Evidences of evolution from the Embryological point of View

Sexual reproduction in multicellular animals results in the formation of zygotes. Each Zygote is  developed into a new offspring  by embryonic  development. The entire sequence of this development is called ontogeny. The development of a species from its ancestors called Phylogeny.

A comparative study of ontogeny  of various forms of animals reveal their phylogenic relationship and this confirms the process of evolution. 

The zygote which was responsible for the development of all the bodies of Metazoan was single celled. The structure of this single-cell zygote is  quite comparable with the bodies of simple protozoans. This  indicates that the origin of metazoan took place from the protozoan. This indicates that metazoan has evolved from the protozoan. 

The early stage of embryonic development like morula gastrula blastula are similar in structure. This similarity indicates the  monopolytic origin. 

In fish, the young individuals are  developed from  gastrula. This gastrula is almost like an adult. Tadpole larvae of  amphibians are quite similar to young fish. This indicates that amphibians have evolved from the Pisces. 

Even after the gastrulation  in the vertebrates, the  post gastrula stages are quite similar in members of all the different classes like fishes, Amphibians, Reptiles Bird and mammals. The embryo of vertebrates passes through the later embryonic stage of lower  vertebrates before attaining the characteristic of their own classes. For Ex- in all birds and mammals including men the embryo passes through stages resembling those of embryos of fish amphibians and reptiles before finally attaining the characters of their respected classes. This proves that all vertebrates have evolved from common fish-like ancestors and also that both birds and mammals have evolved from Reptiles.

The presence of pharyngeal gill slits and gill pouches is one of the three typical characters of chordates.

In most primitive chordates, protochordates,  slits are present in the adult but in adult fishes, not only the gill slits are  present but even Gill develop upon them for aquatic respiration. In adult amphibian reptiles, birds and mammals, gills are not required because respiration takes place by the lung  due to its terrestrial habitat. 

Remember👌👌 Gills slit  and Gill pouches appear in the embryonic stage of all vertebrates and soon disappear or modify into their structure.

When the heart was first developed in the embryo of amphibian reptile birds and mammals. The heart has two Chambered in embryo and adult fish. In later stages the heart became three chambers in amphibians and reptiles but due to the course of evolution, in birds and mammals, there are four chambered hearts. 

According to the basic principle of embryonic development given by Von bear - The  general character appears before special characters. From the more general the less general and finally the special characters are developed. 

The young stage of an animal is like the young or embryonic stage but not like the adult of other animals lower in evolutionary scale. 

Later on Ernest Hackle gave the theory of recapitulation or Biogenetic law.

According to this law individuals of a species  repeat  the history of their successive evolutionary ancestor during and their embryonic  development. This states ontogeny repeats phylogeny.