Physiological Foundations of Behavior

In many of the lower plants, such as liverworts, in which a single apical cell constitutes the growing tip proper, dichotomy results froin the equal division of this cell in a plane vertical to the plane of the two resulting axes. We know nothing of the intracellular physiological conditions determining such division, but it is evident that they must be different in some way from those which determine the other divisions, for these latter take place in other planes, are usually unequal and give rise, not to growing tips but to the other cells of the thallus. In the dichotomous divisions the plane of dichotomy is commonly the plane of the flattened thallus (Fig. 137), therefore it seems probable that differential] physiological conditions of some sort in definite relation to this plane determine both occurrence and plane of dichotomy.

In plants with multicellular growing tip dichotomy involves the division of the tip into two multicellular tips, but cell division is not necessarily directly concerned. In animals also a dichotomy of the apical or anterior dominant region obviously involves a division of a dominant region or growing tip into two. Except in certain colonial, normally branching forms such dichotomous division of the chief body axes in animals occurs under either pathological. or experimental conditions and gives rise to so-called double monsters, partial, or sometimes complete twins, in short to all degrees of duplication.

The case of the armadillo. — In the nine-banded armadillo investigated by Patterson and Newman and discussed at length with references to the original papers by Newman (717 b, Chaps. TE Lie 23; Chap. VIII), the single embryonic vesicle gives rise normally to four complete embryos. In certain other species of armadillo larger numbers of embryos are produced normally from the single ovum. Since this embryonic division in the armadillo is a normal phenomenon and has become well known through the work of Patterson and Newman and since Newman has advanced a physiological interpretation of it, it serves well as a starting point for interpretation. According to Newman the division is the consequence of a quiescent period in early development. During this period in which development is retarded or inhibited for some unknown reason, the original dominant region loses its dominance because of decreased activity and parts of the embryonic vesicle previously subordinate, or perhaps parts of the dominant region itself, become physiologically isolated. In consequence of such isolation four new radially situated dominant regions arise and develop into four embryos.

In Newman’s earlier account he conceives this process of reduplication as the simultaneous formation of a number of equivalent radially arranged apical regions in the ectodermic vesicle. Two of these favored by position with respect to the bilaterality of the uterus, become the “primary” embryos, two others the “secondary”? embryos, but all four are equivalent as regards origin (Newman, 717 b, pp. 50, 51). According to his later views, however, the process is apparently conceived as consisting of two successive fissions of the vesicle, the direction of the first determined by a differential of some sort associated with the bilaterality of the uterus, that of the second by undetermined conditions (Newman, ’23, Chap. VIII). Patterson (713) regarded the process of reduplication as a double budding and Stockard (’21) has accepted his view. Newman’s contention that the process is rather a double fission than budding is based on the supposed dichotomous character of the divisions without persistence of the original dominant region. As regards the first duplication this’ contention seems well founded for, as far as the evidence goes at present, this duplication appears to be a true dichotomy. As regards the second duplication, however, the evidence is somewhat less conclusive, though it seems to suggest dichotomy, rather than lateral budding with persistence of the dominant region of the primary embryo.

Newman’s assumption of obliteration of an original single dominant region may be correct, but it is not the only way of accounting for the facts. If there is a differential in the uterine environment of the embryonic vesicle such that two lateral regions of it are more favorably situated, for example, as regards respiratory exchange, or other physiological conditions, these two regions may be directly determined as dominant apical regions in spite of the presence of the original dominant region. Such a process of duplication is essentially a process of physiological isolation by “excitation of subordinate parts” (p. 159), except that the parts accelerated in this case may be parts of the original dominant region. If the first duplication in the armadillo occurs in this way, the original dominance is obliterated, not by the quiescent period, but directly by the bilateral differential in the uterus. Very probably both factors are concerned. The chief purpose of this discussion is to call attention to the fact that the data of observation, even in a case so extensively investigated as this of the armadillo, do not yet permit us to reach definite conclusions concerning the physiological factors involved, but merely Serve as a basis for pointing out the possibilities, It appears that we do not certainly know whether this case of double twinning represents two successive dichotomies or a simultaneous tetratomy or a polytomy with persistence of four members. Nor is it certain whether one, or both of the duplications, if there are two, results from a loss of dominance and physiological isolation during a quiescent period, or whether the two primary, or all four polarities are directly determined by local differentials in spite of a preéxisting original polarity.

As the facts stand, they suggest that the axes of the two primary embryos are directly determined by the bilateral environmental differential in the uterus, but whether after obliteration of an original dominance by a quiescent period, or in spite of it, I do not think we can as yet determine. In any case, however, this interesting embryonic reduplication, in spite of its uniformity and constancy, cannot be accounted for in terms of heredity alone. It represents the behayior under certain physiological conditions of a specific protoplasm with certain hereditary potentialities.

Other processes of dichotomy and twinning. — Turning to other cases of dichotomous duplication, twinning, ete., in multicellular forms, it is evident that we know no more concerning the physiology of the process than in the case of the armadillo. Whenever a growing tip or an axis divides dichotomously, the position of the two new tips or axes with reference to each other must be determined either by a differential of some sort, such as a double Symmetry gradient already present in the region concerned, or by some differential in en-

_ vironmental conditions within the organism or external to it, which localizes two active regions instead of one. The physiological processes concerned in dichotomy may differ in different cases. In dichotomy in the lower plants there is usually no evidence that any inhibition, quiescent period, or loss of dominance is necessary. The apical cell divides equally, the division plane coinciding with the median plane of the thallus, and each half remains an apical cell. The plane of division is usually definitely related to the bilaterality of the thallus, which in turn may be determined originally by light or by some other environmental factor. In plants with multicellular growing tip and in multicellular animals dichotomy is not primarily a matter of cell division, but involves changes in rate of activity in cell masses, and essentially the same questions arise as in the case of the armadillo.

It appears to be true that flattened growing tips and blastoderms present conditions relatively favorable for the occurrence of di- Fics. 139-141.—Diagrams illustrating dichotomy of a growing region in consequence of growth and flattening in a plane at right angles to the main axis. Further explanation in text. chotomy in the plane of the flattening. A brief consideration will serve to indicate the conditions which may be concerned in such a case. A growing region of this sort may be at first radially symmetrical (Fig. 139), but differential growth, determined either by its relations to other parts of the organism, or directly by some external differential, brings about a flattening at right angles to the polar axis (Fig. 140). Such flattening may itself determine a differential in the growing region between the middle portion, c, and the two lateral regions, a and b (Fig. 140). The regions a and b are more favorably situated than c as regards nutrition, since they are nearer the less active cells of lower levels from which they can obtain food, while c is surrounded on all sides by cells with a high rate of metabolism and growth. As regards respiratory exchange also the conditions are more favorable at a and b than at ¢, particularly if the surface of these regions is curved as indicated in the diagram. If such a differential becomes sufficient, the activity of c may in time be somewhat inhibited,

while a and 6 retain or increase their activity and so become distinct growing and dominant regions (Fig. 141, a, b), separated by a less active region, c. In this way a true dichotomy may result from differential growth of a dominant region determined by conditions acting at right angles to the polar axis. It seems probable that many of the dichotomies which occur normally or occasionally in nature are of this sort, the differential transverse growth of the dominant region being determined by different factors in different cases. In certain plants, for example, dichotomy occurs at right angles to the direction of incident light, while in animals other conditions, probably in many cases conditions connected with respiratory exchange, are concerned.

Moreover, if a flattened dominant region is inhibited by an external chemical or physical agent and then undergoes acclimation or Figs. 142, 143.—Diagrams of early developmental stages of fish to illustrate dichotomy of anterior end of embryonic area: Fig. 142, normal; Fig. 143, dichotomy. recovery, it is evident that conditions are more favorable for such acclimation or recovery at a and b than at c (Fig. 140). Consequently dichotomy of an axis may sometimes be brought about experimentally by the action of inhibiting factors.

The occurrence of different degrees of dichotomy and twinning as a teratological phenomenon in the embryonic development of fishes has been noted by various investigators. Such dichotomy gives rise to all degrees of twinning, from double-headed monsters to complete normal twins. Recently Stockard (21) has described such dichotomies experimentally determined by inhibiting conditions and has interpreted them in terms of loss of dominance and physiological isolation of a subordinate region with budding. Even though some eases of unequal twins may be accounted for in this way, the frequent cases of equal twins present difficulties to this interpretation. It seems much more probable that these are true dichotomies and that they

‘arise in some one of the ways described above. The fish embryo presents exceedingly favorable conditions for the occurrence of such dichotomy. The dominant region of the embryo develops as a broad flattened area upon the yolk (Fig. 142), 7. e., it is normally flattened transversely and is therefore a favorable substratum for the appearance of dichotomy in the manner indicated in Figures 140 and 141. Stockard has produced such dichotomous fish embryos by low temperature and by insufficient oxygen. It is evident at once that in the flattened embryonic area the differential exposure of lateral borders and median. portion of the dominant region may determine physiological differentials like those indicated in Figure 141 above. Something of this sort apparently happens in many cases. The originally single dominant region becomes two, not simply because of loss of dominance of one region over another, but because the shape of the dominant region favors the appearance of a differential between lateral and median portions and this differential determines two dominant regions (Fig. 143) and consequently a greater or less degree of twinning, according to conditions.

It seems to be true that those forms in which the dominant region in early stages is a flattened area spread out over yolk give rise more frequently to such dichotomies than other forms. In the chick, for example, such dichotomous twinning is not infrequent. There may be other conditions not yet recognized which also favor dichotomy, but, taking the facts as they stand, the shape of the dominant region appears to be a factor of considerable importance in the occurrence of dichotomy in both plants and animals, consequently the conditions determining the shape must be regarded as conditions predisposing to dichotomy.

In many cases of complete twinning from a single ovum the embryos may be more or less widely separated from each other. The possibility may be pointed out that in some such cases duplication may have been brought about in some way at a stage before a single embryonic area was established, rather than by fission of such an area. What’ conditions have been concerned in such cases and whether such twins represent physiological isolation in consequence of inhibition of a single dominant region in the early blastoderm or earlier, or direct determination of axes by environmental differentials in spite of an original dominance, we do not know.

Situs inversus viscerum and mirror imaging in twinning. — In partial duplications in vertebrates it is often found that the usual visceral asymmetry is reversed in one of the components. Mirror imaging in the reduplication of amphibian limbs has been discussed in the preceding chapter. In his recent book Newman (23, Chaps. XII-XIV) has described and figured numerous cases of these symmetry relations, consequently detailed description is unnecessary here. We have as yet no experimental evidence for analysis of these relations and little more than surmise is possible as regards the conditions concerned (see p. 127). I wish, however, to point out that both situs inversus and mirror imaging indicate that the components concerned affect each other or are affected by a common factor. In short, the symmetry relations, whether determined by bioelectric, or by other conditions associated with the physiological gradients, represent in each case a reaction of the specific protoplasm to environmental conditions.

The problem of polytomy. — It is a question of some interest whether a number of new axes, either of individuals or of organs, with a definite localization and arrangement, can arise simultaneously through polytomy of a single dominant region or of a general sub- | stratum. Many apparent cases of such polytomy occur. In many plants, for example, branches, the parts of flowers, etc., appear in whorls and among animals the tentacles of ccelenterates and the rays of the starfish apparently represent equivalent parts which, so far as appearances are concerned, must have originated simultaneously. A priori, the simultaneous orderly origin and localization of such parts in a definite and constant pattern appears physiologically impossible without preéxisting differentials of some sort determining the radius in which each part shall arise. But when we examine the origin of such parts more closely we find that, at least in many cases, they do not arise simultaneously. Among plants many whorls and multiple radial parts are known to originate as compressed axes, the radial arrangement and equivalence being secondary. That is to say, the parts arise successively in a regular order and sequence in relation to an extremely short axis. There is considerable evidence of similar conditions in animals. In the development of certain sea anemones the tentacles do not appear simultaneously, but in a definite order and position with respect to each other. In the buds of hydra also and in pieces undergoing reconstitution it can often be seen that the tentacles do not appear simultaneously, but in order, and small isolated pieces often give rise to one tentacle only. In many other cases among the ccelenterates there js at present no indication of such a definite order in appearance of tentacles and the process of their localization appears to be a true polytomy. In the reconstitu-

tion of a piece of Tubularia stem, for example, the tentacle ridges seem to appear simultaneously about the whole circumference (Figs. 67-69) and in the development and reconstitution of hydranths in various other hydroids the tentacle buds seem to arise simultaneously about the whole circumference (Figs. 91-98, 117-121). If these processes are true polytomies, as they appear to be, and not successive buddings, we have as yet no basis for physiological inter- ' pretation of them. As regards the echinoderms, the earlier development shows clearly enough that the more or less equiradiate form of later stages is secondary and not the result of a true polytomy.

Polytomy in anaxiate forms is of frequent occurrence, ¢. 9., the multiple cell divisions in protozoan and other cysts, but in such cases we merely have the localization of cell boundaries with relation to the positions of nuclei rather than the origin of new axes. It is perhaps still a pertinent question whether true polytomy, 7. e., simultaneous multiple division of an axis, does occur in any case of orderly and definite axial multiplication, and if it does occur, we have then to inquire how the particular axes are localized and how the arrangement of the whole series of axes, ¢. g., a ring of tentacles, is determined.

If the conception of dominance and physiological isolation discussed in this chapter is correct, it follows that unless other factors are concerned, the length of any individual axis must be determined by the range of dominance. If the length of the axis increases beyond the range of dominance, or if the range of dominance decreases below the length of the axis, or if dominance is otherwise interfered with, physiological isolation occurs and a new individual may arise. In the plants and the simpler animals these are apparently the chief factors in determining the size of the individual and, as might be expected, individual size shows very wide variation according to conditions.

Another factor or group of factors is concerned in determining size, particularly in the more highly specialized organs and organisms. This is the decrease and cessation of growth with the progress of differentiation. Even in the plants such organs as leaves and parts of flowers usually lose the ability to grow during the course of development, though in some cases it reappears after processes of dedifferentiation, as in adventitious buds from epidermal cells of leaves. In animals which do not reproduce asexually the rate of

growth decreases during later development and increase in size finally ceases or becomes very slow. - There is no doubt that in animals with well-developed nervous system the range of dominance increases greatly in the course of development and sooner or later exceeds by far the actual size of | even the largest individual. The nerves of the higher, if not of all vertebrates, are capable of conducting impulses to very great, perhaps to indefinite distances (see Chap. XI), but the size of the vertebrate body is very definitely limited. The limiting factors in these cases are the factors which limit growth, not those which limit dominance.

Even in the higher animals, however, the range of dominance may be very low in early developmental stages and particularly during the development of the nervous system the increase in range must be very great. If this is true, physiological isolation may occur in early stages, even though the individual is of very small size, and may be impossible later, although the size is much larger. For example, segmentation, which is probably a limited reproductive process in which a certain degree of physiological isolation is concerned, may occur, even in the higher vertebrates in early developmental stages, but not in advanced life.

Nevertheless certain cases of apparent physiological isolation do occur in mature life and particularly in old age, viz., the neoplasms, especially the malignant forms. In these the isolating factor seems usually, if not always to be an irritation of the cells concerned, 7. e., it is isolation by local action on the subordinate part (see p. 159), though perhaps some decrease in the effectiveness of dominance with advancing age may also play a part. In some cases the cells of neoplasms show more or less differentiation, but in the extreme malignant type they grow and divide indefinitely and this indefinite growth constitutes their malignancy. Whether such growth is the result of continued action of the factor which initiated the growth, or whether the cells have lost their ability to differentiate, is not certain. The point of chief importance for present purposes is that physiological isolation is apparently concerned in the initiation of many, if not all sorts of neoplasms, as well as in normal reproductive processes.

The evidence along both morphological and physiological lines indicates that the sex cells are as much a part of the body as other cells, that the genesis of the egg and spermatozodn from the primitive germ cell is a process of differentiation and of senescence comparable to that occurring in other body cells, and finally, that the fully developed gamete, whether male or female, is a physiologically old cell, approaching death, and that the initiation of development leads to regressive development and rejuvenescence (Child, ’15 b, Chaps.

The sex cell may retain physical continuity with the protoplasm of other body cells up to a late stage, or it may lose this connection early and undergo its later development in the fluid of the gonad, or, as in many annelids, in the body cavity. Its loss of continuity with other cells seems at first glance to be a process quite different - from physiological isolation. Certainly neither growth beyond the range of dominance nor decrease in dominance are involved and it seems highly improbable that dominance is blocked from the sex organ. If physiological isolation is concerned apparently it must occur through local action on the subordinate part, 7. e., the sex cell. It seems to be true that the loss of direct continuity with other cells follows the loss of physiological significance of such continuity for the growing egg or sperm. In many annelids, for example, connection of the egg with other cells has no appreciable nutritive or respiratory function and is lost very early, the fluid of the body cavity serving as nutritive and respiratory medium. In many other forms eggs receive their nutrition through the region of attachment, and con- _-tinuity persists through the whole erowth period, as in the case of Sternaspis (Fig. 63), the sea urchin and starfish, most ccelenterates and many other forms. In the case of the male cell the nutritive requirements are usually much less than for the egg and the fluids of the gonad adequate as nutritive and respiratory medium, consequently continuity with other cells may be lost very early. According to this view the isolation of the sex cells within the parent body is essentially the atrophy or interruption of a connection which has become functionless or nearly so, and it is the environment of the sex cell, 7. e., the nutritive and respiratory environment, which has made the connection functionless.

If this interpretation is correct, the isolation of the sex cell may be regarded as a physiological isolation resulting from direct local action on the cell. The extrusion of the sex cells from the parent body is perhaps in its original, purely physiological significance a process of getting rid of waste material. The sex cells have completed their development and are approaching death from old age: any endocrine, or other functional relations to other parts of the body which they possess in earlier stages have reached a minimum and when they accumulate to a certain degree, or under the action of certain external stimuli, they are cast off. In the mammal, however, the fertilized egg enters into a new functional relation with the parent body and profound modification results. Here again, however, a change in the significance of these functional relations results in the separation of embryo and parent.

The question of the significance of excitation and transmission as the primary correlative factor in organismic integration has already been touched upon in Chapter V and we have seen in later chapters that the axial physiological gradients, the physiological axes of individuals, exhibit primarily all the physiological characteristics of excitation-transmission gradients and that they may be determined by local or differential excitation or apparently by any environmental differential which gives rise to a persistent local difference in rate of fundamental metabolism (Chap. IX). In “The Origin and Development of the Nervous System” the various lines of evidence were brought together which indicate that the physiological conditions determining the structural and functional origin and development of the nervous system in the individual themselves originate in excitation and transmission. Certain aspects of the problem of excitation were considered in Chapter IV of that book, but for present purposes a somewhat more extended survey, particularly of the development of present conceptions of excitation, transmission and the more highly specialized conduction seems necessary.

In the consideration in Chapter V of excitation and transmission as factors in physiological correlation, attention was called to the following characteristics of the excitation-transmission change in protoplasms: first, all living protoplasms are to some extent irritable or excitable and capable of transmission. Second, excitation is a dynamic change involving increased energy-liberation in the protoplasmic system. Third, it is initiated by the impact from without of some form of energy upon the protoplasm excited | and the relation between the exciting factor and the process of excitation is non-specific, in other words, the same excitatory changes may be induced by different forms of external energy. Fourth, transmission

1 The question whether or to what extent “‘self-excitation”” may occur and whether the so-called automatic tissues are really self-exciting is considered results from the fact that an excited region is capable of inducing in some way excitation in adjoining regions within a certain distance. Fifth, in non-nervous transmission and in nervous transmission under — certain conditions a decrement in intensity or energy, in short of physiological effectiveness, of the excitation very generally, if not always, occurs. In other words, the excitatory change loses in effectiveness in the course of its progress, so that finally at a greater or less distance from the point of origin, it is no longer effective in ex- — citing further points and so transmission ceases. And finally, no specialized structure of any sort beyond that of a living protoplasm with limiting surface is necessary for the occurrence of excitation and transmission. The excitation itself makes the excited region different at least temporarily from unexcited regions and the transmission of excitation brings different and often widely distant regions into correlation. The action of the external exciting factor determines regional differences and relations of organismic character and magnitude in the protoplasm. In short, as pointed out in Chapter V and in “The Development of the Nervous System” (Chap. V) excitation and its transmission, apparently give rise to a temporary pattern and relation of the simplest and most primitive organismic character possible between regions or parts, and this pattern is in its simplest terms a physiological gradient. So far as determined, this gradient differs from the axial physiological gradients of axiate organisms chiefly as. being less permanent and more immediately dependent on the action of the external factor. Apparently the excitation-transmission gradient and the physiological axis in its simplest terms are fundamentally similar or identical phenomena in living protoplasms.