Introduction
Prenatal development refers to a series of development stages that begins from the fertilization of the egg by the sperm cell to the onset of birth process. It begins with the formation of the zygote to the development of the fetus. It involves three major stages namely, germinal stage, embryonic stage and fetal stage. (Balinsky, 1975) The developmental stages take place partly in the fallopian tube and in the uterus. It usually takes a period of nine month in human being but varies from one species of animal to another.
There are several principles of prenatal development which basically guides the whole process. These principles are as follows. Development occurs in head to toe direction also known as cephalocaudal. That is the head develops first before the toes therefore the head and parts of the upper extremity are always more developed than the lower limb at any particular time during development. The basic forms of organs are first developed before they specialize to make functional organs. Therefore development occurs from basic form to specialized form. Development follows the order of importance. Important organs are the first to be developed followed by organs that are less important. Consequently the brain and the heart are the first organs to develop. (Carlson, 1999)
The fusion of the egg and the sperm to form a single cell called zygote marks the start of the germinal period and ends after the attachment of the blastocyst to the uterine wall leading to the establishment of pregnancy. It takes a period of about two weeks. Embryonic period picks up in the middle of the second week up to the end of the eighth week. So it takes about six weeks. This period is marked with the formation of the major organs of the body. Stem cells undergo division and differentiation to form specialized organ. Fetal period begins from week nine up to parturition. (Dudek & Fix, 1998). During this period the organism is called fetus. Continuation of differentiation and development occurs to make more structured organs.
Germinal developmental stage
The germinal period is probably the most important period in human and animal life since it is the initial stage of growth and development that eventually leads to formation of a live form that can interact with outside environment and exists on its own. I would actually be very right to say that life begins at this stage of life. Although conventional reasoning would suggest that life begins at birth and often the process that occurs before is disregarded. The first stage of the germinal development occurs in the fallopian tube whereby a mature egg is fertilizes by a successful sperm cell to form a single cell zygote, which undergoes successive divisions to produce two cells, four cells, eight cells, sixteen cells until a large mass of cells is produced. (Dudek & Fix, 1998) The kind of cell division that occurs during prenatal development is mitosis which applies only to somatic cells. The germ cells, which bear the genetic code that, determine the gene and phenotype of an organism are the ones that undergo meiotic division during their development.
The logic behind fertilization still puzzles many people since only one sperm out of a million is able to fertilize an egg, after which the rest cannot fertilize the same egg. Scientist has suggested this is possible due the formation of zona pellucida immediately after fertilization. (Prenatal Development and Birth). It encapsulates the zygote and prevents the entry of sperm. The sperm and the egg are germ cells that contain genetic information needed to form a new life form. The mitotic division of the zygote is referred to as cleavage and it start approximately twenty four hours after fertilization The zygote undergoes several mitotic divisions to form a mass of cell called a morula. This occurs within a period of two days after fertilization. The cells continue to divide and form two distinct mass of cells that is the outer cell and the inner cell. This phenomena leads to the formation of blastocyst, which is a morula containing a cavity filled with fluid. The blastocyst is made up of three parts namely endoderm, mesoderm and the ectoderm. The inner layer is called the endoderm. It is the precursor for the growth and development of most of the internal organs such as intestines, stomach, heart, liver and the lungs. Mesoderm is the middle layer and it differentiates to form connective tissues, bones, teeth, muscles, kidneys, blood and the circulatory system. The ectoderm is the outer layer. It differentiates during prenatal development to form; the integumentary system that is the skin, hair, sweat glands and tooth enamel; the salivary glands; the brain and the whole nervous system. ( Marieb, 2006). It should be noted that differentiation and development of the three sections of blastocyst to form various organs and tissues occurs in the embryonic period. The cells (blastocyst) moves from the fallopian tube into the womb as it divides down where it gets attached in to the uterine wall a process which is scientifically referred to as implantation. After five to seven days of implantation a placenta develops from the blastocyst to facilitate the supply of oxygen and essential nutrients needed for development. It also produces hormones that are responsible for preventing menstruation and the mother’s immunity from attacking the blastocyst as it is likely to be regarded as a foreign matter.
After the formation of morula the embryo does not increase in size but each successive cell division results into smaller cells in order to maintain the size. This is made possible due to the presence of a glycoprotein called zona pellucida. Zona pellucida actually is a coat on the blastocyst and does not allow an increase despite the fact that cells are undergoing rapid division. The lysis of zona pellucida occurs when the blastocyst has reached the uterus to permit its attachment to the endometrium. This occurs at approximately five days after fertilization. The blastocyst must have the right positioning and orientation during its attachment at the uterus for the pregnancy to be successful. In fact scientific community estimate that about sixty percent of pregnancy never goes through due to improper attachment of the blastocyst to the endometrium. This phenomenon usually happens without it being realized by the victim.
The science behind prenatal development had remained a mystery for centuries. Due to advance in science and the growing interest of unveiling the unknown mysteries of development, several researches have been conducted to find out exactly what happens in the biological environment of the womb that enables the process of fertilization, mitotic division of cells, growth, differentiation and specialization of cells to form tissues and organs that constitute a whole human being. Advance in imaging techniques and development of in vitro science have facilitated proper understanding of these processes. It is now possible to view images of developing embryo by employing the magnetic resonance imaging technique. (National Center for Biotechnology Information, 2011). In vitro fertilization has been conducted by taking the germ cells from their natural environment and doing artificial fusion in a highly biologically controlled environment to mimic the natural environment of the womb. This is usually referred to as IVF. Monitoring is usually conducted to ensure that fertilization has taken place and cell division has started. It is usually as successful as the In vivo fertilization. After the IVF the resultant cell is placed in incubator where it undergoes division to form an embryo. The embryo is taken into the uterus after approximately spending five days in the incubator. In vivo study involving living organisms have also been extensively conducted in order to monitor the whole process of antenatal development. (: IVF : American Pregnancy Association ).
Conclusion
Prenatal development is characterized by three major periods which includes; germinal period, embryonic period and fetal period. Germinal period is the initial stage that begins with conception and is concluded by the attachment of the blastocyst to the endometrium lining of the uterus. Embryonic period starts after the second week of fertilization. It is the period in which organogenesis occur leading to the formation of mature body organs such as the heart. It takes a period of six weeks. Fetal period is the longest since it takes about six months ultimately ends upon delivery. The germinal period is very important since it is the period in which a pregnancy is successfully established.Once fertilization occurs the zona pellucid develops on the zygote. It serves to preveent entry of other sperms and to maintain the size of the morula once the zygote has undergone several mitotic division. A blastocyst is formed when a fluid filled cavity develops on the morula. The inner cell mass of the blastocyst is called the trophoblast.
Advancement and development in scientific research has enable scientist to study and come up with findings on prenatal life thereby creating an understanding of what seemed to mysteries back then. Scientist have enabled to answer to answer some puzzles that have always intrigued people concerning prenatal life but still many are yet to be understood.

Reference
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