Oogenesis and spermatogenesis both involve the chromosomal-reduction divisions of meiosis, which takes the gametes from the full complement of chromosomes to number of chromosomes.

Oogenesis and spermatogenesis both involve the chromosomal-reduction divisions of meiosis,which takes the gametes from the full complement of chromosomes to the Haploid number of chromosomes. Meiosis is the  type of cell division that take place in the gamete cells, where diploid cells of the germ lines undergo cell division to yield haploid cells, normally the gametes are made through this process, that is, the ovum through Oogenesis, and the sperm through spermatogenesis. During fertilization the two combines to produce a diploid cell once again that undergoes continuous mitosis to form a zygote.

Hydrosphere is the correct answer