Jain Biology: Reproduction In Animals

Asexual Reproduction

1. Fission: Reproduction in its simplest form involves the splitting of the whole organism into equal or unequal parts. This is called fission. It is binary if it results in two daughters and multiple if the result is more than two daughters. The division of the cytoplasm is accompanied by the mitotic division of the chromosomes. Both binary and multiple types of fission are usual and characteristic method of reproduction among protozoans. In parasitic Protozoa where multiple type is more common, the products are called schizonts. The malarial parasite forms schizonts inside red blood cells of the host. When they are liberated, each one penetrates a fresh cell.
2. Fragmentation: When processes similar to fission take place in some metazoans also, they are called fragmentation. In sea anemones and most of the corals, it is the normal method.
3. Budding: This process is quite common among coelenterates such as hydra. It results in the production of a group of new cells which eventually form new individuals. The rate of growth of buds is determined by the availability of food.
4. Regeneration: In this process restoration of a complete, functioning individual from some part of the parent body other than the gamete, takes place. Protozoa, Porifera, Coelenterata and Planaria can regenerate fully from as small as 0.5% of the parent body. Planarians show remarkable powers of regeneration. If an individual is cut into two, the anterior end will regenerate a tail and the posterior end develops a new head. A middle piece will regenerate both a head and a tail, earthworms also have considerable powers to regenerate. In general the regenerative capacity is greater in animals of lower grades of evolution. The exact mechanism of the regulation of regeneration is far from fully understood.

Sexual Reproduction

In addition to the asexual methods of reproduction described above, some protozoans also show sexuality accompanied by reproduction. As stated before, the basic characteristic of sexual reproduction is the fusion of two specialized sex cells called gametes. The body formed by the fusion of the two gametes is called a zygote and the term common for all types (and there are many) of sexual reproduction involving a zygote is ZYOOGENESIS.

In protozoa, this is done by two whole animals uniting temporarily for nuclear transfer—conjugation.

Gametic union requires two types of gametes. In metazoans„one of the gametes is large and heavy, usually with incorporated food material (called the egg or ovum) while the other is smaller and mobile usually with a flagellum (called spermatozoa or sperms). The fusion of such dimorphic gametes is called oogamy, which is generally the rule in metazoans. Despite differences in their appearance, the gametes are alike in one important respect: each has in its nucleus a haploid set of chromosomes, the only difference possible being in the sex-chromosomes.

The development of eggs and sperms is carried out in special organs called ovaries and testes respectively (common term gonad). In the great majority of metazoans an individual possesses either testes or ovary only (unisexual) and thus two types of individuals—female and male - are present in metazoans. There are other differences, called secondary sexual characters, also but they have nothing to do with the reproductary system as such.

The union of gametes—sperm with the egg—is called fertilization. It includes both the union of gametic nuclei and the cytoplasm and leads to restoration of deploy and initiation of cell division. In some animals, fertilization is completed only after the sperm penetrates the egg membrane and enters the cytoplasm. In its simplest form—common in many aquatic animals—the process of fertilization requires only the synchronized discharge of male and female products into a particular limited are of the surrounding water where union takes place. This is external fertilization and to achieve positive results animals have to produce hundreds of thousands of eggs and millions of sperms.

Considering the slim chances and the wastes involved, species belonging to higher phyla have evolved mechanisms to introduce the sperms directly into the body of the female—internal fertilization. Males possess special structures for depositing the sperms into the female and the process is called copulation or mating. Internal fertilization is unavoidable for all animals which lay eggs on land as they must be protected with impervious coverings. After fertilization the eggs are invested with their outer coverings while still in the oviduct and then they are laid. Thus it is that completely terrestrial animals from earthworm to mammals all have internal fertilization. All animals which lay eggs are called oviparous.

Some animals retain the fertilized eggs inside the mother's body till the development is complete so that, fully formed miniature adults and not helpless eggs come out of the female body. During development, embryo derives nourishment directly from the mother's body. For this purpose it forms a structure called placenta through which diffusion of nutrients and oxygen to the embryo as well as excretory products from the embryo takes place. Animals which never lay eggs are called viviparous (Indian shark and most mammals).

Gradations between oviparous and viviparous exist. Some animals retain the fertilized eggs inside the mother's body till the development is completed but don't form a placenta and so the connection between the mother and the embryo is less intimate than in the viviparous forms. This condition, common among a-number of lizards and snakes, is called ovoviviparity.

In some cases both ovary and testis may be present in the same individual. This condition is called hermaphroditism, which is of two types—a) both reproductive organs may be permanently present in an individual as in the case of earthworm or, b) there may be a single gonad which alternately produce sperms and ova. Hermaphrodites are considered primitive and some are found in every phylum. Among vertebrates some fishes are seasonally so. Oyster,^[1] the bivalve mollusk found in the sea, is a good example of seasonal hermaphrodite. The larva which attains adulthood will be a male and will discharge sperms in its first season. Later, the same individual produces ova instead of sperms. Sex-reversal may occur several times if conditions are favorable.

Reproduction from either male or female gamete without the cooperation of a gamete of the opposite sex is called Parthenogenesis. Reproduction by unfertilized ovum (female parthenogenesis) occurs in at-least some animals belonging to all phyla but in some it is obligatory, i.e., is the sole form of reproduction. In bees, rotifers and some others, males are produced pathenogenetically; the fertilized eggs develop into females, or into the queen and the workers as in the case of honey-bee. How exactly the egg is activated in parthenogenesis is still not clear.