Parthenogenesis

Reproduction is of two types, sexual and asexual. In sexual or biparental reproduction union of two sex cells, the sperm and the egg, is involved. In asexual reproduction offspring are produced without the involvement of sex cells. Asexual reproduction includes fission, sporulation, budding and parthenogenesis. Parthenogensis is the development of an egg without fertilization. Although included under asexual reproduction, parthenogenesis is strictly speaking a form of sexual reproduction since it involves sex cells.

Parthenogenesis has only been defined as the development of an egg which has been activated by some agent other than the sperm. Sometimes, however, a sperm may stimulate an egg into development, without contributing its nucleus. This form of development it also parthenogenesis.

Certain conditions are intermediate between parthenogenesis and fertilization, e.g. partial fertilization, gynogensis, androgensis and merogony. In partial fertilization the egg may be fertilization by only a part of the sperm. In the sea urchin egg, Boveri has described the fertilization of the egg by the sperm aster only. The sperm nucleus fuses with the egg nucleus only in the two-cell stage, or even in the four-cell or eight cell stages. In gynogenesis the sperm penetrates the egg but takes no part in development. It degenerates in the egg without fusion with the egg nucleus. In androgenesis the egg is activated by the sperm and development takes place without the participation of the egg nucleus. If the eggs of frogs and toads are treated with radium and then fertilized by normal sperms, the eggs nucleus does not take part in development. In eggs developing by androgenesis, the nuclei of the embryo are of paternal origin only. In merogony egg fragments devoid of a nucleus develop when fertilization by a normal sperm. If sea urchin egg are shaken to small pieces the fragments round up to form spheres. Some of these spheres are without nuclei. If such spheres are fertilized by normal sperms they may develop into dwarf larvae.

Parthenogenetic development of the egg under natural conditions is called natural parthenogenesis. Parthenogenesis experimentally produced in the laboratory is called artificial parthenogenesis. It may result in an adult, or development may proceed only to some embryonic stage.