Fertilization is the fusion of the male and the female gametes with each other to give rise to a single-celled zygote. The zygote then undergoes repeated divisions to give rise to a new individual.
It may be external as in fishes where it takes place in water or internal as in mammals where it takes place in side body of female .In human being and other higher mammals although only one sperm enters egg, but many sperms are needed to clear way for one sperm by digesting theca interna cells with help of acrosomal enzymes .During fertilization head and proximal centriole of sperm enter egg , while distal centriole and tail remain out .Thud a sprm contributes one nucleus , one centriole and little cytoplasm while egg contributes one nucleus one centriole all mitochondria and most other cellular organelles, which is reason of maternal effect in animals
