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MORPHOLOGY (Gr. yopdsil, form)

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Originally appearing in Volume V18, Page 867 of the 1911 Encyclopedia Britannica.
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MORPHOLOGY (Gr. yopdsil, See also:form)  , a See also:term introduced by See also:Goethe to denote in See also:biology the study of the unity of type in organic See also:form (for which the Linnaean term " See also:Metamorphosis " had formerly been employed) . It now usually covers the entire See also:science of organic form . There are numerous restricted senses of the term in various sciences, but here we shall See also:deal with it as a substantive See also:side of See also:zoology and See also:botany . See also:Historical Outline.—If we disregard such vague likenesses as those expressed in the popular classifications of See also:plants by See also:size into herbs, shrubs and trees, or of terrestrial animals by See also:habit into beasts and creeping things, the See also:history of See also:morphology begins with See also:Aristotle . Founder of See also:comparative See also:anatomy and taxonomy, he established eight See also:great divisions (to which are appended certain See also:minor See also:groups)—Viviparous Quadrupeds, Birds, Oviparous Quadrupeds and Apoda, Fishes, Malakia, See also:Malacostraca, Entoma, and Ostracodermata—distinguishing the first four groups as Enaima (" with See also:blood ") from the remaining four as Anaima (" bloodless ") . In these two divisions we recognize the See also:Vertebrata and Invertebrata of J . B . P . A . See also:Lamarck, the first four groups corresponding with the Mammals, Birds, See also:Reptiles, Fishes, whilst the others agree more loosely with the Cephalopods, See also:Crustacea, Insecta, and Echinoderms with See also:Mollusca other than the Cephalopods . Far from committing the See also:mistake attributed to him of reckoning Bats as Birds, or Cetaceans as Fishes, he discerned the true See also:affinities of both, and erected the latter into a See also:special yfm'or beside the Viviparous Quadrupeds, more on See also:account of their See also:absence of limbs than of their aquatic habit . Not only is his method inductive, and his groups founded on the aggregate of known characters, but he foreshadows such generalizations as those of the correlation of See also:organs, and ofthe progress of development from a See also:general to a special form after-wards established by G .

L . See also:

Cuvier and K . E. von See also:Baer respectively . In the See also:correspondence he suggests between the scales of Fishes and the feathers of Birds, or in that hinted at between the fins of Fishes and the limbs of Quadrupeds, the See also:idea of homology is nascent; and from the compilation of his See also:disciple Nicolaus of See also:Damascus, who regards leaves as imperfectly See also:developed fruits, he seems almost to have anticipated the idea of the metamorphosis of plants . Even after the reappearance of Aristotle's See also:works in the 13th See also:century, little can be recorded but revivals of his conclusions . Monographs on groups of plants and animals frequently appeared, those of P . See also:Belon on Birds and G . Rondelet on Fishes being among the earliest; and in the former of these (1555) we find a comparison of the skeletons of See also:Bird and See also:Man in the same posture and as nearly as possible See also:bone for bone—an idea which, despite the contemporaneous See also:renaissance of human anatomy initiated by Vesalius, disappeared for centuries, unappreciated See also:save by the surgecn Ambroise See also:Pare . B . See also:Palissy, like Leonardo da See also:Vinci before him, discerned the true nature of fossils; and such flashes of insight continued to appear from See also:time to time during the 17th century . Thus, See also:Joachim See also:Jung recognized " the distinction between See also:root and See also:stem, the difference between leaves and foliaceous branches, the transition from the See also:ordinary leaves to the folia See also:floris," and W . See also:Harvey anticipated the generalizations of See also:modern See also:embryology by his researches on development and his theory of epigenesis .

The encyclopaedic See also:

period of which See also:Gesner is the highest representative was continued by See also:Aldrovandi and others in the 17th century; but, aided by the Baconian See also:movement, then influencing all scientific minds, it developed into one of genuinely systematic aim . At this See also:stage of progress the most important See also:part was taken by See also:John See also:Ray, whose classificatory labours among plants and animals were crowned with success . He first expelled the fabulous monsters and prodigies of which the encyclopaedists had handed on the tradition from See also:medieval times, and succeeded, particularly among plants, in distinguishing many natural groups, for which his own terms sometimes survive—e.g . See also:Dicotyledons and Monocotyledons, See also:Umbelliferae and See also:Leguminosae . The true precursor of See also:Linnaeus, he introduced the idea of See also:species in natural history, and reformed the practice of See also:definition and terminology . Of the works which followed up Ray's systematic labours, none can be even named until we come to those of his great successor Linnaeus, whose grasp of logical method and lucidity of thought and expression enabled him to reform and reorganize the whole labours of his predecessors into a compact and definite " systema naturae." The very See also:genius of See also:order, he established modern taxonomy, not only by the introduction of the See also:binomial nomenclature and the renovation of descriptive terminology and method, but by the subordination of the species under the successive higher categories of genus, order and class, so reconciling the See also:analytic and synthetic tendencies of his predecessors . Although the See also:classification of plants by the number of their essential organs is highly artificial, it must be remembered that this artificiality is after all only a question of degree, and that he not only distinctly recognized its provisional See also:character but collected and extended those fragments of the natural See also:system with which A. de See also:Jussieu soon afterwards began to build . His classification of animals, too, was largely natural, and, though on the whole he See also:lent his authority to maintain the notion of three kingdoms of nature, he at least at one time discerned the fundamental unity of animals and vegetables, and See also:united them in opposition to the non-living See also:world as Organisata . At the same time he was still far more a scholastic naturalist than a modern investigator . While the artificial system was at the See also:zenith of its usefulness, See also:Bernard de Jussieu was arranging his gardens on the lines afforded by the fragmentary natural system of Linnaeus . His ideas were elabcrated by his See also:nephew See also:Antoine de Jussieu, who published diagnoses of the natural orders, so giving the system its modern character . Its subsequent elaboration and definite See also:establishment are due mainly to the labours of Pyrame de See also:Candolle and See also:Robert See also:Brown .

The former concentrated his own See also:

long See also:life and that of his son upon a new " systema naturae," the See also:colossal Prodromus systematis naturalis (20 vols., 1818-1873), in which 8o,000 species were described and arranged . Mean-while the penetrative genius of Brown enabled him to unravel such structural complexities as those of Conifers and Cycads, See also:Orchids and Proteaceae, thus demonstrating the possibility of ascertaining the systematic position of even the most highly modified floral types . Both Candolle and 3rown were thus no See also:mere systematists, but genuine morphologists of the modern school . The labours of Bernard and Antoine de Jussieu initiated a parallel advance in zoology, the See also:joint memoir on the classification of mammals with which Cuvier and See also:Geoffroy St-Hilaire almost began their career receiving its dominant impulse from the " genera " of Antoine . Cuvier's works correspond in zoology to those of the whole period from the Jussieus to Brown, and epitomize the results of that See also:line of advance . Although in some respects preceded by A. von See also:Haller and J . See also:Hunter, who compared, though mainly with physiological aim, the same parts in different organisms, and much more distinctly by Vicq d' Azyr, the only real comparative anatomist of the 18th century, he opens the era of detailed anatomical See also:research united with exact comparison and clear generalization . The Regne See also:animal (1817) and the theory of types (vertebrate, molluscan, articulate, and radiate) are the results of this See also:union of See also:analysis and See also:synthesis and See also:mark the reconstitution of taxonomy on a new basis, henceforth to be no longer a See also:matter of superficial description and nomenclature but a See also:complete expression of structural resemblances and See also:differences . In See also:Germany, L . H . Bojanus, J . F .

Meckel, C . T . E. von See also:

Siebold and Johannes See also:Muller, with his many pupils, carried on the See also:work; in See also:France, too, a See also:succession of brilliant anatomists, such as A . De Quatrefages, A . Milne-See also:Edwards and H. de Lacaze-Duthiers, were his intellectual heirs; and in See also:England he has been admirably represented by See also:Sir R . See also:Owen . It is now necessary to return to Linnaeus, whose more speculative writings contain, though encumbered by fantastic hypo-theses, the idea of floral metamorphosis . About the same time, and quite independently, C . F . See also:Wolff, the embryologist, stated the same theory with greater clearness, for the first time distinctly reducing the plant to an See also:axis bearing appendages—the vegetative leaves—which become metamorphosed into bud-scales or floral parts through diminution of vegetative force . See also:Thirty years later the same view was again independently developed by Goethe in his now well-known pamphlet (Versuch See also:die Metamorphose der Pflanzen zuerklaren, See also:Gotha, 1790) . In this brilliant See also:essay the See also:doctrine of the fundamental unity of floral and foliar parts is clearly enunciated, and supported by arguments from anatomy, development and teratology .

All the organs of a plant are thus modifications of one fundamental See also:

organ—the See also:leaf—and all plants are in like manner to be viewed as modifications of a See also:common type—the Urpflanze . Whether, as some historians hold, his " Urpflanze " was a mere ideal archetype, bringing forth as its See also:fruit the innumerable metaphysical abstractions of the Naturphilosophie, and leading his countrymen into all the extravagances of that system; or whether, as E . H . See also:Haeckel maintains, it represented a See also:concrete ancestral form, so anticipating the view of modern evolutionists, it is certain that to him F . W . S. von See also:Schelling was indebted for the See also:foundation upon which he erected his philosophic edifice, as also that Goethe shared the same ideas . It must be remembered that he lived and made progress for See also:forty years after the publication of this essay, that he was See also:familiar with the whole scientific movement, and warmly sympathized with the evolutionary views of Lamarck and Geoffroy St-Hilaire; it is not therefore to be wondered at that his writings should furnish See also:evidence in favour of each and every See also:interpretation of them . His other morphological labours must not be forgotten . Independently of Vicq d' Azyr, he discovered the human premaxillary bone; independently of L . See also:Oken, he proposed the vertebral theory of the See also:skull; and before S . C . See also:Savigny, he discerned that the jaws of See also:insects were the limbs of the See also:head .

In 18r3 A . P. de Candolle published his Theorie elementaire de la botanique, which he developed into the classic Organographie vegetale (1827) . He established his theory of symmetry, reducing all See also:

flowers to " symmetrical " groupings of appendages on an axis and accounting for their various forms by cohesion and See also:adhesion, by arrested or excessive development . The next advance was the investigation by W . P . Schimper and A . Braun of phyllotaxis-the ascending See also:spiral arrangement of foliar and floral organs—thus further demonstrating their essential unity . The term morphology was first introduced by Goethe in 1817, in a subsequent essay (Zur Naturwissenschaft uberhaupt, besonders zur Morphologic) . It did not come into use in botany until its popularization by Auguste de St-Hilaire in his Morphologic vegetale (1841), and in zoology until later, although De See also:Blainville, who also first employed the term type, had treated the See also:external forms of animals under " morphologie . Though the Naturphilosophie of Schelling and its countless modifications by his followers, its mystic theories of " polarization " and the like, its apparatus of See also:assumption and See also:abstraction, See also:hypothesis and See also:metaphor, cannot here be discussed, its undoubted services must not be forgotten, since it stimulated innumerable reflective minds to the See also:earnest study of natural science, gave a powerful impulse to the study of comparative anatomy and vindicated the claims of philosophic synthesis over those of analytic See also:empiricism . Among its many adherents, some are of more distinctly theological type; others metaphysical, others mystical or poetic, others, again, more especially scientific; but its most typical and picturesque figure is Lorenz Oken, who epitomizes alike the best and the worst features of the school, and among whose innumerable pseudo-morphological dreams there occasionally occurred suggestions of the greatest fruitfulness—notably, for instance, the See also:independent statement of the vertebral theory of the skull . By far the most distinguished anatomist of the transcendental school is Geoffroy St-Hilaire, who being comparatively See also:free from the extravagances of Oken, and uniting a See also:depth of morphological insight scarcely inferior to that of Goethe with greater knowledge of facts and far wider See also:influence and reputation in the scientific world, had greater influence on the progress of science than either .

He started from the same studies of anatomical detail as Cuvier, but, influenced by See also:

Buffon's view of unity of See also:plan and by the evolutionary doctrines of Lamarck, diverged into new lines, and again reached that idea of serial homology of which we have so frequently noted the independent origin . His greatest work, the Philosophic anatomique (1818-1823), contains his See also:principal doctrines . These are: (1) the theory of unity of organic See also:composition, identical in spirit with that of Goethe; (2) the theory of analogues, according to which the same parts, differing only in form and in degree of development, should occur in all animals; (3) the " principe See also:des connexions," by which similar parts occur everywhere in similar relative positions; and (4) the " principe du balancement des organes," upon which he founded the study of teratology, and according to which the high development of one organ is allied to diminution of another . The advance in morphological theory is here obvious; unfortunately, however, in eager pursuit of often deceptive homologies, he wandered into the See also:transcendentalism of the Naturphilosophie, and seems utterly to have failed to appreciate either the type theory of Cuvier or the discoveries of Von Baer . He defended Buffon's and See also:Bonnet's earlier view of unity of plan in nature; and the controversy reached its See also:climax in 183o, when he maintained the unity of structure in Cephalopods and Vertebrates against Cuvier before the See also:Academy of Sciences . On the point of fact he was of course utterly defeated; the type theory was thenceforward fully accepted and the Naturphilosophie received its See also:death-See also:blow . Such was the popular view; only a few, like the aged Goethe, whose last See also:literary effort was a masterly critique of the controversy, discerned that the very See also:reverse interpretation was the deeper and essential one, that a veritable " scientific revolution " was in progress, and that the supremacy of homological and synthetic over descriptive and analytic studies was thenceforward assured . The irreconcilable See also:feud between the two leaders really involved a reconciliation for their followers; theories of homological anatomy had thenceforward to be strictly subjected to anatomical and embryological verification, while anatomy and embryology acquired a homological aim . This union of the solid matter and rigorous method of Cuvier with the generalizing spirit and philosophic aims of Geoffroy is well illustrated in the works of Owen . The further See also:evolution of the idea of homology is sketched below, while the extent and rapidity of the subsequent progress of the knowledge of all the structural aspects of plants and animals alike make a historical survey impossible up to the See also:appearance of the Origin of Species (1859) . The needful See also:solution was effected by See also:Darwin . The " Urpflanze " of Goethe, the types of Cuvier, and the like, at once became intelligible as schematic representations of ancestral organisms, which in various and varying environments, have undergone differentiation into the vast multitude of existing forms .

All the enigmas of structure become resolved; " representative " and " aberrant," " progressive " and " degraded," " synthetic " and "isolated," " persistent " and " prophetic " types no longer baffle comprehension; conformity to type represented by differentiated or rudimentary organs in one organism is no longer contradicted by their entire disappearance in its near See also:

allies, while systematist and morphologist become related simply as specialist and generalizer, all through this See also:escape from the Linnaean See also:dogma of the fixity of species . The phenomena of individual development receive interpretation in terms of ancestral history; and embryology thus becomes divided into ontogeny and phylogeny—the latter, too, coming into intimate relation with palaeontology—while classification seeks henceforth the reconstruction of the genealogical See also:tree . All these results were clearly developed in Haeckel's Generelle Morphologic (1866), while the valuable contemporaneous Principles of Biology of See also:Herbert See also:Spencer also gave special See also:attention to the relation of morphology to See also:physiology . Individuality.—Probably no subject in the 'whole range of biology has been more extensively discussed than that of the nature of organic individuality . The history of the controversy is of See also:interest, since besides leading up to solid results it serves, perhaps better than any other See also:case, to illustrate the slow emergence of the natural sciences from the influence of scholastic thought . Starting from the obvious unity and indivisibleness of Man and other higher animals, and adopting some definition such as that of C . F . B . Mirbel, " Tout titre organise, complet dans ses parties, distinct et separe des autres titres, est un individu," it was attempted times without number to discover the same conception elsewhere in nature, or rather to impose it upon all other beings, plants and animals alike . The results of different inquirers were of course utterly discrepant . It seemed easy and natural to identify a tree or See also:herb corresponding to the individual animal, yet difficulties at once arose . Many apparently distinct plants may arise from a common root, or a single plant may be decomposed into branches, twigs, shoots, buds or even leaves, all often capable of See also:separate existence .

These, again, are decomposable into tissues and cells, the cells into See also:

nucleus, &c., and ultimately into protoplasmic molecules, these finally into atoms—the inquiry thus passing outside organic nature altogether and See also:meeting the old dispute as to the ultimate divisibility of matter . In See also:short, as Haeckel remarks, scarcely any part of the plant can be named which has not been taken by some one for the individual . It is necessary, therefore, briefly to See also:notice some of the principal works on the subject, and these may conveniently be taken in descending order . While H . See also:Cassini practically agreed with Mirbel in attempting to regard separate plants as individuals, the widest interpretation of the individual is that of G . Gallesio (1816), who proposed to regard as an individual the entire product of a single See also:seed, alike whether this developed ihto a uni-axial plant extended continuously like a See also:banyan, or multiplied asexually by natural or artificial means like the weeping-See also:willow or the See also:Canadian See also:pondweed, of each of which, on this view, there is only a single individual in See also:Britain, happily discontinuous . At once the See also:oldest and most frequently maintained view is that which regards the bud or shoot consisting of a single axis with appendages as the plant-individual, of which the tree represents a See also:colony, like a branched hydroid See also:polyp . This conception, often attributed to Aristotle, but apparently without foundation, appears distinctly in the writings of See also:Hippocrates and See also:Theophrastus—the latter saying, " The bud grows on the tree like a plant in the ground." The See also:aphorism of Linnaeus, " Gemmae totidem herbae," is. well known ; and in this view C . F . Wolff and See also:Humboldt concurred, while See also:Erasmus Darwin supported it by an See also:appeal to the facts of anatomy xvIII . 28and development . The most influential See also:advocate of the bud theory during the first See also:half of the 19th century was, however, Du See also:Petit-See also:Thouars, who, although starting much as usual with a " principe unique d'existence,"• supported his theory on extensive though largely incorrect observations on stem structure and growth .

For him the tree is a colony of phytons, each being a bud with its axillant leaf and fraction of the stem and root . Passing over numerous minor authors, we come to the central work of Alex . Braun (1853), in which, as See also:

Sachs has clearly pointed out, the illegitimate See also:combination of Naturphilosophie with inductive morphology reaches its extreme . He reviews, however, all preceding theories, admits the difficulty of fixing upon any as final, since the plant, physiologically considered, is rather a dividuum than an individuum, and proposes as a See also:compromise, or indeed as a partial cutting of the See also:knot, the See also:adoption of the shoot, as the morphological individual, comparable to an animal, especially because, unlike the See also:cell, leaf, &c., it includes all the representative characters of the species . Darwin and Spencer on the whole also accept the bud or shoot as at any See also:rate the most definite individual . The theory of metamorphosis naturally led Goethe, ()See also:ken and others to regard the leaf as the individual, while Johannes Muller, J . J . S . Steenstrup and others adopted the same view on various physiological grounds . C . Gaudichaud elaborated a theory inter-mediate between this view and that of Du Petit-Thouars. according to which the plant was built up of individuals, each consisting of a leaf with its subjacent internode of stem, which was regarded as the leaf-See also:base, and this was supported by See also:Edward See also:Forbes and others, while the nominally converse view—that of the leaf as a mere outward expansion of the stem-segment—was proposed by C . F .

Phoenix-squares

See also:

Hochstetter . Though sundry attempts at identifying various tissues, such as the fibro-vascular bundles, as the constituent individuals may be passed over, those associated with the cell theory are of great importance . T . See also:Schwann decided in favour of the cell and regarded the plant as a cell-community, in which the separate elements were like the bees of a swarm—a view virtually concurred in in all essential respects by M . See also:Schleiden, R . See also:Virchow and other founders of the cell theory . Yet, although the structure and functions of the plant are ultimately and specially cellular, it is impossible to ignore the fact that, save in the very lowest organisms, these are subordinated and differentiated into larger aggregates, and form virtually but the bricks of a See also:building, and hence the later theories outlined above . Of attempts to find the individual in the nucleus or the See also:protoplasm granules it is unnecessary to speak further . So far the theories of See also:absolute individuality . The conception of relative individuality was first clearly expressed by See also:Alphonse de Candolle and Schleiden, both of whom take the cell, the shoot and the multi-axial plant as forming three successive and subordinated categories . K . W. von Nageli too recognized not only the See also:necessity of establishing such a See also:series (cell, organ, bud, leafy axis, multi-axial plant) but the distinction between morphological and physiological individualities afterwards enunciated by Haeckel .

Passing over the difficulties which arise even among the See also:

Protozoa we find that a similar controversy (fully chronicled in Haeckel's Kalkschwdmme) has raged over the individuality of See also:sponges . While the older observers were content to regard each sponge-See also:mass as an individual, a view in which J . N . Lieberkiihn and other monographers substantially concurred, the application of the See also:microscope led to the view suggested by See also:James See also:Clark, and stoutly supported by Saville See also:Kent, that the sponge is a See also:city of amoeboid or infusorian individuals . H . J . See also:Carter looked upon the separate ampullaceous sacs as the true individuals, while others, defining the individual by the See also:possession of a single exhalent See also:aperture, distinguish sponges into solitary and social . For the higher animals the problem, though perhaps really even more difficult, is less prominent . As Haeckel points out, the earlier discussions and even the comparatively See also:late essay of Johannes Muller take an almost purely psychological or at least a physiological point of view; and the morphological aspect of the inquiry only came forward when the study of much See also:lower forms, such as Cestoid See also:worms (see See also:PLATYELMIA) or Siphonophores (see See also:HYDROZOA), had raised the difficulties with which botanists had so long been familiar . With the rapid progress of embryology, too, arose new problems; and in 1842 Streenstrup introduced the conception of an ' See also:alternation of generations " as a mode of origin of distinct individuals by two methods, for him fundamentally similar, the sexual from impregnated See also:females and the asexual from unimpregnated " nurses " —a view adopted by Edward Forbes and many other naturalists, but keenly criticized by W . B . See also:Carpenter and T .

H . See also:

Huxley . In R . Leuckart's remarkable essay on polymorphism (1853) the Siphonophora were analysed into colonies, and their varied organs shown to be morphologically See also:equivalent, while the alternate generations of Steenstrup were reduced to a case of polymorphism in development . Leuckart further partly distinguished individuals of different orders, as well as between morphological and physiological individuals . In .1852 Huxley, starting from such an undoubted homology as that of the See also:egg-producing See also:process of See also:Hydra with a free-See also:swimming Medusoid, pointed out that the See also:title of individual, if applied to the latter, must logically be due to the former also, and avoided this confusion between organ and individual by defining the individual 11 animal, as Gallesio had done the plant, as the entire product of an impregnated ovum—the swarm of See also:Aphides or free Medusae which in this way might belong to a single individual being termed Zooids . In See also:Carus's System of Animal Morphology (1853) another theory was propounded, but the problem then seems to have fallen into See also:abeyance until 1865, when it formed the subject of a prolonged and fruitful discussion in the Principles of Biology . Adopting the cell (defined as an aggregate of the lowest order, itself formed of physiological See also:units) as the morphological unit, H . Spencer points out that these may either exist independently, or gradually exhibit unions into aggregates of the second order, like the lower See also:Algae, of which the individuality may be more or less pronounced . The union of such secondary aggregates or See also:compound units into individuals of a yet higher order is then traced through such intermediate forms as are represented by the higher seaweeds or the liverworts, from the thallus of which the axes and appendages of Monocotyledons and Dicotyledons are ingeniously derived . The shoot of a flowering plant is thus an aggregate of the third order; it branches into an aggregate of the See also:fourth or higher order, and finally as a tree " acquires a degree of composition too complex to be any longer defined." Proceeding to animals, the same method is applied . The Protozoa are aggregates of the first order .

These, like plants, exhibit transitions, of which Radiolarians, See also:

Foraminifera and sponges are taken as examples, . to such definite compound wholes as Hydra; and such secondary aggregates multiply by gemmation into permanent aggregates of the third order, which may exhibit all degrees of integration up to that of the Siphonophora, where the individualities of the Polyps are almost lost in that of the aggregate form . The whole series of articulated animals are next interpreted as more or less integrated aggregates of the third order, of which the lower Annelids are the less developed forms, the Arthropods the more highly integrated and individualized . Molluscs and vertebrates are regarded as aggregates of the second order . In 1866 appeared a morphological classic, the Generelle Morphologie of Haeckel . Here pure morphology is distinguished into two sub-sciences—the first purely structural, tectology, which regards the organism as composed of organic individuals of different orders; the second essentially stereometric, promorphology . To tectology, defined as the science of organic individuality, a large See also:section of the work is devoted . Dismissing the theory of absolute individuality as a metaphysical figment, and starting from the view of Schleiden, De Candolle and Nageli of several successive categories of relative individuals, he distinguishes more clearly than heretofore the physiological individual (or See also:bion), characterized by definiteness and See also:independence of See also:function, from the morphological individual (or morphon), characterized similarly by definiteness of form; of the latter he establishes six categories, as follows: I . Plastides (cytodes and cells), or elementary organisms . 2 . Organs (cell-See also:stocks or cell-fusions), See also:simple or homoplastic organs (tissues), or heteroplastic organs . Organ-systems, organ-apparatuses . 3 .

Antimeres (opposite or symmetrical or homotypic parts), e.g. rays of radiate animals, " halves of bilaterally symmetrical animals." 4 . Metameres (successive or homodynamous parts), e.g. stem-segments of Phanerogams, segments or zoonites of Annelids or vertebrates . 5 . Personae, shoots or buds of plants, polyps of Coelenterates, &c., " individuals " in the narrowest sense among the higher animals . 6 . Corms (stocks or colonies), e.g. trees, chains of Salpae, polyp-stocks, &c . In his subsequent monograph on calcareous sponges, and in a final See also:

paper, he somewhat modifies these categories by substituting one See also:category of extreme comprehensiveness, that of the idorgan, in See also:place of the three separate orders of organs, antimeres and metameres . The idorgan (of course clearly distinguished from the physiological organ or biorgan) is finally defined as a morphological unit consisting of two or more plastids, which does not possess the See also:positive character of the See also:person or stock . These are distinguished into homoplasts or homo-organs and alloplasts or alloe-organs, the former including, as subdivisions, plastid-aggregates and plastid-fusions, the latter idomeres, antimeres and metameres . The former definition of the term antimere, as denoting at once each separate ray of a radiate, or the right and See also:left halves of a bilaterally symmetrical animal, is corrected by terming each ray a paramere, and its symmetrical halves the antimeres . Thus an ordinary Medusoid has four parameres and eight antimeres, a See also:starfish five and ten . The conception of the persona is largely modified, not only by with-See also:drawing the comparison of the animal with the See also:vegetable shoot and by omitting the antimere and metamere as necessary constituents, but by taking the central embryonic form of all the Metazoa—the gastrula (fig .

I) and its assumed ancestral representative, the gastraea—as the simplest and oldest form of persona . The different morphological stages to which it may attain are classified into three series: (I) Monaxonial inarticulate personae, i.e. uniaxial and unsegmented without antimeres or metameres, as in sponges or lowest Hydroids; (2) Stauraxonial inarticulate personae with antimeres, but without metameres, e.g. See also:

coral, See also:medusa, turbellarian, trematode, bryozoon; (3) Stauraxonial articulate personae with antimeres andmetameres, e.g. annelids, arthropods, vertebrates . The colonies of protozoa are mere idorgans . True corms, composed of united personae, occur only among sponges, hydroids, siphonophores, See also:corals, bryozoa, tunicates and echinoderms, of which the apparent parameres are- regarded as highly central