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Fertilization and Development

Fertilization is the union (fusion) of a monoploid sperm nucleus (n) with a monoploid egg nucleus (n).

During meiosis the chromosome number (2n) was reduced.

In fertilization, the resulting diploid zygote has it’s number of chromosomes restored.

n + n = 2n (male monoploid) (female monoploid) (zygote diploid)

Fertilization must occur in a moist environment.

There are two types of fertilization. They are:

External Fertilization and Internal Fertilization

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External FertilizationReproduction of many aquatic vertebrate animals such as fish and amphibians is characterized by external fertilization. This is fertilization that occurs outside the body of the female, in the water. Usually, because of a lack of protection from consumers and a harsher environment, large numbers of eggs are released into the water to ensure species survival.

Internal FertilizationReproduction in most terrestrial vertebrates, such as birds, reptiles, and all mammals, occurs in the moist reproductive tract of a female. This is internal fertilization. In this case, only one or two eggs are released, because the environment is protected.

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Embryonic DevelopmentThere are a few stages of development after the egg is fertilized. After fertilization, the fertilized egg (zygote) undergoes a series of rapid mitotic cell divisions. This is called cleavage.

Stage 1: CleavageDuring cleavage there is no increase in cell size, just an increase in cell numbers.

First the many new cells forms a mass called the morula. Then, as the mitotic cell division continues, the cells form a blastula, made of hundreds of cells. The blastula is a single layer of cells with a hollow ball-type structure.

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Stage 2: Gastrulation

In certain animals, one side of the blastula becomes indented, forming the gastrula.

There is an inner layer called the endoderm.

There is an outer layer called the ectoderm.

There is a third layer between the ectoderm and endoderm called the mesoderm.

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Stage 3: Differentiation

Three embryonic layers differentiate and give rise to various tissues, organs, and systems of the multicellular animal.

The muscles, circulatory system, skeleton, excretory system, and gonads originate from the mesoderm layer.

The nervous system and the skin originate from the ectoderm layer.

The lining of the digestive and respiratory tracts, and portions of the liver and pancreas, originate from the endoderm layer.

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Stage 4: GrowthGrowth is an increase in cell number and cell size. Early development consists chiefly of the differentiation, growth and development of the specialized cells, tissues, and organs.

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External DevelopmentExternal Development occurs outside of the female’s body in both terrestrial and aquatic environments.

In Water:

The eggs of many fish and amphibians are fertilized externally and develop externally in an aquatic environment. The survival rate is generally low, which accounts for the large number of fertilized eggs produced. The developing embryo’s source of food is the yolk stored in the egg.

On Land

Eggs of birds, many reptiles, and a few mammals develop externally on a land environment after internal fertilization. Since there is a better survival rate, a somewhat fewer number of fertilized eggs are produced. The developing embryo’s source of food is the yolk.

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Amniotic Egg

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Some adaptations for animals which develop externally on land are a shells. Shells provide protection during embryonic development. The shell membranes we need to know are:

Amnion – contains the amniotic fluid. This fluid provides a watery environment, protects the embryo from shock, and prevents the tissues from tearing.

Yolk Sac – surrounds the yolk. Blood vessels which penetrate the yolk sac transport food to the developing embryo.

Allantois – functions as a respiratory membrane and a storage site for the nitrogenous waste, uric acid.

Chorion – the outer membrane surrounding the other embryonic membranes that separates them from the environment.

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Internal DevelopmentThis involves the growth of the embryo within the body of the parent which provides nutrition and protection. This has a higher survival rate than external development.

Placental MammalsThe embryo develops in the uterus. The eggs of mammals have little yolk and are therefore very small.

Within the uterus, an organ called the placenta is formed from the embryonic and maternal tissues. The placenta is where the exchange of respiratory gases, nutrients, and wastes occurs between the mother and embryo.

There is no direct connection between maternal and embryonic bloodstreams. The blood does not mix. Transport is accomplished by both active transport and diffusion. An umbilical cord, containing blood vessels, attaches the developing embryo to the placenta.

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Internal DevelopmentMarsupials

Marsupials are non-placental animals. Their internal fertilization and internal development occur without direct nourishment from the parent.

The source of food is the yolk stored in the egg.

The embryo is born at a relatively premature stage compared to the placental mammals and completes its development externally in a pouch which contains mammary glands. The mammary glands produce milk for the very small baby.