Theories of embryonic development

Theory of preformation,
Epigenetic theory,
Theory of pengenesis,
Recapitulation theory,
Germplasm theory,
Mosaic theory,
Regulated theory,
Gradient theory
Theory of organizers.

1. Dr. Dinesh C. Sharma,
Associate Professor and Head-Zoology
K.M. Govt. Girls Post Graduate College,
Badalpur, G.B. Nagar-U.P., INDIA

2. Theory of preformation,
Epigenetic theory,
Theory of pengenesis,
Recapitulation theory,
Germplasm theory,
Mosaic theory,
Regulated theory,
Gradient theory
Theory of organizers.

3. Some Puzzling Things……..
WilliamHarveyconcludedthatthewombconceivestheembryoasthebrain
conceivesathought.Harveybelievedthatcoitusmerelyexcitesconceptionin
theuterusasdesireisgeneratedinthebrain,andthepresentuseofthe
wordsconceiveandconceptionisnotwithoutsignificanceinthisregard.
Pythagorasregardedsemenasafoamofthepurestbloodandasanexcessof
nutriment,andwomanalsoproducessemen.
Thisideamayhavehadavariousorigin,butsincetheyknewthespawnofthe
femalefishandthemiltofthemale,aswellasthesexualdischargesof
amphibia,itisnotimprobablethatthesuperficialsimilaritybetweenthe
testisandtheovaryandbetweenthecervical,uterineandtubalsecretions
andsemen,mayhavebeenpartlyresponsibleforthisidea.
Anaxagorasthought that male individuals arise from sperm produced by the
right and females from that produced by the left testis,
Hippocrates, held that semen is a product of the whole body, He thought that
maleness and femaleness are determined by the excess of male or female
semen present at the time of conception

4. Aristotlestated“thereisevidencethatthesemenisinthecatamenia,for,as
saidbefore,thissecretionappearsinthemaleatthesametimeoflifeasthe
catameniainthefemale”andcontinuestosaythat“thespermaticsecretions”
areproducedb_v“theuterusandpudendaandbreasts,”includingmilkbecause
itisanutriment.Aristotleheldthatthelossofsemenisexhaustingbecause
“thebodyisdeprivedoftheultimategaindrawnfromthenutriment.”
TheembryotoAristotlewas“thefirstmixtureofmaleandfemale”andthe
ovumanoviformbodyfoundintheuterus,asitwasalsotoHarvey.
AristotlesaysEmpedocles thoughtthatsexwasdetermined bythe
temperatureintheuterus.Ifhotamaleresults,ifcoolafemale.

5. The Theory of Preformation or Preexistence
ThepreéxistencetermwasfirstusedbySirKenelmDigbyin
1644,Thispreformationideawasalsocalledthetheoryof
evolution,butaccordingtoitorganismswerenotthoughtof
asslowlyunfoldingorevolving,butmerelyasincreasingin
sizefromamicroscopicminiaturetotheadult.
Inthehistoryofbiology,preformationism(orpreformism)isa
formerly-popular theorythatorganisms develop from
miniatureversionsofthemselves.Insteadofassemblyfrom
parts,preformationistsbelievedthattheformoflivingthings
exist,inrealterms,priortotheirdevelopment.Itsuggests
thatallorganismswerecreatedatthesametime,andthat
succeedinggenerationsgrowfromhomunculi,oranimalcules,
thathaveexistedsincethebeginningofcreation.
Atinypersoninsidea
sperm,asdrawnby
NicolaasHartsoekerin
1695
JosephofAromatari,whowasenthusedovertherevelations
ofthemicroscope,andwhileexaminingseedswasimpressed
bytheresemblances ofthegermandcotyledonstoaplant
andhenceannounced,in1625,thatallplantswerecontained
inminiaturewithintheseed.

6. Frompainstakingandreallyveryskilleddissectionsofthelarvae
andpupaeoffliesandofbutterflies,Swammerdam alsowasled
toconcludethatallthepartsoftheseadultanimalsare
containedinminiatureintheimmature forms.Hisskillin
dissectionandrepresentationwasunsurpassed,butheallowed
hisimagination-tocarryhimsofarthat,accordingtoBoerhaave,
heactuallydemonstrated allpartsofthebutterflyinthebodyof
acaterpillaratameetingofscientists.
SurelytheremusthavebeensomedoubtingThomases present!
Swammerdam apparentlywasmisledbywhathesawinthe
pupalstage,andfromthepresenceofallpartsconcludedthatall
organsalsoexistinthelarvaandtheovum.Thisseemedonlya
smallandlogicalstepfromhisobservationsupondissections
andthisSwammerdam took.Hewronglyopposedtheideaof
metamorphosis, asillustratedinthedevelopmentofthebutterfly
whichhestudied,butitseemsthathewasthefirsttorepresent
developingfrogeggsshowingcleavage.

7. Spallanzanisaysheannounced hisdiscoveryofthe
preéxistenceofthegerminaspeciesoffrog,inhis
Prospectusconcerninganimalreproduction,published
in1768.Intheintroductiontohisdissertationsrelating
tothe“NaturalHistoryofAnimalsandVegetables,”he
wrote:
“Havingexaminedotheranimals,andhavingfoundthatthesamethingistrue
withrespecttothem,Ihavestillstrongerreasonforpresumingthatthe
existenceofthegerminthefemalebeforefecundationisoneofthemost
generallawsofnature....Ihavebeenledbyobservations,whichshowthe
preexistenceofthegerm,todiscoverthatanorderofanimals,consideredby
naturalistsasoviparous,isinrealityviviparous.”
He not only believed that the embryo preéxisted in the ovum. but that the
amnion and umbilical cord also did so even before fertilization , and insisted
that ova, hence, were not such, but fetuses. He held that tadpoles of frogs and
toads were likewise contained in the ova before fertilization while still in the
ovary, saying:

8. Epigenetic theory
Development beforebirth,includinggametogenesis, embryogenesis, andfetal
development,istheprocessofbodydevelopmentfromthegametesareformed
toeventuallycombineintoazygotetowhenthefullydevelopedorganismexits
theuterus.Epigeneticprocessesarevitaltofetaldevelopmentduetotheneed
todifferentiatefromasinglecelltoavarietyofcelltypesthatarearrangedin
suchawaytoproducecohesivetissues,organs,andsystems.
Thetermepigenetics(orepigenesis)
referredtothehypothesis that
embryo development occursasa
progressive and gradual
differentiationoftheunstructured
egg.Thiscontrastswiththeother
classicalhypothesisofpreformation,
wherebytheembryowasarguedto
developbyaprocessofenlargement
and elaboration ofpreexisting
structurespresentwithintheegg.

9. Theory of pengenesis,
CharlesDarwin'spangenesistheorypostulatedthateverypartof
thebodyemitstinyparticlescalledgemmuleswhichmigratetothe
gonadsandaretransferredtooffspring.Gemmules werethoughtto
developintotheirassociatedbodypartsasoffspringmatures.The
theoryimpliedthatchangestothebodyduringanorganism'slife
wouldbeinherited,asproposedinLamarckism.

10. Pangenesis mirroredideasoriginallyformulatedby
Hippocratesandotherpre-Darwinianscientists,but
builtoffofnewconcepts suchascelltheory,
explainingcelldevelopment asbeginning with
gemmules whichwerespecifiedtobenecessaryfor
theoccurrenceofnewgrowthsinanorganism,both
ininitialdevelopmentandregeneration.Darwinwrote
thatHippocrates'pangenesiswas"almostidentical
withmine—merelyachange ofterms—andan
applicationofthemtoclassesoffactsnecessarily
unknowntotheoldphilosopher
Darwinhypothesizedthatgemmules mightbeabletosurviveandmultiply
outsideofthebodyinalettertoJ.D.Hookerin1870.Somegemmules were
thoughttoremaindormantforgenerations,whereasotherswereroutinely
expressedbyalloffspring.Everychildwasbuiltupfromselectiveexpressionof
themixtureoftheparentsandgrandparents'gemmules comingfromeither
side.
Darwin's description of cell proliferation using
pangenesis theory

11. HugodeVriescharacterizedhisownversionofpangenesistheoryin
his1889bookIntracellularPangenesiswithtwopropositions,ofwhich
heonlyacceptedthefirst:
I.Inthecellstherearenumberlessparticleswhichdifferfromeach
other,andrepresenttheindividualcells,organs,functionsand
qualitiesofthewholeindividual.Theseparticlesaremuchlarger
thanthechemicalmoleculesandsmallerthanthesmallestknown
organisms;yettheyareforthemostpartcomparabletothelatter,
because,likethem,theycandivideandmultiplythroughnutrition
andgrowth.Theyaretransmitted,duringcell-division,tothe
daughter-cells:thisistheordinaryprocessofheredity.
II.Inadditiontothis,thecellsoftheorganism,ateverystageof
development,throwoffsuchparticles,whichareconductedtothe
germ-cellsandtransmittothemthosecharacterswhichthe
respectivecellsmayhaveacquiredduringdevelopment.
DeVriesalsocoinedtheterm'pangene'which20yearslaterwas
shortenedbyWilhelmJohannsentogene.

12. Recapitulation theory,
Thetheoryofrecapitulation,alsocalledthe
biogeneticlaworembryological parallelism—often
expressedusingErnstHaeckel'sphrase"ontogeny
recapitulatesphylogeny"—isahistoricalhypothesis
thatthedevelopment oftheembryoofananimal,
fromfertilizationtogestationorhatching(ontogeny),
goesthroughstagesresembling orrepresenting
successive adultstagesintheevolutionofthe
animal'sremoteancestors(phylogeny).
Itwasformulatedinthe1820sbyÉtienneSerresbasedontheworkofJohann
FriedrichMeckel,afterwhomitisalsoknownasMeckel–Serreslaw.
Sinceembryosalsoevolveindifferentways,theshortcomingsofthetheoryhad
beenrecognizedbytheearly20thcentury,andithadbeenrelegatedto
"biologicalmythology"bythemid-20thcentury

13. Modernstatus
Modernevolutionarydevelopmental biology
(evo-devo)followsvonBaer,ratherthan
Darwin,inpointingtoactiveevolutionof
embryonic development asasignificant
meansofchangingthemorphologyofadult
bodies.Twoofthekeyprinciplesofevo-devo,
namely thatchanges inthetiming
(heterochrony)andpositioning(heterotopy)
withinthebodyofaspectsofembryonic
development wouldchangetheshapeofa
descendant's body compared toan
ancestor's,werehoweverfirstformulatedby
Haeckelinthe1870s.Theseelementsofhis
thinkingabout development havethus
survived, whereas his theory of
recapitulationhasnot.
TheHaeckelianformofrecapitulationtheoryisconsidereddefunct.Embryosdo
undergoaperiodwheretheirmorphology isstronglyshapedbytheir
phylogeneticposition,ratherthanselectivepressures,butthatmeansonlythat
theyresembleotherembryosatthatstage,notancestraladultsasHaeckelhad
claimed.

14. Germplasm theory,
Germ-plasm theoryis a concept of the physical basis of
heredity, put forward by August Weismann. According to
histheory,germplasmis the essential element of germ
cells (eggs and sperm) and is passed from one generation
to the other.
Histheorystatesthatmulticellularorganismsconsistofgermcellsthat
containandtransmitheritableinformation,andsomaticcellswhichcarryout
ordinarybodilyfunctions.Inthegermplasmtheory,inheritanceina
multicellularorganismonlytakesplacebymeansofthegermcells:the
gametes,suchaseggcellsandspermcells.Othercellsofthebodydonot
functionasagentsofheredity.Theeffectisone-way:germcellsproduce
somaticcells,andmoregermcells;thegermcellsarenotaffectedbyanything
thesomaticcellslearnoranyabilitythebodyacquiresduringitslife.Genetic
informationcannotpassfromsomatogermplasmandontothenext
generation.
ThisisreferredtoastheWeismann barrier.Thisidea,iftrue,rulesoutthe
inheritanceofacquiredcharacteristicsasproposedbyLamarckandimpliedby
CharlesDarwin'spangenesistheoryofinheritance.

15. Mosaic theory,
RouxtoformulatetheMosaicTheoryofdevelopment, orthetheorythatthecell
separatedhereditarymaterialsindifferentamountstodaughtercellsatcelldivision,a
processcalledqualitativedivision
EdmundBeecherWilsonexperimentedwithAmphioxus(Branchiostoma)
embryosin1892toidentifywhatcausedtheircellstodifferentiateinto
newtypesofcellsduringtheprocessofdevelopment.Wilsonshook
apartthecellsatearlystagesofembryonic development, andhe
observedthedevelopmentoftheisolatedcells.Heobservedthatinthe
normaldevelopmentofAmphioxus,allthreemaintypesofsymmetry,or
cleavagepatternsobservedinembryos,couldbefound.
Wilsonproposed ahypothesisthatreformedtheMosaicTheory
associatedwithWilhelmRoux(1893)inGermany.Wilsonsuggestedthat
cellsdifferentiatedintoothercellswheninfluencedbyphysiological
(dynamic)changesinthehereditarysubstancecontainedincells,and
notbecauseofthequalitativedivision,orparcellingout,ofthe
substanceintodaughtercells.

16. Gradient theory
Itindicatesthattherateofmetabolismismoreintheanimalpolethanthe
vegitalpolewhichiscalledasmetabolicaxialgradient.Horstadius&
Runnstromcalleditasdoublegradientthoery.Theyexplaineggdevelopment
onthebasisofanimalpolegradient&vegitalpolegradient.
ItwasgivenbyT.Boveri&wassupportedbyM.C.Child.Childcalleditas
metabolicaxialgradienttheory.
Accordingtoit,theegghasdistinctanimal&vegitalpoles.
Thecytoplasmoftheanimalpoledividesrepidlybuttherateofcleavageinthe
vegitalpoleisslow.Itindicatesthattherateofmetabolismismoreinthe
animalpolethanthevegitalpolewhichiscalledasmetabolicaxialgradient.
Horstadius&Runnstromcalleditasdoublegradientthoery.Theyexplainegg
developmentonthebasisofanimalpolegradient&vegitalpolegradient.

17. Theory of organizers.
TheSpemann-Mangoldorganizerisagroupofcellsthatareresponsibleforthe
inductionoftheneuraltissuesduringdevelopmentinamphibianembryos.First
describedin1924byHansSpemann andHildeMangold,theintroductionofthe
organizerprovidedevidencethatthefateofcellscanbeinfluencedbyfactors
fromothercellpopulations.Thisdiscoverysignificantlyimpactedtheworldof
developmental biologyandfundamentallychangedtheunderstanding ofearly
development.
HansSpemannshowedthattransplantingpresumptiveepidermisintotheareaofpresumptiveneuraltissuewouldchange
thefateofthetransplantedcellstothatoftheirnewdestination,andlikewisewhenhetransplantedpresumptiveneural
tissuetowherethepresumptiveepidermiswasforming.Spemannalsoshowedthatbytransplantingapiecefromtheupper
blastoporelipintoanareaofpresumptiveepidermis,asecondaryembryonicprimordiumformed,includingasecondary
neuraltube,notochordandsomites.Additionally,splittingtheembryoinhalfandrotatingtheanimalpoleinrespecttothe
vegetalpoleresultedindeterminationspreadingfromthelowervegetalpole,wheretheupperblastoporelipwaslocated,
totheupperanimalhalf.Healsofusedtogethertwoidenticalhalvesfromdifferentembryosandobservedformationofthe
neuralplate.Thisworkprovidedtheinitialevidencetosupportthenotionthatthereexistedsome“organizationcenter”
thatwasdeterminedpriortotheotherembryonictissueandinfluencedthedeterminationofsurroundingcells

18. Totestthishypothesis,Spemann, alongwithHildeMangold,performedexperiments
between1921and1922usingembryosfromTrituruscristatusandTriturustaeniatusthat
wereundergoinggastrulation.Theexperimentperformedresembledtheonedonein1918,
howeverinsteadofahomoplastictransplantationtheyusedembryosfromtwospeciesof
newtthatarecloselyrelated.Oneofthebenefitsofusingthecristatusandtaeniatus
embryoswasthatthecristatusembryocellslackedpigmentsothefateofthetransplant
couldbeeasilytrackedwhenplacedamongthepigmentedtaeniatuscells.Apiecefromthe
upperblastoporelipwasremovedfromthecristatusembryoandtransplantedintoaventral
regionofpresumptiveepidermisinthetaeniatusembryo,awayfromthedevelopinghost
blastopore.Followingthistransplant,theyobservedtheformationofasecondary
embryonicprimordium,consistentwiththeirpreviouswork.Thissecondaryembryohadthe
normalfeaturesoftheprimaryembryo,includingstructuressuchastheneuralplateand
notochord,althoughtheylaggedslightlyindevelopment.Sectioningoftheembryoshowed
thatcellsfromthetransplantwereincorporatedintothemesoderm, theneuralplate,and
constitutedalmosttheentirenotochordofthesecondaryembryo.Itwasfurthershownthat
theneuralplatewasalmostentirelycomposed ofcellsfromthehosttaeniatusembryo.
Theseexperimentsconcludedthatapieceoftheupperblastoporelipcanbetransplanted
intotheindifferenttissueofanotherembryoandinducethehosttissueintotheformation
ofasecondaryembryo,thereforeimplicatingthetransplantedtissueasan“organization
center”.
The discovery of the Spemann -Mangold Organizer is considered one of the most influential
findings in the field of developmental biology and resulted in Hans Spemann being awarded
the Nobel Prize in 1935 for his work.The mechanisms of how this organizer operates has
been the subject of decades of follow up research.

19. Regulated theory,