Individuality in Organisms

In all cases developmental gradients of some sort appear sooner or later as expressions of the metabolic axial gradients and usually become more distinct as morphological development proceeds. The so-caUed law of antero-posterior development is a partial recognition of this fact. This ''law" is merely a statement of the observed fact that in the development of the animal from the egg organs first become morphologically \asible in that region which becomes the anterior or apical end, and from this region morphogenesis proceeds posteriorly or basally in a regular, orderly manner. In short, a gradient in morphogenesis exists along the major axis of the body, the apical end preceding. In addition to this major gradient more or less definite

Figs. 10, 11. — Two stages of cleavage of frog's egg, showing axial gradient in cell size resulting from gradient in rate of division. morphogenic gradients appear in relation not only to the minor axes of the whole body, but also in relation to the axes of particular organs or parts. In fact the law of antero-posterior development is merely a statement for the major axis of the more general law of axial developmental gradients Embryonic stages of a flatworm among the invertebrates and the chick among the vertebrates will serve to show these developmental gradients. Fig. 12 is a diagrammatic outline of the adult stage of a small bilaterally symmetrical flatworm, showing "brain,"

pharynx, and alimentary tract; Fig. 13 is a longitudinal section, almost in the median plane, of an embryo of the same species. The anterior end is toward the left. Figs. 12, 13. — Axial developmental gradients in flatworm, Plagiostomitm giradi: Fig. 12, outline of adiJt worm, showing eyes, cephalic ganglia, pharj^nx, and alimentary tract (after von Graff); Fig. 13, longitudinal section near median plane of embr>'o, head at left, showing the apico-basal or longitudinal and ventro-dorsal gradients in rate of development (from Bresslau).

The organs of the anterior end, the brain and pharynx, consist of nmnerous cells, and the morphological arrangement is already apparent, while the whole postpharyngeal region, which in the adult is by far the larger part of the body, is very short and consists of but few cells. This major gradient is very distinct, but the ventrodorsal gradient is also evident. The section shows that multiplication of cells and structural development are proceeding chiefly in the ventral region, while the dorsal region consists of relatively few cells. Examination of transverse sections of embryos would show the transverse gradients: we should find that the development was proceeding more rapidly in the median ventral region than in the lateral regions. The transverse and the ventro-dorsal gradients are in reality different components of the same gradient. The fact is that a developmental gradient extends laterally and dorsally from the median ventral region. In such a bilaterally symmetrical animal there are then two chief developmental gradients, a major, from the anterior region posteriorly, and a minor, from the median ventral, or in some cases most of the ventral region, laterally and dorsally. In other bilaterally symmetrical invertebrates relations are in general similar. In Fig. 2 (p. 38) the relations in a simple case of this sort are diagrammatically indicated.

In the vertebrates the longitudinal gradient is similar to that in the invertebrates, but instead of a ventro-latero-dorsal gradient, as in the invertebrates, the gradient is dorso-latero-ventral in direction. Fig. 14 represents an early stage of the chick embryo in wliich the head is just becoming morphologically distinct, but other organs are not yet formed, while in Fig. 15, a later embryonic stage, the head region is advanced in development, and differentiation of the body is

progressing posteriorly, the successive formation of the somites or segments being a conspicuous feature of this progress. Fig. i6 is a transverse section of an early Figs. 14, 15. — Surface views of two early stages in embryonic development of chick, showing progress of development in basal direction from the head-region (upper end) and laterally from the median region; 5, somites (from F. R. Lillie). stage before distinct organs have begun to form. At this time cells are separating from the outer layer of the body in what will later become the median dorsal

region, and passing inward to form the mesoderm. Most of the region of the embryo behind the head in Figs. 16, 17. — Transverse sections of chick embryo at ditTcrcnt levels, to show developmental gradients. Fig. 14 and the extreme posterior region of the embr\'o in Fig. 15 are at about this stage of development. Fig. 17 is a transverse section at, a stage of development corresponding to that attained at the level of the sixth somite of the embryo in Fig. 15. At this stage the embryonic nervous system is present in the form of a tube open dorsally, and differentiation has progressed both laterally and ventrally from the median dorsa! region. In the other vertebrates, including the mammals, the developmental gradients are similar.

Differences in rate of growth constitute another feature of these developmental gradients, but the relation between the axial metabolic gradient and rate of growth is not simple, for the period of highest growth rate occurs at different times in different parts according to the time of their formation, and it may happen at certain stages of development that the rate of growth at the apical end of a metabolic gradient is lower than at the basal, because the region at the apical end began its growth iirst, has grown at a more rapid rate, and is therefore completing its growth earlier than the region at the basal end. Nevertheless, so far as it is possible to compare corresponding stages in the development of different parts, along an axial gradient, differences in rate of growth corresponding to the gradient do appear. The head-region, for example, at the stage of highest, growth rate grows more rapidly than the posterior region of the body at its stage of highest rate, and similar relations exist with reference to other gradients.

In the egg of the plant as well as in that of the animal developmental gradients usually appear in early stages. In the eggs of many of the lower plants the first division is transverse, the two cells thus formed representing apical and basal regions of the plant, and in most of the plant groups a more or less definite relation exists between the directions o( the early divisions and the major axis of the embryo. In these cases a more or less distinct gradient in division rate, cell size, and cellular constitution usually appears either at the beginning of development or in early stages. On the one hand, this gradient shows a definite relation to the position of the Qgg with respect to surrounding parts of the parent organism, and, on the other, the region of smallest size and most rapid division of the cells and most abundant and deeply staining protoplasm is the region of highest rate of reaction and becomes the apical region of the embryo. Fig. 1 8 shows this gradient in the embryo of a moss, the uppermost cell in the figure representing the apical region of the embryo.

In most of the higher plants only a portion of the egg takes part in the formation of the embryo, the remainder forming a suspensor, a stalk on which the embryo is carried. Fig. 19 shows the cellular gradient in the early developmental stage known as the proembryo of Ginkgo, a gymnosperm related to the conifers. The embryo proper arises later from the small-celled tissue in the lower part of the developing egg. Some of the cycads also show a very definite gradient of this sort. In the angiosperms, the higher seed plants, where the egg is attached to the wall of the embryo-sac, the embryo arises from its free apical end.

A characteristic feature of the plant individual in all except the simplest forms is the growing or vegetative tip. This growing tip is the region of most active nuclear division and growth and with rare exceptions forms the free end of the individual and gives rise to other parts of the plant body. In the complex higher plant, stems, branches, buds, roots, and various other parts possess a growing tip, at least during earlier stages, and each such part is to a certain extent an individual.

Figs. i8, 19. — Axial developmental gradients in embryonic stages of plants: Fig. 18, embryo of moss, apical cell at upper end (from preparation loaned by W. J. G. Land); Fig. 19, proembryo of g>'mnosperm (Ginkgo); apical region of plant arises from lower end (from Lyon). In most of the lower plants a single cell forms the apex or center of the growing tip, and it may be larger than ' other cells with a gradient of decreasing size extending from it, as in the stem of the alga in Fig. 20, but during the course of plant evolution the apical cell gradually

gives place to an apical region, consisting of several or many cells, and in the course of this change the apical cell itself becomes relatively smaller, and a gradient of increasing size extends from the apical region (Figs. i8, 36). The gradients in size in different forms depend Fig. 20. — Axial gradient in ceU size in alga Cladostephiis (from Pringsheim) . upon the relation between frequency of division and growth in size of the apical cell, and this relation shows a characteristic range in each form. Even where the whole plant body is a single multinucleate cell, the apical regions of stem and branches are undoubtedly physiologically growing tips. In the higher plants the

growing tip consists of several or many cells. Figs. 21 and 22, longitudinal sections through the growing tips of a stem and a root respectively, show the gradients in cell Figs. 21, 22. — Axial developmental gradients in growing tips of seed plants: Fig. 21, stem-tip of Hippuris; Fig. 22, root-tip of Tradescantia (from preparations loaned by Department of Botany, University of Chicago). size and protoplasmic condition which extend from the growing tips. In the stem-tip these gradients extend to a much greater distance than in the roottip and Fig. 21 shows only a fraction of them.

In the development of the plant the growing tip is the first part of the individual to become distinguishable, and from it other parts arise. In the moss em]:)ryo in Fig. 18 the growing tip is already present as the uppermost cell and other cells have arisen from it in an orderly way. In the higher plants the growing tip is not usually localized until later stages. In Gingko, for example, the growing tip of the plant is not yet distinguishable at the stage of Fig. 19, although the small-celled region is the growing tip of the whole proembryo and in this the growing tip of the plant-stem later appears. In certain algae the major axial gradient in the egg is apparently determined by external factors, such as light, but in most plants this gradient is determined by the relation of the egg to the parent body, the growing tip of the plantstem arising from the apical region of this gradient.

The vegetative stages of certain liverworts and the sexual generation of various ferns show a high degree of bilateral symmetry and often consist, at least during the earlier stages of their growth, of single elongated flattened individuals (Figs. 23, 24) with a growing tip, a, at one end, often with a thickened longitudinal midrib and with root-like outgrowths on the ventral surface, the surface facing the substratum as the plant grows. In many cases these individuals undergo division by branching or by the formation of buds on the surface in later stages. In these plants, as in bilaterally symmetrical animals, three axes — longitudinal, transverse, and dorso-ventral — are distinguishable; in other words, order is apparent in three directions. Various indications of gradients in activity appear in the same directions. As regards the major axis, the rate of cell division and growth is highest

in the apical region and decreases basally; as the plant grows older, death may even begin at the basal end and proceed apically while the apical end is still growing actively. Evidences of a transverse gradient in activity appear in a decrease in growth toward the lateral margins and in many forms in a decrease in thickness of the body in the same direction. In the direction of the Figs. 23, 24. — Bilaterally symmetrical prothallia of lix^erwort, Marchanlia (dorsal view), and a fern (ventral view).

dorso-ventral axis which is determined by the action of light and perhaps other external factors, the differences in metabolic activity are indicated by the outgrowth of root-like structures and the sexual organs, and in some forms of scales or leaf-lilve structures on the ventral surface, and also in some forms by the greater density of cellular structure iri the ventral region. Among the lower animals and most plants new individuals arise, not only by the process of gametic or sexual reproduction, but by various agamic or asexual processes, such as division, budding, etc. These processes vary greatly in different forms and even in the same individual under different conditions, but their essential feature is the formation of a new individual from a part of a pre-existing individual, a process which usually involves more or less dedifferentiation and redifferentiation in a new direction. Although these agamic reproductive processes differ more or less widely from embryonic development, the metabolic gradients characteristic of the individual either persists from the original individual or arise anew in each case, and developmental gradients of some sort appear in relation to them.

In the formation among animals of new individuals by budding, as, for example, in the hydroid, Pennaria (Figs. 25-27), the hydranth becomes distinguishable first, the stem later, and closer examination shows that apical regions of the hydranth are somewhat in advance of basal. In Figs. 26 and 27, for example, the apical tentacles are more advanced in development than the basal. In the flatworm, Stenostomum, division occurs after the body attains a certain length, the first visible indication of the new individual being the appearance of a new head-region (Fig. 28) at a certain distance from the original head. This new head-region acquires control of parts posterior to it and finally separates as a new animal. By continued division before separation of

each new individual thus formed chains of from eight to sixteen individuals or zooids, as they are usually called, in various stages of development may result (Fig. 29). Many other cases of division among animals are essentially similar. In many of the lower animals agamic reproduction can be induced experimentally by isolating pieces. In the flatworm Planaria (Fig. 30) a piece such as a or b, or almost any other piece, cut from the body will develop into a whole animal of small size by the formation of a new head at one end and a new tail at the other and a transformation and redifferentiation of the internal organs of the piece into those of a whole animal as indicated in Figs. 31-33. In the outgrowth of the new tissue at the two cut surfaces the axial gradients appear as gradients in rate of

Figs. 28, 29. — Asexual reproduction in llatworm, Stcnostomum: Fig. 28, stage of two zooids; Fig. 29, chain of several zooids. In agamic reproduction in plants each new individual arises as a localized region of growth and the growing tip is the first region to become clearly defined. New Figs. 30-34. — Planaria dorotocephala: Fig. 30, structure of alimentary tract and arrangement of central nervous system; a, b, two regions indicating pieces for reconstitution; Figs. 31-33, stages of reconstitution; Fig. 34, side view of early stage.

buds, new roots, and other parts arise in this way in nature and under experimental conditions. The small outgrowths along the sides of the growing stem-tip in Fig. 21 (p. 76) are stages in the formation of leaves and the developmental gradients appear to some extent in them,. In many plants new "adventitious" individuals arise, either in nature or under experimental conditions, from cells already difTerentiated as part of an individual. In the liverwort, Melzgeria, new individuals may arise either by division of the growing tip resulting in bifurcation of the flat body, as shown in Fig. 35, a, a, or after injury to, or removal of, the growing tip by a renewal of division and growth in dilTerentiated cells. Fig. 36 shows the cellular structure of the growing tip in a well-developed individual and Fig. 37 the early stage of a new individual formed from a differentiated cell. In both figures the gradient in cell size is clearly evident.

Among the higher seed plants, as well as among lower forms, the "adventitious" formation of new individuals from differentiated cells occurs, as for example in the begonias, where buds capable of producing new plants arise under certain experimental and natural conditions from the epidermal cells of leaves. The epidermal cells which take part in the formation of such a bud lose their differentiated, vacuolated condition, become filled with protoplasm, like embryonic cells, and divide rapidly. Fig. 38 is a surface view of the formation of such a bud involving several epidermal cells, but centered chiefly in parts of four cells, and Fig. 39 is a longitudinal section through a bud formed from two cells The double contours in Fig. 38 show the thickened cellulose walls of the original epidermal cells, the single contours within them the cells formed by their repeated division,

and the shading indicates in a general way the disappearance of the vacuoles and the filling of the cells with Figs. 35-37. — Metzgeria, a liverwort: Fig. 35, portion of prothallium, showing midrib and apical regions, a, a; Fig. 36, cell structure of growing tip, showing apical cell, a, and gradient in cell size; Fig. 37, cell structure of an adventitious bud, showing apical cell, a, and gradient in cell size (Figs. 36 and 37 from Goebel). protoplasm. A gradient in cell size and protoplasmic condition appears in both cases, in Fig. -^Z from the center to the periphery of the region involved and in

Fig. 39 from the upper part at the free surface of the leaf downward. These gradients are evidently the Figs. 38-41. — Origin of adventitious buds in seed plants: Fig. 3S, surface view and Fig. 39, section of bud arising from dilTerentiated epidermal cells of leaf of Begonia (from Regel); Figs. 40, 41, development of bud in callus (from Simon). visible expression of gradients in metabolic activity, the smallest, most protoplasmic cells indicating the region of most intense activity, and it is from tliis most active

region that the apical vegetative tip of the new plant individual develops. *In many woody plants the cut end of a stem or branch develops a mass of wound tissue, the callus, and in this callus new buds arise independently of other parts of the plant and become connected with them secondarily. In all such cases the differentiation of the vascular bundles which connect the new buds with the old parts proceeds from the buds. Fig. 40 shows an early stage of bud-formation in the poplar at the periphery of a mass of callus on the cut end of a stem, and Fig. 41, a later stage in which vascular connection with other parts has been established. In such cases the appearance of the new bud is the first step in the formation of the new individual ; it is followed by the appearance of a gradient in growth and differentiation from the bud toward other parts.

In isolated pieces of plants the formation of new growing tips or the outgrowth of resting buds occurs in certain more or less definite portions with relation to the axes. The removal of the chief growing tip of a stem results in outgrowth or altered growth of the uppermost buds or branches. When these are removed those lower down react, and so on. Evidently a gradient in the capacity to respond or in the rate of response to the altered conditions exists along the major axis, and those buds or branches which react first dominate those below them and prevent them from reacting in the same way.

In isolated pieces of the bilaterally symmetrical liverworts, such as Marchantia (Fig. 23, p. 78), the position of the new buds evidently represents the region of highest metabolic rate in the piece as a resultant of the three axial gradients (see Figs. 99-102, p. 167), and the formation of new individuals in these regions inhibits their formation elsewhere, although practically every cell of the plant-body is capable under proper conditions of giving rise to a new individual.

All the various lines of evidence considered agree in showing that axial gradients in the dynamic processes are characteristic features of organisms and that a definite relation exists in each individual between the direction of the gradient in any axis and the physiological and structural order which arises along that axis. In the major axis the region of highest rate in the metabolic gradient becomes the apical or anterior region of the individual, and in the minor axes also the regions of highest rate in the gradients represent particular features of the order in each case. Along any axis particular parts apparently represent particular levels in the gradients. The variety, extent, and agreement of the evidence is all the more interesting in view of the fact that such gradients have not heretofore been recognized 2VS characteristic features of organic constitution.

According to the theory outlined in chap, ii, the organic individual is fundamentally a dynamic relation of dominance and subordination, associated with and resulting from the establishment of a metabolic gradient or gradients. In the present chapter some of the evidence for the existence of dominance in the process of individuation is considered. This evidence is obtained primarily from the experimental reproductions, because only here is it possible to analyze and control the process of individuation to any considerable degree. The egg is usually a more or less highly specialized individual at the time embryonic development begins, and the earlier stages of its individuation commonly occur in such relations to the parent body that they are not readily accessible to experimental investigation. Nevertheless, the evidence indicates very clearly that the process of organic individuation is fundamentally the same in the egg and embryo and in experimental reproduction.