Regents Prep: Living Environment: Reproduction:

Initial Development and Differentiation
The processes of gamete production, fertilization, and development follow an orderly sequence of events. Zygotes contain all the information necessary for growth, development, and eventual reproduction of the organism.

The zygote, which is a fertilized egg consisting of one cell, will begin to divide rapidly by mitosis forming the early developing human embryo.   Fertilization and the initial stages of this mitotic cell division occur in the oviduct.  The early embryo is migrates down the fallopian tube and completes most of its development in the wall of the uterus.

            Fertilization and Initial Development of the Embryo

The placenta is a combination of maternal and fetal tissue which allows for the exchange of materials with the fetus and mother.   Needed materials such as food and oxygen diffuse through the placenta to the fetus, while wastes from the fetus diffuse to the mother.   The umbilical cord is a fetal structure containing blood vessels which allows materials to be carried between the fetus and placenta in both directions.   The amniotic fluid surrounds the fetus and helps to provide a shock absorber to protect the fetus against mechanical injury in the event the mother is shaken or injured in some manner.

                    Fetal Development in the Uterus

The embryo will eventually develop into a three cell layered structure.   This structure is called a gastrula and will eventually differentiate to form the specialized cells.  Differentiation means that the cells will develop specific jobs and develop into specific tissues in the maturing organism.   An example of this is that the outer cell layer of the developing gastrula will develop into the skin and nervous system of a mature human organisms.  Most multicellular animals undergo a similar pattern
of development and differentiation.

Fetal Development
Development is a highly regulated process   After this small cluster of cells called the gastrula forms in humans, tissues begin to form. In humans, the embryonic development of essential organs occurs in early stages of pregnancy.  During the first three months of human development, organs begin to form.  The human embryo is usually referred to as a fetus when human like features become visible in its structure.    All organs and body features are developed by the end of the sixth month.   During the last three months of pregnancy, organs and features develop well enough to function after birth.

 Human Development at 6 weeks Human Development at 4 months

The embryo (or fetus) may encounter risks from faults in its genes and from its mother's exposure to environmental factors such as inadequate diet, use of alcohol, tobacco, drugs, other toxins, or infections.   

While the patterns of development discussed previously hold true for humans, these developmental patterns vary between different plants and different animals.  

Aging is a complex series of developmental changes which occur with the passage of time.  This
process is influenced by both heredity and the environment.   This process eventually leads to the
death of the organism.

Reproduction and development are subject to environmental impact. Human development, birth, Fertilization occurring in a petri dish.and aging should be viewed as a predictable pattern of events.

Reproductive Technologies
Reproductive technology has medical, agricultural, and ecological applications.  In many instances, these technologies have progressed at a faster rate than the ethical considerations resulting from these technologies.  Some of these techniques include birth control methods used to block the process of fertilization.   Many technologies now exist to enhance the process of fertilization and development in humans and other organisms. Hormone therapy can cause increased egg production.  Surgery can open blocked fallopian tubes in females and the vas deferens in males. In vitro fertilization (test-tube babies) is a widely used technique to aid infertile couples, allowing them to have children where this otherwise would not be possible.

The in vitro Fertilization Process


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