Embryological Evidences These evidences are based on the

Embryological Evidences

• 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. • 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.

• Such 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

• (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.

• The Morula develops into a single layered hollow Blastula which later transformed into either two or three layered Gastrula. • Zygote Gastrula Morula Blastula • The animals which have two layered gastrula are said to be diploblastic (ectoderm and endoderm).

• Later 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.

• Thus 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.

• (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.

• Limb 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.

• (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.

• It 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 fourchambered. • 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.

• Soon 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

• (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.

• (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.

• (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.

• (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”.

• Later 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 organism • While Phylogeny is the evolutionary history of the race of that organism. • Thus an organism repeats its ancestral history during its development.

• Examples: • (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.

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