Updated by: Arabinda Naik
Similarity in early development: In all multicellular animals the zygote undergoes cleavage to produce morula. The morula develops into a single layered hollow blastula. The blastula changes into two or three layered gastrula. The animal having two layered gastrula are said to be diploblastic as in coelenterate. The animals in which three layered gastrula are said to be triploblastic as in frog, lizard. These two or three layers of gastrula are called primary germ layers, which give rise to the entire animal. Such a similar early development establishes a close relationship among all multicellular animals.
Resemblance among vertebrate embryos: The early embryos of all the vertebrates resemble in shape and structure. They have similar head with eye and ear rudiments, similar visceral arches and visceral furrows and a similar embryonic tail. This similarity among the early embryos shows that all the vertebrates have evolved from a common ancestor and they possess a common heredity that expresses their similarity in the embryonic life. The embryos of closely related vertebrates resemble more and for longer period. The embryos of a fish and salamander will resemble more and for a longer duration than the embryos of a fish and a mammal.
Temporary non-functional embryonic structure: Embryos of certain animals develop non-functional structures which disappear before hatching. The embryos of birds develop tooth buds which disappear before hatching. The embryos do not take any food through the mouth. Their teeth are therefore of no use to them. The adult bird feeds on hard, solid food such as grains nuts, and meat without teeth. The birds have evolved by modification of toothed reptiles. They have lost teeth in the course of modification. Their embryos repeat for a short period the ancestral character of the presence of teeth.
Development of vertebrate organs: The development of certain organs such as heart, brain and ear indicates their common ancestry. The heart during its development in birds and mammals passes through two chambered and three chambered stages before becoming four chambered. This indicates that both bird and mammals have originated from fishes which have two chambered heart through the amphibians and reptiles which have three chambered heart.
Biogenetic Law: These observation has led by Ernst Haeckel. The law is also known as The theory of Recapitulatioin which states that Ontogeny repeats Phylogeny. This means that an organism during its development repeats its ancestral history. The fertilized egg of human being is comparable to unicellular protozoa. At gastrulation stage it compared with coelenterate, development of mesoderm comparable to flatworm, development of coelomic cavity in mesoderm can be compared with Annelida. Early embryo with dorsal hollow nerve cord, series of pharyngeal gill slits represent the fundamental chordate character. With development of two chamber heart, paired aortic arches and a tail it resembles the fish embryo. Later it somewhat resembles the reptilian embryo and finally develops the mammalian characters. In the development of the frog a fish like tailed larva is formed which swim with the tail and respires by the gills. This indicates that the frog has been evolved from a fish like ancestor. Herdmania larva shows characters of chordates that is presence of notochord and central nervous system and tail. At adult stage it does not have notochord and tail.Thus the larva shows its ancestral characters. The protonema at an early stage of development resembles the filamentous green algae in structure. This indicates an algal ancestry of the bryophytes and pteridophytes.