These evidences are based on the comparative study of the embryos of various animals Von Baer noticed remarkable similarities among vertebrate embryos however their adults were quite different ID: 932380
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Slide1
Embryological Evidences
Slide2These evidences are based on the comparative study of the embryos of various animals.
Von Baer
noticed
remarkable similarities
among
vertebrate embryos
, however, their adults were quite different.
Darwin
and other scientist also studied the development of animals and concluded that
their early developmental stages
are
more
conservative or evolutionary stable
than
late stages or adults
thus an individual
during its development
reflect its ancestral history
.
Slide3Such evidences can be obtained by studying the following:1. Development of early
embryo in all
multicellular
animals.
2. Development of vertebrate
embryo
.
3. Development of
Organs
4. Recapitulation
Slide4(i) Development of
early
embryo in all multicellular animals:
In all the multicellular animals the
development of Embryo start from a single cell fertilized egg called as Zygote.This Zygote after repeated cell division form a solid ball called as the Morula.
Slide5The Morula develops into a single layered hollow Blastula
which later transformed into either
two
or
three
layered Gastrula.Zygote Morula Blastula GastrulaThe animals which have two layered gastrula are said to be diploblastic (ectoderm and
endoderm).
Slide6Later all structures are developed from these two germ layers e.g., coelenterates.
However those animals which
three layered gastrula
are known as
triploblastic
and these three germ layers are called as ectoderm, mesoderm and endoderm.Later these three germ layers produced similar types of structures in all animals starting from platyhelminthes up to chordates.
Slide7Thus the development after gastrula stage become modified in different groups of animals. These germ layers of gastrula are termed as primary germ layers, which give rise to the entire animal
.
Such a similar early development establishes a close relationship among all multicellular animals.
Slide8(ii) Development of vertebrate embryo.If
we made a
comparative study of embryos of the same age of vertebrates, such as a
fish
, a
amphibia (salamander), a reptiles (tortoise), a Birds (chick)and a mammal (man), each resemble one another very closely in:having segmentally arranged somites, similar structures and position of head, ear & eye rudiments, visceral arches, gill
clefts, tail, an internal hollow nerve cord and a notochord.
Slide9Slide10Slide11Slide12Limb buds and various internal organs develop in similar region and notochord is replaced by vertebral column in all vertebrate Although the embryos of all vertebrates resemble with one another but the embryos of closely related groups resemble more closely than the embryos of the distant groups.
This is another evidence establishing close relationship among these divergent vertebrates as well as common ancestory
.
Slide13(iii) Development of Vertebrate Organs
Development of many vertebrate organs (e.g.,
heart
,
brain
, kidney) indicate the possible path of evolution as well as the common ancestry of vertebrates. For example(i) Heart is mesodermal in origin in all vertebrates.
Slide14It is 04 chambered in birds and mammals but during the course of development it is initially two-chambered (as in fishes), then become three-chambered (as in amphibians and some reptiles) and ultimately become four-chambered. It clearly shows that birds and mammals have common ancestory.
(ii)
brain
in all vertebrates, is
ecdodermal in origin and arises as an anterior enlargement of the neural tube.
Slide15Soon it develops two grooves and gets divided into three parts
—
fore brain
,
mid brain
and hind brain. Each of these parts develops further to attain the adult state. For instance: The cerebrum in case of fishes is just like a small lobes but in reptiles it become elongated and branched in birds and become highly convoluted in mammals
Slide16(iii) kidneys in all vertebrates are three types i.e. Pronephric, Mesonephric
and
Metanephric
which are
Endodermal
in origin. During the course of embryonic development of birds and mammals the kidney is initially Pronephric type as found in fishes, than it become Mesonephric type as found in amphibians and ultimately it become Metanephric type.These observations further strengthen the idea that higher vertebrates have evolved from the lower vertebrates over a period of time and have a common ancestory.
Slide17(iv). Recapitulation Theory
In
1828,
Von Baer
, the father of
modern embryology, discovered the primary germ layers of animals and found that the germ layer which produces a particular set of structures in one group of animal produces the same set of structures in others group of animals.He proposed four propositions:(i) During embryonic development, the generalized features/characters (such as brain, spinal cord, axial
skeleton, aortic arches, etc. are common to
all vertebrates) appeared first or earlier
.
Slide18(ii) From these most generalized features
,
less generalized and finally special features
(like hair in mammals only, feathers in birds only, limbs in quadrupeds only)
are formed
.(iii) During course of embryonic development, an animal of one species diverge continuously from that of another species.
Slide19(iv) During development, a higher animal passes through stages which resemble stages of development of lower animals
.
These proposition have given way to
Von Baer’s law
which stated that
“animals resemble one another more and more”.
Slide20Later on, this law was modified as the biogenetic law by Ernst Haeckel
in 1866.
Haeckel’s biogenetic law states that
“Ontogeny repeats phylogeny”
.
Ontogeny is the life history of an organismWhile Phylogeny is the evolutionary history of the race of that organism. Thus an organism repeats its ancestral history during its development.
Slide21Examples:(a) In the development of the frog
a fish like tailed larva (tadpole) is formed, which swims with the tail and respires by the gills. This indicates that the frog has been evolved from a fish like ancestor.
(b)
Tadpole (larva) of
Herdmania
(urochordate) shows characters of chordates i.e., presence of notochord, well developed dorsally placed central nervous system and tail. However adult Herdmania does not have notochord and tail.Nervous system is also very much reduced in adult Herdmania. Thus the larva shows its ancestral characters.
Slide22End