In this article we will discuss about the asexual and sexual methods of reproduction in animals.
A new animal comes into existence by the transformation of some part of a pre-existing animal; this is reproduction.
The animals reproduce their kind by two fundamental methods:
(1) Asexual, and
(2) Sexual.
Asexual reproduction involves only one parent and no special reproductive structures. Sexual reproduction usually involves two parents and the union of two germ cells, or of two cells of some kind, or of two nuclei derived from different cells.
1. Asexual Reproduction:
It occurs only in simpler and lower forms of animals. Vertebrates never reproduce asexually. The more important types of asexual reproduction are: binary fission, multiple fission, and budding.
a. Binary Fission:
In this an individual divides into two equal halves after which each part grows to the original form. When occurring in a protozoon, the division of the cell body is always preceded by the division of the nucleus.
Fission may occur in a transverse plane, as in Amoeba and Paramoecium, or it may be longitudinal, as in Euglena. The most important feature of fission is that the parent disappears as an individual, and the two new individuals take its place.
b. Multiple Fission:
This is found only in protozoa where the nucleus fragments repeatedly and then the cytoplasm divides, so that a part of it surrounds each of the nuclear fragment. Each bit of nucleated fragment is virtually an asexually produced spore; hence multiple fission is also known as sporulation. Multiple fission is usually preceded by encystment as in Amoeba. In Plasmodium, however, there is no encystment.
c. Budding:
When an animal divides into two unequal parts, it is said to reproduce by budding. The larger portion is regarded as the parent and the smaller one as the offspring. A bud usually arises as a small protuberance from the body of the parent; it grows larger and develops similar parts. It may remain attached to the parent or it may separate and live independently. Budding is the usual method of reproduction in Hydra.
2. Sexual Reproduction:
Higher animals usually reproduce sexually; each new organism originates from the union of two germ cells. These are the male and female gametes, and are known respectively as spermatozoa and ova.
The gametes are produced in gonads, the ova in ovaries and the spermatozoa in testes. There is a great interest in the ultimate origin of the gametes, because they serve to transmit hereditary characters from the parent to the offspring.
August Weismann (1834-1914) held that the gametes or germplasm are totally kept apart from the influence of other body cells or Somatoplasm, and are transmitted continuously from generation to generation. It has been definitely established that at least there is a continuity of the chromosomes by which the hereditary characters are transmitted from parents to offspring.
The germ cells are derived from the germinal epithelium of the gonads. The primordial germ cells are known as spermatogonia in the male, and as oogonia in the female. At the approach of sexual maturity these cells multiply rapidly by mitosis and the nucleus of each cell contains a dual set of chromosomes, which is the diploid number and is represented as 2n.
The process by which the spermatogonia become spermatozoa and the oogonia become ova is known as gametogenesis. It involves two successive divisions—one of which is a reduction division or meiosis. As the gametes of the two sexes differ in form, size, and behaviour, the gametogenesis, therefore, occur differently in the two; but the meiotic changes associated with the formation of the gametes are identical.
a. Spermatogenesis:
In male, the spermatogonia are converted into full-grown cells known as the primary spermatocytes, each of which has 2n chromosomes. Every primary spermatocyte divides twice in quick succession. The first division is meiotic, and produces two secondary spermatocytes, each having in chromosomes in its nucleus.
During the following mitotic division each secondary spermatocyte divides into two spermatids. Thus a primary spermatocyte yields four spermatids, each containing n chromosomes, which represents the haploid number.
It may be pointed out here that from each pair homologous chromosomes in a primary spermatocyte, any one spermatid receives only one. The spermatids undergo a process of metamorphosis and are converted into the spermatozoa. The whole process is known as spermatogenesis.
b. Sperm:
The spermatozoon is small and motile. It is usually thread-like in appearance and consists of a head, a middle piece and a tail. The oval head represents the nucleus and the middle-piece encloses the central-body. The flagella-like wavy tail is composed of cytoplasm.
c. Oogenesis:
In females, the oogenesis is slightly different. The oogonia are converted into primary oocytes with 2n chromosomes. Each primary oocyte undergoes two successive maturation divisions of which the first one is meiotic.
In the first division it is converted into two unequal cells—a large secondary oocyte, and a small first polar body with very little cytoplasm. Later division results in the formation of a second polar body and the mature ovum.
The polar bodies are useless and never function as gametes. Thus each primary oocyte yields but one mature ovum, containing n chromosomes; yet the nuclear changes that produce the ovum and the polar bodies are equivalent to those by which four spermatozoa are derived from a primary spermatocyte.
d. Ovum:
The mature ovum is a spherical or oval, non-motile cell. It usually contains a variable quantity of yolk to nourish the future embryo. The hen’s ovum is about 30 mm. in diameter, whereas a toad’s ovum is only 2 mm.
The human ovum contains very little yolk; hence it is as small as 0.15 mm. in diameter. When the quantity of yolk present in the ovum is very large, it is usually located at one pole of the cell, the vegetative pole; such an ovum is said to be telolecithal.
If, on the other hand, the quantity of yolk is very small, the ovum is said to be alecithal or microlecithal. The ovum is surrounded by a thin vitelline membrane which is derived from the oocyte. There is a prominent nucleus embedded in the centre of the cytoplasm. A central body is absent.
e. Fertilization:
The union of a mature spermatozoon with a mature ovum is known as fertilization, and the resulting cell is a zygote. In fertilization, the fusion takes place primarily between the two nuclei, each contributing n chromosomes. This restores the normal diploid number (2n) of the species, in the nucleus of the zygote.
Meeting of the ovum and spermatozoon always takes place in a fluid medium in order to permit the active movements of the motile male gamete. The ovum remains passive during the whole period. There is evidence to believe that the outer layer of the ovum secretes a chemical substance called fertilizing which attracts spermatozoa by positive chemo taxis.
On reaching the ovum, the head of the spermatozoon penetrates through the vitelline membrane along with the middle-piece; the tail, however, is left behind. A central body arises from the middle piece of the sperm and soon forms the spindle. The male and female pro-nuclei now break up into chromosomes which take their position in the equator of the spindle.
During this process the homologous chromosomes lie side by side in pairs. In every such homologous pair, one chromosome is contributed by each pro-nucleus. The fertilized ovum or zygote now divides into cells, containing 2n chromosomes. The products of division are known as the blastomeres which ultimately develop into a young of the same species.
Special Types of Sexual Reproduction:
In some protozoa, sexual reproduction involves union between two similar cells which are known as isogametes. The process is known as conjugation and is found in Monocytes. In other protozoa, such as Plasmodium, the gametes are dissimilar and hence they are anisogametes. The smaller and motile microgamete fertilises the larger and passive macrogamete.
In Paramoectum, two individuals conjugate temporarily and make an exchange of nuclear material. After this they separate and reproduce by binary fission. Parthenogenesis s Sexual reproduction usually involves two parents and the fusion of two germ cells.
An ovum, however, may develop into a young individual without being fertilized by a sperm. This peculiar phenomenon is known as parthenogenesis (parthenos =*virgin; genesis—origin).
Parthenogenesis occurs normally in ants, bees, wasps and aphids. The female of these animals lay both fertilized and unfertilized eggs. She can control fertilization by releasing or retaining the sperms which are stored in her spermatheca. In bees the males are derived from the unfertilized eggs, and the females from the fertilized eggs. In certain insects, having no males, reproduction is entirely parthenogenetic.
Parthenogenesis can be induced artificially in the egg of many animals which ordinarily require fertilization. Thus shaking the eggs vigorously, heating them, or pricking them with a needle, all have started development in certain eggs.
Loeb, by pricking thousands of frog-eggs with pointed needles obtained over two hundred tadpoles, and reared nearly one hundred frogs. Recently a rabbit-egg has also been developed parthenogenetically to produce a young rabbit.
Although parthenogenesis involves only one parent, yet by origin the cells giving rise to the new individuals are ova. It is for this reason that parthenogenesis is regarded as a kind of sexual reproduction.
Paedogenesis:
Sexual reproduction is usually carried on by adult animals. But this is not always the case. There are certain species who have remarkable power of reproducing sexually when they are in the larval state.
This peculiar sexual reproduction by a larval animal is known paedogenesis. The axolotl larva of the tiger salamander attains sexual maturity and breeds under certain conditions. Larva of certain flies produce ova which may develop by parthenogenesis. This is an instance of parthenogenetic paedogenesis.
Development in Animals:
As a result of sexual reproduction a new individual starts life as a single cell. It is the fertilized or activated eggs, or zygote. The zygote by repeated mitotic division produces many cells which in due time differentiate into the tissues and organs of the developing embryo.
This process is known as embryogeny and the branch of zoology which deals with the development of individuals is known as embryology. The following is a generalized account of the early development of an embryo.
i. Cleavage:
Soon after an egg is fertilised, the single-celled zygote begins to divide by mitosis. The first division results in two cells, the two divide into four, and so on. The process is known as cleavage and it results in the segmentation of the egg into a large number of smaller cells, each of which is a blastomere.
The solid cell mass, thus produced, is known as the morula. If the ovum is alecithal or microlecithal, the cleavage is total or holoblastic. If, on the other hand, the ovum is telolecithal, then the cleavage is. meroblastic and the blastomeres are unequal.
ii. Blastula:
As cleavage continues, the blastomeres become arranged in the form of a hollow ball, or blastula. The cavity of the blastula is designated as the blastocoele, or segmentation cavity.
Gastrula:
The blastula now undergoes gastrulation. This is a complicated process by which the hollow ball of cells is converted into a double walled cup, the gastrula. The blastocoele is gradually obliterated by pushing in of one side of the ball into the other, that is by invagination. The cavity of the cup constitutes the primitive gut or archenteron which communicates with the exterior by the blastopore.
When complete, the gastrula consists of:
(1) An outer layer of cells, the ectoderm,
(2) An inner layer of cells, the endoderm, and between these two layers develops
(3) A third cell-layer, the mesoderm.
These are the three primary germ layers which by modification and orientation produce the various tissues and organs. The ectoderm forms the outer covering of the body, the nervous system and the sense- organs.
The endoderm is transformed into the lining membrane of the digestive canal and the other structures associated with it. The mesoderm produces the supporting tissues, vascular tissues, muscles, and epithelial lining of the body cavity.
The preceding description of the formation of a new individual is more or less generalized. An outline of the development of a chick-embryo will now furnish a good example to illustrate the principles of embryology.
