Female gametes Female gametes - egg cells (ova) - are one of the very few types of multicellular animal cells that have an ideal spherical sha

Gametes structure [male female, ovum sperm, cell]

Female gametes

Female gametes — egg cells (ova) — are one of the very few types of multicellular animal cells that have an ideal spherical shape. They are immobile. They are the largest cells. Typically, their diameter amounts to a fraction of a millimeter. But there are also giant cells. The diameter of the egg cell of porbeagle, for example, equals to 22 centimeters. The biggest ovum and at the same time the biggest cell organism, now living in the world, is considered to be egg yolk of African ostrich (weight of eggs of this bird varies from 1.5 to 2 kg. Large eggs are a distinctive feature of the vertebrate animals: fish, amphibians, reptiles and birds in particular. In mammals they are relatively small. Human ovum is only 0.2 mm in diameter. Gigantic size of female gametes is due to the fact that in their cytoplasm there is a lot of a reserved substance — the yolk (diagram 8). Taken from http://worldofschool.org

Diagram 8. The structure of the egg cell by the example of bird eggs: 1 — egg containing a yolk; 2 — protein; 3 — the air chamber; 4 — eggshell
Diagram 9. The structure of the birds’ sperm: 1 — head; 2 — neck; 3 — the middle part; 4 — tail

Male gametes

Male gametes, on the contrary, are very small cells, looks quite similar to female. Male gametes of animals, algae and fungi are mobile. They have a special motion organ — flagella. Male gametes of animals are sometimes called spermatozoon and of plants — spermatozoa (sperm). But most often, the latter definition assigns to all male gametes. Sperm of vertebrates has the shape of a long string. There can be distinguished the head, which contains the nucleus, the neck, which contains the centriole, body and tail — flagella (diagram 9). Spermatozoa are almost devoid of cytoplasm.

Now every enlightened person knows well that sperm gives rise to a new life. However, the significance of sperm in the fertilization remained unknown for a long time. The discoverers of sperm were A. Leeuwenhoek and his student J. Ham, who called them animalcules — “little animals”. For a long time the sperm was considered as some parasites and only in the early XIX century was introduced the concept of sperm and sperm importance in fertilization process was proved.

  • What are the features of the structure of the egg cells and the sperm?

  • Why does the size of eggs increase in accordance with the level of organization among vertebrates: fish, amphibians, reptiles, birds, and in placental mammals they are much smaller?