Reproduction is one of the ways organism replicate themselves thereby giving birth to same organisms which shares the same genes. This is always a way organisms are added to the population.
During fertilization, the sperm fuses with the ovum to form a Zygote. The zygote later develops into a new individual. In multicellular animals such as mammals, the zygote attaches itself to the uterine wall and begins to grow and develop by cell division and differentiation into an embryo (or foetus).
STAGES IN THE DEVELOPMENT OF A TOAD
The croaking of the male attracts the female. The male mounts on the female and grasps it. Eggs are laid in strings of jelly in water. The jelly is for protection against injury, predation and water uptake. The male fertilizes the egg by spraying sperms on it. Fertilization is external and it takes place in water. After 1-2 days, the fertilized eggs hatches into tadpoles. The tadpole has a tail, a horny mouth, external gill and a long intestine.
Two to three days later, the external gills are replaced by the internal gills, covered by an operculum. The head becomes distinct with a pair of nostrils, eyes and two tympanic membranes. About 8-9 weeks from hatching, the hind limbs develop. In the 10th week, fore limbs formed under the opercula skin burst through the opercula. At this stage, the tadpole changes from herbivorous to carnivorous feeding which is accompanied by the shortening of the intestine. The lungs now develop and internal gills begin to disappear. The tadpole can now come to surface to breathe. The horny jaw disappears and it’s replaced by a wide mouth, eyes become more prominent and the tail shortens. It takes about 12 weeks for internal organs to develop. The tadpole now become a miniature toad with a tail which later disappears. The metamorphosis is then completed.
THE ROLE OF HORMONES IN THE DEVELOPMENT OF A TOAD
1. THYROXIN: This is important for metamorphosis to occur in toad. When it is absent, metamorphosis will not occur.
2. MELANOCYTE: This hormone stimulates a dramatic darkening of the frog’s skin.
ADAPTIVE FEATURES IN DEVELOPING ANIMALS
This is essential in animals as this helps them to adapt to their changing environment. Here, the animals adapt to different temperatures, hormonal imbalances, struggle for food and survival, etc. Most developing animals have features which help them to survive.
ADAPTIVE FEATURES IN FISH, TOAD, AND BIRD
The yolks in eggs of fish, toad and bird’s embryo are mainly rich in fats which when oxidized supply energy and water to the embryos.
In fish, the oil drop in the yolk serves both for energy source and as flotation organ. Respiration is through the semipermeable egg membrane and shell.
In toads, the eggs are covered by a jelly substance which protects them from injury, predators and water uptake.
In birds, there are four extra embryonic membranes and they are
1. The Yolk Sac which connects the embryo with its food.
2. The amnion which protects the embryo from heat and mechanical damage.
3. The Chorion which helps in the exchange of gases.
4. The Allantois which serves as a store for excretory products.
ADAPTIVE FEATURES IN MAMMALS
In mammals, extra embryonic membranes are developed. These are the placenta, Umbilical cord and the Amnion.
The placenta grows from the tissues of the embryo and joins the uterine wall. It is through the placenta that the developing embryo obtains food and oxygen from the maternal blood and also excretes waste products of metabolism. Placenta also produces the hormone oestrogen and progesterone. Thus, the placenta is nutritive, respiratory, excretory, and endocrine in function.
1. This connects the embryo to the placenta
2. It acts as a support to the embryo.
The amnion fluid cushions the foetus against shock. It reduces friction during birth.
ADVANTAGES OF THE EMBRYO DEVELOPMENT IN MAMMAL’S UTERUS OVER THE DEVELOPMENT IN TOAD’S
Internal development of embryo in mammals provides greater protection against shock or pressure or injury than in the toad’s embryo which develops externally.
Amnion in mammals ensures good aquatic environment for developing embryo unlike water in toad which might dry up exposing eggs to hazards.
Internal environment of mammalian uterus ensures constant environmental condition in contrast to body of water which is liable to fluctuating environmental condition.
In mammals, embryo developing internally in the uterus is protected, the amphibian’s eggs are laid outside in water exposed to predators.