlet-runner bean: Fig. 135, physiological isolation and growth of buds in axils of cotyledons by zone of low temperature on internode next above them (petiole of leaf was broken after outgrowth of buds); Fig. 136, physiological isolation and growth of buds in axils of first foliage leaves by zone of low temperature on internode next above them. of low temperature a very effective means of blocking the correlative factor in various plants (Child and Bellamy, 719; Child, ’20 b, 21 b.) The result of such an experiment on the scarlet-runner bean is shown in Figures 135, 1386. In the normal bean plant the buds in the axils of the cotyledons and of the first foliage leaves do not grow out because they are inhibited by the chief growing tip, or in its absence by other growing tips apical to them which reacts to isolation more rapidly than they do. The cooling to 3°-5° C. of a zone of the stem twenty mm. or more in length blocks the dominance of the growing tip or other buds and the buds of the axil next below the cooled zone grow, as Figures 135, 136 show. Such cooling produces no perceptible physical injury and after removal of the low temperature the inhibiting action of the growing tip is again effective below

the cooled zone, unless the isolated buds have become so active during the period of isolation that the chief growing tip is no longer able to inhibit them. In the course of these experiments it has been found that within a certain range of temperature the effectiveness of the block varies with the length of the cooled zone. For example, at a temperature of 4°-5° a cooled zone 25-30 mm. in length may be ineffective as a block, but with the same temperature a cooled zone 80-100 mm. is effective. With sufficiently low temperature, e. g., 2°, a cooled zone 25-30 mm. long is effective as a block at any level of the stem, but with higher temperatures, e. g., 4—-5°, the farther away the cooled zone from the buds to be isolated, the less effective it is as a block. These facts suggest that with the higher temperature the inhibiting factor is not completely blocked and that in the normal parts of the stem below the cooled zone it gradually recovers its normal intensity or effectiveness. When the cooled zone is near the buds to be isolated the length of normal stem below the zone is not sufficient for such recovery, but when the zone is far away it is sufficient. Another possibility is that the correlative factor itself is weaker at the lower level and therefore more completely blocked by a given length of zone at a given temperature than at the higher level.

And finally, it is important to note that the blocking by low temperature of the inhibiting action of the growing tip on buds below does not block the passage of water and salts in the opposite direction. Plants with a cooled zone of stem do not wilt and the stem continues to grow above the cooled zone. ‘To determine whether the cooled zone blocks the downward transport of substances is not so easy, but by using different plants and different parts it has been possible to make it highly probable that the blocking of such transport is not the essential factor in the physiological isolation (Child, 21 b.)

In the course of his experiments on Bryophyllum Loeb ! has advanced various hypotheses in the attempt to interpret the relation of dominance and subordination in plants. These hypotheses are in terms of transportative correlation, an attempt to account for the facts by assumptions concerning the flow or distribution or flow of sap or of certain nutritive substances in it or by the assumption of the production by the dominant growing tip of inhibiting substances. As I have pointed out elsewhere, the results of the low temperature experiments indicate, if they do not demonstrate, that the re-

lation of dominance and subordination in plants is primarily rather a matter of transmission of excitation, than, as Loeb maintains, of transportation of substance. On the basis of other experiments Harvey (’20) has recently reached essentially the same conclusion. It has long been known that nerve impulses in animals may be blocked by cooling, compressing or partially anesthetizing a zone of the nerve, but the blocking of the correlative factor concerned in the general relation of dominance and subordination in the physiologieal gradients of animals has not yet been attempted. There is, however, every reason to believe that with the proper technique such blocking can be accomplished in the same way as in the plant.

Local action on the subordinate parts. — Finally, physiological isolation may occur in still a fourth way, viz., by direct excitation of, or other local action on, a subordinate part of such a degree that it is no longer sensitive to the control of the dominant region. Physiological isolation occurs in such cases because the subordinate part after excitation itself becomes the high end of a new gradient, and the excitation-transmission changes from the originally dominant region are obliterated in passing up this gradient (pp. 195, 200), or, if they do reach the part in question are no longer effective in controlling it because of its increased activity.

This sort of physiological isolation sometimes occurs in the buds of plants through the action of external factors favorable to growth. In the case of Bryophyllum, for example, the buds in the notches of leaves are ordinarily inhibited by the chief growing tip, even in moist air, but when a leaf is immersed in water while still attached to the parent plant some of the buds will often develop new plants even though the chief growing tip of the parent plant is growing actively and inhibiting all other buds. Similarly, the direct action of external factors on the growing tips of runners may induce their transformation into new plants before the length of the runner has become sufficient to bring about physiological isolation of the tip in the ordinary way.

The development of biaxial forms from pieces in various hydroids and in Planaria (Figs. 92-101) and in other forms are cases in which this factor of physiological isolation plays a part. The development of a new hydranth or a new head at the basal or posterior end of the piece occurs because the external differential determines a new gradient from the cut surface inward, 7. ¢., in the opposite direction to the original gradient, and in spite of its presence. The new gradient determines first the new apical end and then so much of the body

as the length of the piece and the lengths of new and old gradients permit. In these cases the new polarity arises more readily because the removal of the original hydranth or head and the isolation of the short piece have weakened dominance in the original direction. Physiological isolation in development of repetitive series.—Physiological isolation is also unquestionably a factor in the development of repetitive series of parts, e. g., Segments, series of tentacles, the mesenteries of sea anemones and corals, the “gills” of mushrooms, the order of leaves, branches, and other parts in plants. Each region of growth dominates a certain area and a new region of growth can arise only outside this area. In this way physiological isolation of neighboring cells from the dominance of a growing part such as a tentacle, usually in consequence of growth of the organism, may determine the localization and development of another tentacle at a certain distance from the first in the region which reacts most rapidly to the isolation. In short, the principle of physiological isolation applies to the process of repetition of parts as well as to the repetition of whole individuals.

Combinations of the different factors. — It may be noted, in conclusion that a particular case of physiological isolation is not necessarily determined by the action of a single one of the four factors, but two or more of them may be concerned. For example, growth may partially isolate a region and a temporary or permanent decrease in dominance or a direct stimulation of the region, or both may complete the process. Again, conditions bringing about decrease in dominance may also partially block the passage of the correlative factors, and so on. In all probability many cases of physiological isolation in nature represent such combined action of more than one of the four factors. But, however it is brought about in any particular case, the occurrence of physiological isolation shows, not only that dominance is a real physiological factor in development, but also that in the less highly specialized protoplasms it is limited in range and that this range can be controlled and altered in various ways.

In the preceding section it has been shown that physiological isolation plays a fundamental part in the reproduction of new individuals and the reduplication of parts from regions which are originally subordinate parts of an individual. There are, however, certain reproductive processes which involve the dominant region more or less directly, rather than the more distant subordinate parts. In many plants the growing tip divides at certain stages, or periodi-

cally, into two, or sometimes more than \a two growing tips of equal physiological ‘ rank which give rise to two or more similar and equivalent axes. Such division of an axis into two is commonl called dichotomy (> Z and in various | < a alge, liverworts WS a and other forms xs e it is a normal ae form of branching (Fig. 187). In various animals also such equal Fia. 137.—Dichotomy in livernormal developwort, Metzgeria: the growing tip ment or under divides periodically into two grow- 5 ing tips, (a, a,) of equal physiologipathological or eal rank and each of these gives experimental con-

rise to a new axis. sis ditions. It may give rise in some cases to complete and separate individuals, in others to partially double or multiple individuals, e. g., double-headed forms, or to double or multiple parts, such as the reduplicated legs of amphibia (Figs. 127, 128, pp. 126-129), crustacea and various other Sera pene forms. It is a familiar fact that dichotomous ee te ee bee duplication of an axis can be experimentally tial longitudinal splitting. : : otted line indicates approduced in many cases by partial or complete proximate boundary bephysical isolation of the halves of the dominant tween old and regenerated i ° ° : ° : tissue. region. If a planarian is: split longitudinally into two halves, each half reorganizes into a whole animal, or, if we split only the anterior end and prevent the cut surfaces from uniting, each half of the head, or of the separated portion, becomes a whole and a form with two anterior ends results (Fig. 138). Dichotomous division of the posterior end may be accomplished in the same way.

In the sea anemones and various other ccelenterates forms with double apical ends can also be produced by such physical isolation — of the halves of the apical region. The amphibian limb-bud which would normally give rise to a single leg can be made to produce two legs by splitting it longitudinally at an early stage. In various plants dichotomous division of the axis can be experimentally induced by splitting the growing tip longitudinally for a short distance. Numerous other cases might be mentioned, but these suffice to call attention to the fact that such experimental dichotomy is induced by physical isolation from each other of two parts of the dominant or growing region.

It should be noted, however, that the occurrence of duplication does not depend on complete separation of the two halves of the body or part, but rather upon the loss of direct transverse continuity between the halves of the dominant or growing region. The indirect physical continuity between the two halves through the levels basal or posterior to the split is evidently not equivalent to direct transverse continuity, since, in spite of such indirect continuity, the two halves become wholes.