- Last updated
- Save as PDF
- Page ID
- 93585
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\)
\( \newcommand{\vectorC}[1]{\textbf{#1}}\)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}}\)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}}\)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)
After the ovum or egg ripens and is released from the ovary, it is drawn into the fallopian tube and reaches the uterus in 3 to 4 days. When fertilized, the egg is typically fertilized in the fallopian tube and continues its journey to the uterus. At ejacul*tion, millions of sperm are released into the vagin*, but only a few reach the egg, and typically, only one sperm fertilizes the one egg. Once a single sperm has entered the egg’s wall, the wall becomes hard and prevents other sperm from entering. The tail of the sperm that is inside the egg breaks off, and the head of the sperm, containing the genetic information from the father, unites with the nucleus of the egg. As a result, a new cell is formed. This cell, including the combined genetic information from both parents, is called a zygote. Azygoteis a new single-celled organism formed through fertilization.
The Stages of Prenatal Development
Prenatal development is divided into three stages: the germinal period, the embryonic period, and the fetal period. These stages vary in terms of length and developmental outcomes.
Germinal Stage
The germinal period, about 14 days in length, is the shortest stage and lasts from conception to implantation of the zygote (fertilized egg) in the lining of the uterus. During this stage cell division with no significant growthproduces a cluster of cells that is the same size as the original zygote. This processis called cleavage. At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula. After the fourth doubling, differentiation of the cells also begins to occur. About 60 percent of natural conceptions are estimated to fail to implant in the uterus. The rate is higher for in vitro conceptions.
Embryonic Stage
This period begins once the organism is implanted in the uterine wall. It lasts from the third through the eighth week after conception. During this period, cells continue to differentiateand22 days after conceptionthe neural tube forms, becoming the brain and spinal column. Growth during prenatal development occurs in two primary directions: from head to tail (cephalocaudal development) and from the midline outward (proximodistal development). This means that those structures nearest the head develop before those nearest the feet, and those nearest the torso develop before those away from the center of the body (such as hands and fingers). The head develops in the fourth week, and the precursor to the heart begins to pulse. In the early stages of the embryonic period, gills and a tail are apparent. But by the end of this stage, they disappear, and the organism takes on a more human appearance.
About 20 percent of organisms fail during embryonic development due to gross chromosomal abnormalities. As in the case of the germinal period, often the mother does not yet know that she is pregnant. During this stage, the major structures of the body take form, making the embryonic period the time when the organism is most vulnerable to the most significant damage if exposed to harmful substances. (We will look at this in the section on teratology below.) The embryo is approximately 1 inch long and weighs about 4 grams at the end of this period. The embryo can move and respond to touch at this time.
Fetal Stage
From the ninth week until birth, the organism is called a fetus. During this stage, the major structures are continuing to develop. By the twelfth week, the fetus has all its body parts, including external genitalia. In the following weeks, the fetus will build hair, nails, and teeth, and the excretory and digestive systems will continue to develop. At the end of the twelfth week, the fetus is about 3 inches long and weighs about 28 grams.
During the fourth to sixth months, the eyes become more sensitive to light, and hearing develops. The respiratory system continues to grow. Reflexes such as sucking, swallowing, and hiccupping develop during the fifth month. Cycles of sleep and wakefulness are present at that time as well. The first chance of survival outside the womb, known as the age of viability, is reached at about 22 and 26 weeks.Most neurons in the brain have developed by 24 weeks, although they are still rudimentary, and the glial cells that support neurons continue to grow. At 24 weeks, the fetus can feel pain.
The fetus primarily prepares for birth between the seventh and ninth months. It is exercising its muscles; its lungs begin to expand and contract. It develops fat layers under the skin. The fetus gains about 5 pounds and 7 inches during this last trimester of pregnancy, including a layer of fat accumulated during the eighth month. This layer of fat serves as insulation and helps the baby regulate body temperature after birth.
(unable to fetch text document from uri [status: 0 (UnableToConnect), message: "Error: TrustFailure (Ssl error:1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED)"])
