FERTILIZATION

Human zygote during fertilization (fertilization in vitro, optical microscope).
The ovum has been fertilized by the spermatozoa. It has emitted his two polar globules (on the top, between the ovum and the striated membrane). The male and female pronuclei (of the spermatozoa and the ovum) start to smelt (karyogamy).

During ther fertilization, The DNA contained in the head of the spermatozoa (paternal genome) is poured into the cytoplasme of the ovum. This condensed DNA will decondense and be surrounded of a nuclear membrane, to form a "pro-nucleus", the pronucleus. Parallelly, the maternal pronucleus get organized around the maternal genome. Each pronucleus contains a batch of chromosomes, paternal or maternal, which means that it is haploid (23 chromosomes).
Then, the two pronuclei come closer and smelt so as to engender a single diploid nucleus (46 chromosomes, contening the maternal and paternal genome). The zygote is formed.