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Evolutionary patterns in ontogenetic transformation: from laws to regularities

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Fig. 1. Ontogeneticlawsof VanBaer (A) andof Haeckel(8).(AI Van Baer'slaw assumesthat all organismscan be classifiedmto a small numberof basicbodyplans,thatarediscreteandindependentfromeachother.Thispre-evolutionaryconceptis similarto Cuvier'sgeneralclasses,R.Owen'sarchetypesor similarmetaphysicalconceptsassociatedwith the morphologyof the times.Given this precondition,VanBaerlaw can besummarized as "the embryo develops from a more generalizedcondition of its type to the more particularizedfeaturesof its species_" The result isa processof progressive differentiation with a concomitant divergenceIn morphological slmilantyas the embryosdevelop.(8) Haeckelspousedtheviewthat bothan individual'sontogenyand a speciesphylogenywereanalogousprocessesof morphogenesisthroughdifferenttemporalscales.Evolutionproceededby additionof newstagesto theendofthe ancestralontogeneticsequence.To preventontogeniesfrombecominginordinatelylong in time as well as in stages,he had to add a secondprinciple,besidesterminaladdition,whichcfaimedthat as an embryodevelopsit recapitulates an accelerated and a much abbreviated sequence of ancestralstages.Haecke/'sbiogeneticlawreqUiresthata homologybeestablishedbetweenan embryonicstageof a descendantand an adult ancestralstage.Whetherembryosrecapitulated"adult"stageswas a majorissueat thetime.Note,however,thatif a personbelievesin Van Baer'slawsand,unlikethe nineteenthcenturyembryologist,alsoacceptsbiologicalevolution,somesort of recapitulationwouldbe observedduringphylogeny.
Fig. 2. Skullsfromtwoyoungspecimensof theEuropeanfire-salamander,Salamandrasalamandra,cleared and stainedwith Alcian Blue(cartilage)and AlizarinRed (calcified bone)
Fig. 4. Limbbranching pattern

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