There are certain changes in spermatozoa which must take place before fertilization. These are capacitation and acrosome raction. Fertilization is fusion of male and female gametes (both having haploid number of chromosome) to form a single cell ‘zygote’, having diploid number of chromosome. After fertilization the zygote passes through various stages of development like 2-4-8 cell embryo, morula, blastocyst. In blastocyst the cells are divided in inner and outer cell mass. The inner cell mass forms embryo proper while outer cell mass forms accessory structures like chorion, amnion, etc. During development, inner cell mass forms bilaminar embryonic disc with ectoderm and endoderm. The third layer ‘mesoderm’ is added later to form trilaminar embryonic disc.