See also:

• METAMERISM (Gr. writ, after, µEpos, a part)

METAMERISM (Gr. See also:

• WRIT (O. Eng. gewrit, writ, from writan, to write)

writ, after, µEpos, a See also:

• PART

part)  , a technical See also:

• TERM

term used in natural See also:

• SCIENCE (Lat. scientia, from scire; to learn, know)

science: In See also:

• CHEMISTRY
• CHEMISTRY (formerly "chymistry"; Gr. xvµela; for derivation see ALCHEMY)

chemistry it denotes the existence of different substances containing the same elements in the same proportions and having the same molecular See also:

• WEIGHT

weight; it is a See also:

• FORM (Lat. forma)

form of See also:

• ISOMERISM

isomerism . In See also:

• ZOOLOGY (from Gr. Nov, a living thing, and ?byos, theory)

zoology, See also:

• METAMERISM (Gr. writ, after, µEpos, a part)

metamerism is the repetition of parts in an organized See also:

• BODY

body, a phenomenon which is, as E . See also:

• HAECKEL, ERNST HEINRICH (1834- )

Haeckel, W . See also:

• BATESON (BATSON or BETSON), THOMAS

Bateson and others have recognized, only a See also:

• SPECIAL

special See also:

• CASE
• CASE, JOHN (d. 1600)

case of a tendency to repetition of structural See also:

• UNITS, DIMENSIONS OF
• UNITS, PHYSICAL

units or parts which finds one expression in bilateral symmetry . It occurs in almost every See also:

• GROUP

group of the See also:

• ANIMAL
• ANIMAL (Lat. animalis, from anima, breath, soul)

animal See also:

• KINGDOM

kingdom, but is most conspicuous in segmented See also:

• WORMS

worms, arthropods and vertebrates . In certain worms (the Cestoidea and some See also:

• PLANARIANS

Planarians) metameric segmentation is accompanied by the separation of the completed metameres one by one from the older (anterior) extremity of the See also:

• CHAIN (through the O. Fr. citable, chcene, &c., from Lat. catena)

chain (strobilation), but it by no means follows that metameric segmentation has a necessary origin in such completion and separation of the " meeey." On the contrary, metamerism seems to arise from a See also:

• PROPERTY

property of organisms which is sometimes more (eumerogenesis) and sometimes less (dysmerogenesis) fully exhibited, and in some See also:

• GROUPS
• GROUPS, THEORY

groups not exhibited at all . The most See also:

• COMPLETE

complete and, at the same See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time, simplest instances of metameric segmentation are to be seen in the larger Chaetopods, where some hundreds of segments succeed one another—each practically indistinguishable in structure from the segment in front or from that behind; muscles, right and See also:

• LEFT

left appendage or parapodium, See also:

• COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour-See also:

• PATTERN

pattern of the skin, gut, See also:

• BLOOD

blood-vessels, coelom, nephridia, See also:

• NERVE (Lat. nervus, Gr. vevpov, a bowstring)

nerve-ganglion and nerves are precisely alike in neighbouring segments . The segment which is least like the others is the first, for that carries the mouth and a See also:

• LOBE

lobe projecting beyond it—the prostomium . If (as sometimes happens) any of the hinder segments completes itself by developing a prostomium, the chain breaks at that point and the segment which has See also:

• DEVELOPED

developed a prostomium becomes the first or See also:

• HEAD (in 0. Eng. heafad; the word is common to Teutonic languages; cf. Dutch hoofd, Ger. Haupt, generally taken to be in origin connected with Lat. caput, Gr. KerbOvi7)
• HEAD, SIR EDMUND WALKER, BART
• HEAD, SIR FRANCIS BOND, BART

head-bearing segment of a new individual . Compare such an instancc'.,pf metameric segmentation with that presented by one of the higher Arthropods—e.g. the See also:

• CRAYFISH (Fr. ecrevisse)

crayfish . Here - the somites are not so clearly marked in the tegumentary structures; nevertheless, by examining the indications given by the paired parapodia, we find that there are twenty-one somites See also:

• PRESENT

present—a limited definite number which is also the precise number found in all the higher See also:

• CRUSTACEA

Crustacea . We can See also:

• STATE
• STATE, GREAT OFFICERS OF

state as a FIRST See also:

• LAW
• LAW (0. Eng. lagu, M. Eng. lawe; from an old Teutonic root lag, " lie," what lies fixed or evenly; cf. Lat. lex, Fr. loi)
• LAW, JOHN (1671—1729)
• LAW, WILLIAM (1686-1761)

LAW 1 of metamerism or somite formation 1 The word " Law " is used in this See also:

• SUMMARY

summary merely as a convenient heading for the statement of a more or less See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

general proposition .

that it is either indefinite in regard to number of metameres or somites produced, or is definite . Animals in the first case we See also:

• CALL (from Anglo-Saxon ceallian, a common Teutonic word, cf. Dutch kallen, to talk or chatter)

call anomomeristic; those in the second case, nomomeristic . The nomomeristic See also:

• CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition is a higher development, a specialization, of the anomomeristic condition . The SECOND LAW, or generalization, as to metamerism which must be noted is that the See also:

• MERES, FRANCIS (1565-1647)

meres or somites (excepting the first with its prostomium) may be all practically alike or may differ from one another greatly by modification of the various constituent parts of the See also:

• MERE
• MERE, ADALBERT (1838-1909)

mere or somite . Metamerized animals are either homoeomeric or heteromeric . The reference of the variation in the form of the essential parts contained in a " metamere " or " somite " introduces us to the See also:

• NECESSITY (Lat. necessitas)

necessity of a general term for these constituent or subordinate parts; they may be called " meromes (µEpos) . The meromes present in a metamere or somite differ in different annulate or segmented animals according to the general organization of the group to which the animal belongs . As a See also:

• MATTER

matter of convenience we distinguish in the Arthropod as meromes, first, the tegumentary chitinized plates called terga, placed on the dorsal aspect of the somites; second, the similar sternal plates . In Chaetopods we should take next to these the masses of circular and logitudinal See also:

• MUSCULAR

muscular See also:

• FIBRES
• FIBRES (or FIBERS, in American spelling; from Lat. fibra, apparently connected either with filum, thread, or findere, to split)

fibres of the body-See also:

• WALL (O. Eng. weal, weall, Mid. Eng. wal, wane, adapted from Lat. vallum, rampart; the original O. Eng. word for a wall was wag or tenth)
• WALL, RICHARD (1694-1778)

wall and the dorso-ventral muscles . The latter form the third sort of merome present in the Arthropods . The See also:

• FOURTH

fourth See also:

• KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind of merome is constituted by the parapodia or appendages; the fifth by the coelomic pouches and their ducts and See also:

• EXTERNAL

external apertures (coelomo-ducts), whether renal or genital . The See also:

• SIXTH

sixth by the blood-vessels of the somite; the seventh by the See also:

• BIT (from the verb " to bite," either in the sense of a piece bitten off, or an act of biting, or a thing that bites or is bitten)

bit of alimentary See also:

• TRACT (from Lat. tractare, to treat of a matter, through Provencal tractat and Ital. trattato)

tract which traverses it; and the eighth by the neuromere (nerve ganglion pair, commissures, connectives and nerve branches) .

The THIRD LAW of metamerism is that heteromerism may operate in such a way. as to produce definite regions of like modification of the somites and their appendages, differing in their modification from that observed in regions before and behind them . It is convenient to have a special word for such regions of like meres, and we call each a tagma (See also:

• RAY (Lat. raia)
• RAY (or WRAY, as he wrote his name till 1670), JOHN (1628-1705)

ray ta, a See also:

• REGIMENT (from Late Latin regimentum, rule, regere, to rule, govern, direct)

regiment) . The word " tagmosis " is applicable to the formation of such regions . In the Chaetopods tagmosis always occurs to a small extent so as to form the head . In some Chaetopods, such as Chaetopterus and the sedentary forms, there is marked tagmosis, giving rise to three or even more tagmata . In Arthropods, besides the head, we find very frequently other tagmata developed . But it is to be noted that in the higher members of each See also:

• GREAT

great class or See also:

• LINE

line of descent, the tagmosis becomes definite and characteristic just as do the See also:

• TOTAL

total number of meres or somites, whilst in the See also:

• LOWER

lower grades of each great class we find what may be regarded as varying examples of tentative tagmosis . The terms nomotagmic and anomotagmic are applicable with the same kind of implication as the terms nomomeristic and anomomeristic . The FOURTH LAW of metamerism (auto-heterosis of the meromes) is that the meromes of a somite or See also:

• SERIES (a Latin word from serere, to join)

series of somites may be separately and dissimilarly affected by heteromerism . It is See also:

• COMMON

common enough for small changes only to occur in the inner visceral meromes whilst the appendages and terga or sterna are largely changed in form . But of equal importance is the See also:

• INDEPENDENT

independent " heterosis " of these visceral meromes without any corresponding heterosis of the body wall . As instances, we may cite the gizzards of various earthworms and the special localization of renal, genital and gastric meromes, with obliteration elsewhere, in a few somites in See also:

• ARTHROPODA

Arthropoda .

The FIFTH LAW, See also:

• RELATING

relating also to the See also:

• INDEPENDENCE
• INDEPENDENCE, DECLARATION OF
• INDEPENDENCE, WAR OF

independence of the meromes as compared with the whole somite, is the law of autorhythmus of the meromes . Metamerism does not always See also:

• MANIFEST (Lat. manifestus, clear, open to view)

manifest itself in the formation of complete new segments; but one merome may be repeated so as to suggest several metameres, whilst the remaining meromes are, so to speak, out of See also:

• HARMONY (Gr. dpµovia, a concord of musical sounds, dpA'ew to join; &plan midi (sc. TeXvil)

harmony with it and exhibit no repetition . Thus in the hinder somites of the body of Apus the Crustacean we find a series of segments corresponding apparently each to a complete single somite; but when the appendages are examined we find that they have multiplied without relation to the other meromes of a somite: we find that the somites carry from two to seven pairs of appendages, increasing in number as we pass backwards from the genital segment . The appendages are autorhythmic meromes in this case . They take on a quasi-independent metamerism and are produced in See also:

• NUMBERS, BOOK OF
• NUMBERS, PARTITION OF

numbers which have no relation to the numbers of the body-rings, muscles and neuromeres . This possibility of the independent metameric multiplication of a single merome must have great importance in the case of dislocated meromes, and no doubt has application to some of the metameric phenomena of Vertebrates . The SIXTH LAW is the law of dislocation of meromes . This is a very important and striking phenomenon . A merome, such as a pair of appendages (Araneae) or a neuromere or a muscular See also:

• MASS (O.E. maesse; Fr. messe; Ger. Messe; Ital. messa; from eccl. Lat. missa)
• MASS, IN

mass (frequent), may (by either a See also:

• GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

gradual or sudden See also:

• PROCESS

process, we cannot always say which) quit the metamere to which it belongs, and in which it originated, and pass by actual See also:

• PHYSICAL

physical transference to another metamere . Frequently this new position is at a distance of several metameres from that to which the wandering merome belongs in origin . The See also:

• MOVEMENT

movement is more usual from behind forwards than in the See also:

• REVERSE

reverse direction; but this, probably, has no profound significance and depends simply on the fact that, as a See also:

• RULE

rule, the head must be the See also:

• CHIEF (from Fr. chef, head, Lat. caput)

chief region of development on See also:

• ACCOUNT
• ACCOUNT (through O. Fr. acont, Late Lat. comptum, cornputare, to calculate)

account of its containing the sense See also:

• ORGANS

organs and the mouth . In the See also:

• VERTEBRATA

Vertebrata the independence of the meromes is more fully developed than in other metamerized animals .

Not only do we get auto-heterosis of the meromes on a most extensive See also:

• SCALE
• SCALE (1) A

scale, but the dislocation of single meromes and of whole series (tagmata) of meromes is a common phenomenon . Thus, in fishes the pelvic fins may travel forwards to a thoracic and even jugal position in front of the See also:

• PECTORAL

pectoral fins; the branchiomeromes lose all relation to the position of the meromes of muscular, skeletal, coelomic and See also:

• NERVOUS

nervous nature, and the See also:

• HEART
• HEART, LUNG

heart and its vessels may move backwards from their See also:

• ORIGINAL

original metameres in higher Vertebrates carrying nerve-loops with them . The SEVENTH LAW of metamerism is one which has been pointed out to the writer by E . S . See also:

• GOODRICH, or GOODRICKE, THOMAS (d. 1554)
• GOODRICH, SAMUEL GRISWOLD (1793-1860)

Goodrich . It may be called the law of " See also:

• TRANSLATION

translation of heterosis." Whilst actual physical transference of the substance of meromes undeniably takes See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place in such a case as the passage of the pelvic fins of some fishes to the front of the pectorals, and in the case of the backward movement of the opisthosomatic appendages of See also:

• SPIDERS

spiders, yet the more frequent mode in which an alteration in the position of a specialized See also:

• ORGAN

organ in the series or scale of metameres takes place is not by See also:

• MIGRATION

migration of the actual material organ from somite to somite, but by translation of the quality or morphogenetic peculiarity from somite to sornite, accompanied by correlative See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change in all the somites of the series . The phenomenon may be compared to the transposition of a piece of See also:

• MUSIC

music to a higher or lower See also:

• KEY
• KEY (in O. Eng. tang; the ultimate origin of the word is unknown; it appears only in Old Frisian kei of other Teutonic languages; until the end of the 17th century the pronunciation was kay, as in other words in O. Eng. ending in aeg; cf. daeg, day; clang
• KEY (the phraseograms being indicated by hyphens)
• KEY, SIR ASTLEY COOPER (1821-1888)
• KEY, THOMAS HEWITT (1799-1875)

key . It is thus that the lateral fins of fishes move up and down the scale of vertebral somites; and thus that whole regions (tagmata), such as those indicated by the names cervical, thoracic, lumbar and sacral, are translated (accompanied by terminal increase or decrease in the total number of somites) so as to occupy differing numerical positions in closely allied forms (cf. the varying number of cervical somites in allied See also:

• REPTILES (Lat. Reptilia, creeping things, from reptilis; ref ere, to creep; Gr. Ep7rety, whence the term " herpetology," for the science dealing with them)

Reptiles and Birds) . What, in this rapid enumeration, we will venture to call the EIGHTH LAW of metamerism is the law of homoeosis, as it is termed by W . Bateson . Homoeosis is the making of a merome into the likeness of one belonging to another metamere, and is the opposite of the process of " heterosis "—already mentioned . We cite this law here because the result of its operation is to simulate the occurrence of dislocation of meromes and has to be carefully distinguished from that process .

A merome can, and does in individual cases of abnormality, assume the form and See also:

• CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character of the corresponding merome of a distant somite . Thus the antenna of an See also:

• INSECT

insect has been found to be replaced by a perfectly well-formed walking See also:

• LEG
• LEG (a word of Scandinavian origin, from the Old Norwegian leggr, cf. Swed. lagg, Dan. laeg; the O. Eng. word was sceanca, shank)

leg . After destruction of the See also:

• EYE
• EYE (O. Eng. edge, Ger. Auge; derived from an Indo-European root also seen in Lat. oc-ulus, the organ of vision (q.v.)

eye-stalk of a See also:

• SHRIMP

shrimp a new growth appears, having the form of an antenna . Other cases are frequent in Crustacea, as individual abnormalities . They prove the existence in the mechanism of metamerized animals, of structural conditions which are capable of giving these results . What those structural conditions are is a matter for See also:

• SEPARATE

separate inquiry, which wecannot even See also:

• TOUCH (derived through Fr. toucher from a common Teutonic and Indo-Germanic root, cf. " tug," " tuck," O. H. Ger. zucchen, to twitch or draw)

touch here . We now come to the questions of the See also:

• PRODUCTION (Lat. productionem, from producere, to pro-duce)

production of new somites or the addition of new somites to the series, and the converse problem of the suppression of somites, whole or partial . We state as the NINTH LAW of metamerism " that new somites or metameres are added to a chain consisting of two or more somites by growth and gradual elaboration— what is called ' budding '—of the anterior border of the hindermost somite . This hindermost somite is therefore different from all the other somites and is called the telson.' However See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

long or See also:

• SHORT, FRANCIS JOB (1857– )

short or heteromerized the chain may be, new metameres or somites are only produced at the anterior border of the telson, except in the Vertebrata." That is the general law . But amongst some groups of metamerized animals partial exceptions to it occur . It is probably absolutely true for the Arthropoda from lowest to highest . It is not so certain that it is true for the See also:

• CHAETOPODA (Gr. xairq, hair, aous, foot)

Chaetopoda, and would need modification in statement to meet the cases of fissiparous multiplication occurring. among Syllids and Naidids .

In the Vertebrata, where tagmosis and heterosis of meromes and dislocation of merones and tagmata are, so to speak, rampant, new formation of metameres (at any See also:

• RATE

rate as represented by important meromes) takes place at more than one point in the chain . Such points are found where two highly diverse ' tagmata " abut on one another . It is possible, though the See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

evidence at present is entirely against the supposition, that at such points in Arthropoda new somites may be formed . Such new somites are said to be " intercalated." The question of the intercalation of vertebrae in the Vertebrata has received some attentipn . It must be remembered that a vertebra even taken with its muscula' vascular and neural accessories is only a partial metamere—a merome—and that, so far as complete 'metameres are concerned, the Vertebrata do conform to the same law as the Arthropods . Intercalation of meromes, branchial, vertebral and dermal (fin-supports) seems to have taken place in Vertebrata in the fishes, while in higher groups intercalation of vertebrae in large series has been accepted as the only possible explanation of the structural facts established by the comparison of allied groups . The elucidation of this matter forms a very important See also:

• PART

part of the See also:

• WORK

work lying to the See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

hand of the investigator of vertebrate See also:

• ANATOMY
• ANATOMY (Gr. avaroyil, from ava-silo c', to cut up)

anatomy, and it is possible that the application of Goodrich's law (the seventh of our See also:

• LIST (O.E. lisle, a Teutonic word, cf. Dut. lust, Ger. Leiste, adapted in Ital. lista and Fr. lisle)
• LIST, FRIEDRICH (1789-1846)

list) may throw new See also:

• LIGHT

light on the matter . In regard to the diminution in the number of somites in the later See also:

• STAGE (Fr. 6/age; from Lat. stare, to stand)

stage of growth." The See also:

• FUSION

fusion of neighbouring meromes is often preceded by more or less extensive See also:

• ATROPHY (Gr. It- priv., rpo4, nourishment)

atrophy of the somites concerned, and by See also:

• ARREST (Fr. arrester, arreeter, to stop or stay)

arrest of development in the individual ontogeny . Thus, a case of fusion of partially atrophied somites may simulate the See also:

• APPEARANCE (from Lat. apparere, to appear)

appearance of incipient merogenesis or formation of new somites, and, See also:

• VICE

vice versa, incipient merogenesis may be misinterpreted as a case of fusion of once separate and fully-formed somites . A very.complete fusion of somites is that seen on the head of Arthropoda . The head or prosoma of Arthropoda is a tagma consisting of one, two, or three prosthomeres or somites in front of the mouth and of one, two, three, up to five or six opisthomeres . The cephalic tagma or prosoma may thus be more or less sharply divided into two subtagmata, the prae-oral and the See also:

• POST

post-oral .

(E . R .