Essay on Sexual Reproduction Among Animals

The terms and concepts that are necessary to an understanding of reproduction as it occurs among organisms are described below with examples.

A few selected examples will be presented at this point. It should be kept in mind, of course, that there is a great deal of individual variation in the several species of the animal kingdom, and these types which are discussed constitute only a scant survey. In general, however, they may be considered representative.

Paramecium

If one obtains a quart or so of water from any pond found in temperate climates and immerses a handful of hay or straw in it, large numbers of Paramecium are almost true is relatively simple, an outer epidermis having developed from the embiyonic ectoderm, and with endoderm forming the inner gastrodermis. There is a network of nerve cells and certain contractile fibres that make limited muscular movement possible.

Specialized epidermal cells, concentrated largely on the tentacles, contain bodies called nematocysts, each of which releases a coiled thread upon being properly stimulated. These threads cany a paralyzing chemical substance and they enable Hydra to capture smaller animals which are utilized as food.

It is enlightening to place a hydra in a dish, to guide a smaller animal toward it, and to watch the process of capture. If the victim is taken from the hydra and examined micro sonically, it will be seen to have been shot through by some threads and entangled by others.

Unless interrupted, the hydra will guide the smaller paralyzed animal into its mouth by using its tentacles, and subsequent digestion occurs within the gastrovascular cavity.

We have already mentioned that asexual reproduction may occur in Hydra by the process of budding. Sexual reproduction occurs when testes or ovaries are formed from certain epidermal cells. Some species are monoecious, in which case one individual exhibits both types of reproductive organs, while others are dioecious, with individuals being definitely males or females.

In monoecious species, testes and ovaries generally do not form at the same time, and cross-fertilization thus occurs between different individuals. Several sperm from within a testis, with a flagellum, by means of which it swims actively.

Meanwhile, a single large egg forms within each ovary of a given animal, meiosis having occurred with the formation of polar bodies. The egg matures and is finally forced out of the ovary, apparently by a splitting of epidermal cells which were greatly stretched by growth of the egg.

It remains attached to the surface of the parent animal, however, where it is eventually fertilized by a sperm. The haploid sperm and egg unite and form a diploid zygote, and embryogeny of a new individual begins.

A blastula in formed, at about which time a thick wall is secreted around the embryo. Gastrulation occurs, the wall becomes thicker, and the embryo drops from the parent body. At this time, it undergoes a period of dormancy, the length of which depends upon several environmental factors.

Eventually, the thick wall bursts and development continues with the formation of an adult animal with its specialized cells. It is obvious that further overall development is quite limited from the histological standpoint, since the fully formed hydra is little more than a gastrula with tentacles.

Earthworm

Perhaps few animals are more widely known and easily recognized than is the animals are more widely known and easily recognized than is the earthworm, Lumbricus terrestris, a member of the phylum Annelida.

A study of its over-all morphology reveals the presence of advanced structural organization, including well-developed nervous, muscular, and circulatory systems. In its life habits, the earthworm spends most of the time beneath the surface of the soil, where it forms burrows and tunnels by actually eating its way along, although if the soil is quite soft, it may simply push its way through.

The ingestion of soil serves a twofold function; the worm is able to construct an elaborate system of tunnels, eventually depositing the castings on the surface of the soil, and it derives nutritive benefit from such organic materials as are digestible in the gastro-intestinal tract. For both of these reasons, the earthworm is of positive economic importance to the farmer.

Air may penetrate the soil to greater depths by virtue of the tunnels, thus hastening decomposition of organic materials by aerobic microorganisms, and the digestive and metabolic activities of the earthworm contribute to soil fertility. It has been shown that greater crop yields are obtained from soil containing earthworms than from similar soil which is devoid of them.

As is characteristic of all but the lowest animals in the phylogenetic scale, the earthworm does not reproduce asexually. In its sexual reproduction, each individual possesses functional ovaries and testes and is thus hermaphroditic. Self-fertilization does not occur, nor is any mechanism operative whereby sperm may travel freely from one individual to another, as is the case with Hydra, whose habitat is aquatic rather than terrestrial.

Instead, two earthworms establish body contact in the act of copulation, or sexual union, during which the two individuals exchange sperm. These are received by special receptacles in each worm where they are retained for a time.

Eventually, eggs are formed in ovaries which lie behind the sperm receptacles and in front of the clitellum, an obvious band which encircles the worm beginning at the thirty-first or thirty-second segment from the anterior end.

At about the time of egg formation, the clitellum secretes a thick layer of mucous which lies behind the sperm receptacles and in front of the clitellum, an obvious band which encircles the worm beginning at the thirty-first or thirty-second segment from the anterior end. At about the time of egg formation, the clitellum secretes a thick layer of mucous which gradually slips forward. As is passes the oviduct openings this mucous layer receives a number of eggs.

Similarly, sperm are received at the openings of the receptacles, and fertilization occurs. As the mucous layer finally slips off the anterior end and is deposited in the soil, its elasticity causes it to close, forming a pear-shaped cocoon whose greatest diameter is approximately that of a printed capital O.

There is a period of egg production by the worm, during which several cocoons may be produced in a similar manner. It has been reported that only one fertilized egg develops within each cocoon, even though several are present. Blastulation, gastrulation, germ layer formation, and further specialized embiyogency occur, and the young worm thus formed eventually breaks through the cocoon and begins an independent existence.

The Honeybee

Most insects, including the honeybee, exhibit a life cycle in which more than one form of body structure is seen. Because bees are social insects, forming large hives wherein a complex division of labour and a high degree of organization are developed, they are not to be considered typical either in this respect or in their reproductive habits, but several interesting features regarding the latter are worthy of our attention at this point.

Ordinarily, only one reproducing female bee, the queen, is present in a hive. From a single mating early in her life, she receives a supply of sperm which she may viable state over a period of years. As she produces fertilized eggs, which she is able to lay in prodigious numbers, they are placed in special wax cells of the hive by worker bees.

In about three days these eggs develop into larvae, which are small, wormlike forms that bear little resemblance to adult bees. Among the nonsocial insects, the larval form is generally free living but honeybee larvae remain within their wax cells, which are not completely closed. They are fed by “nurse” workers for about five days, during which time they grow tremendously.

Following this period, the larva spins a cocoon, and for about fifteen days it exists as a pupa, in which stage it undergoes drastic body changes. Finally, the adult bee emerges from the cocoon and chews its way out of the cell. It is now ready to take its place in the hive as a worker bee.

Individuals produced in the manner described above are sterile females, incapable of reproduction, and it is obvious that unless functional males and females are present at some point, the species could not persist.

It has been found that fertilized eggs laid by the queen will develop into functional females if they are fed a special diet called “royal jelly” during the entire larval period. This is apparently done when the hive has become sufficiently large to divide.