Neurobiology, a branch of Biology Topics, investigates the structure and function of the nervous system.
What Type of Reproduction Takes Place in Hens ? And Metamorphism — Lifecycle of Frogs And Insects
We will now discuss the reproduction in hen.
The Case Of Hen
The hen is a bird. Internal fertilisation takes place in hen also. But a hen does not give birth to chicks (like human beings give birth to babies). We will now describe how chicks are born.
- After fertilisation takes place inside the body of the hen, the fertilised egg (or zygote) divides repeatedly to form embryo which travels down the oviduct.
- As it travels down the oviduct, many protective layers are formed around the embryo. The hard shell that we see in a hen’s egg is the outermost protective layer (other protective layers being inside the egg shell).
- After the hard egg shell is formed around the developing embryo, the hen finally lays the egg. That is, the hen’s egg comes out of its body.
- The hen then sits on the eggs to provide sufficient warmth to the eggs for the development of the embryo into the chicks. The embryo takes about 3 weeks (21 days) to develop into a complete chick. The development of the chick from egg takes place inside the hen’s egg during this period (see Figure).
- After the chick is completely developed, the egg shell breaks open automatically and the chick comes out of it. This is how a chick is born.
During its development into chick, the embryo gets all its food from the egg yolk. The albumen present in egg helps to protect the embryo from damage. The embryo obtains oxygen by the diffusion of air through the egg shell and other membranes. The embryos of all the birds and reptiles, etc., develop in the same way (like that of hen) inside their shelled eggs.
In animals (such as frogs, fish, etc.) which undergo external fertilisation, development of the embryo takes place in the eggs, outside the female body. In such animals, the embryos continue to grow within the egg coverings. After the embryos develop into complete young ones, the eggs hatch and the alive young ones come out. From this discussion we conclude that :
- Some embryos (like those of humans, cats, dogs, etc.) grow inside the mother’s uterus till they have completely developed into baby animals, but
- Some embryos (like that of birds, reptiles and insects, etc.) grow inside the eggs which the mother has laid, till they have completely developed into baby animals.
Before we go further and discuss ‘metamorphosis’, we should know the meaning of the terms larva, caterpillar and tadpole. Larva is an immature form of an animal (like frog or silk moth) formed by the hatching of its eggs. The embryos present in the eggs of some amphibians (like frogs) and insects (like silk moths) first grow into larvae.
The larvae change into adults later on. The larva of an animal is very different in appearance from that of the adult animal. The larva of silk moth is called caterpillar. The larva of frog is called tadpole. Tadpole is the tailed, aquatic larva of frog breathing through gills and lacking legs, until the later stages of its development.
We know that a number of animals (such as birds, frogs and insects, etc.) are hatched from eggs. For example, chicks are hatched from the eggs of hens. Similarly, the frogs and silk moths are also hatched from their eggs. Now, when a chick comes out of the egg of a hen, its over-all appearance is almost the same as that of an adult hen except that it is much smaller than the hen. So, when a small chick grows and develops to become an adult hen (or cock), then there is not much change in its body appearance.
That is, the young one of hen (chick) hatched from the egg and the adult hen look alike. But this is not so in the case of animals such as frogs and silk moths, etc. For example, the young ones (larvae or tadpole) in frogs hatched from the eggs look very different from the adult frogs. The process of transformation from an immature form of an animal like ‘larva’ to its ‘adult form’ in two or more distinct stages, is called metamorphosis. In most simple words, the change of a larva to an adult animal is called metamorphosis. Metamorphosis occurs in amphibians (like frogs) and insects (such as silk moth, butterfly, housefly and mosquito, etc.).
Metamorphosis in Frog
The hatching of a fertilised egg of frog produces a very immature young one called tadpole (which is a larva) [see Figures (a) and (b)]. The tadpole (or larva of frog) develops gradually and undergoes many drastic changes in appearance before it forms an adult frog [see Figure (c)], The tadpole looks very different from the adult frog. We say that during its life cycle, a frog undergoes metamorphosis. The transformation of ‘larva’ into an ‘adult’ through drastic changes in appearance is called metamorphosis. The change from tadpole to frog is an example of metamorphosis. This can be represented as :
We can see from Figure that a tadpole and the adult frog look very different from each other. The body features which are present in an adult frog are not present in its immature form called tadpole.
Tadpole is adapted to live life in water only. Tadpole has a long tail which helps it to swim easily in water. Tadpole breathes through the gills (like fish). It eats only tiny water plants and small aquatic animals. The tadpole gradually transforms into an adult frog by losing many old features of its body and developing new body features in many stages (which have not been shown in Figure). An adult frog is adapted to live life in water as well as on land. The adult frog develops webbed feet which help it to swim in water and also to hop on land (it has no tail).
The adult frog has lungs for breathing in air on land (it has no gills). The adult frog can also breathe through its thin, moist skin. The frog has a long and forked tongue which can be flicked out to catch insects as food. The eyes in frog move to the top of the head so that the frog can see above the surface of water. In this way, by shedding old body features and developing new body features, a tadpole changes into a frog. This change is called metamorphosis. Metamorphosis changes the aquatic tadpole (water living tadpole) into an amphibian frog (which can live in water as well as on land).
Metamorphosis in Silk Moth
Silk moth is an insect (like butterfly) which undergoes metamorphosis during its life cycle. The silk moth passes through the larva and pupa stages during its development after hatching from the egg and forms an adult silk moth. The various stages in the development of a silk moth can be represented as follows :
The changes during the formation of an adult silk moth from its egg can be shown by diagrams as follows :
Please note that the caterpillar (larva) and pupa stages in the development look very different from the adult silk moth. So, there have been drastic changes in appearance during the transformation of caterpillar into the adult silk moth. Thus, the change from caterpillar to a silk moth is an example of metamorphosis.
We (humanbeings) also observe changes in our body appearance as we grow but these changes are not drastic changes. In human beings, the body parts in adults are similar to those which are present in babies from the time of birth. So, human beings do not undergo metamorphosis. The common animals such as cats, dogs, hens, tiger, lion, deer, horse, cow, etc., also do not undergo metamorphosis during their life cycle. We will now discuss asexual reproduction in animals.