Sexual reproduction
In sexual reproduction a new organism develops from the fusion of two gametes; gametes are formed in the sex glands. This combines the hereditary traits of the father and mother, increases the diversity of offspring and their adaptability to changed conditions, and is very important in evolution. The spermatozoon is small and motile: it consists of a head (the nucleus and the acrosome, a part with enzymes that dissolve the egg’s covering), a neck and a tail, with mitochondria providing energy. The egg cell is large and immobile and stores reserve food (yolk); in birds and reptiles it is especially large. Gametogenesis has four periods: multiplication (cell number increases by mitosis), growth (the cell enlarges, DNA doubles), maturation (meiosis, haploid set) and formation. In spermatogenesis one primary cell gives 4 spermatozoa (in the testis); in oogenesis one primary cell gives one mature egg and three small polar bodies (in the ovary), because the nutrients are concentrated in a single cell.
“Gamete portrait”. Two large posters: spermatozoon and egg cell. Groups stick parts (acrosome, nucleus, yolk, mitochondria, tail) onto the right poster and write each function.