ANIMAL DEVELOPMENT

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ANIMAL DEVELOPMENT

• Mrs. Isaacs
• Biology II

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Unit Outline

• Early Developmental Stages
• Fertilization
• Embryonic Development
• Effect of Yolk
• Neurulation and the Nervous System
• Developmental Process
• Cellular Differentiation
• Homeotic Genes
• Human Embryonic and Fetal Development
• Embryonic Development
• Fetal Development

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Early Developmental Stages-Sperm

• A mature sperm is also known as a spermatozoa.
• Human sperm have three distinct parts a tail, a
  middle piece, and a head.
• The middle piece and the tail contain
  microtubules, in the characteristic 9 cilia to 2
  flagella pattern.
• In the middle piece, mitochondria are wrapped
  around the microtubules and provide the energy
  for movement.
• The head contains a nucleus covered by a cap
  called the acrosome acros-at the tip/somabody
  that stores enzymes needed to penetrate the egg.

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Early Developmental Stages-Sperm

• A single milliliter of semen from a normal human
  male will have between 50 and 150 million sperm.
• If less than 25 million sperm per milliliter are
  released, infertility may result.
• Fewer than 100 sperm reach the vicinity of the
  egg.
• With contractions of the female uterus, it takes
  spermatozoa about two hours to reach the egg
  traveling approximately 12.5 cm per hour.

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Early Developmental Stages-Egg

• The mammalian egg (oocyte) is surrounded by a few
  layers of adhering follicular cells, collectively
  called the corona radiata, which nourish the egg
  when it was in a follicle of the ovary.
• Next the egg has an extra-cellular matrix called
  the zona pullucida just outside the plasma
  membrane, but under the corona radiata.

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Early Developmental Stages-Fertilization

• The ovarian cycle occurs as a follicle changes
  form a primary to a secondary to a vesicular
  follicle.
• Epithelial cells of a primary follicle surround a
  primary oocyte.
• As a follicle matures, oogenesis (meiosis), is
  initiated and continues.
• The primary oocyte divides, producing two haploid
  cells (23 chromosomes). One cell is a secondary
  oocyte (egg), and the other is a polar body.
• The vesicular follicle bursts, releasing the
  secondary oocyte (egg), and develops into a
  corpus luteum.
• The egg enters the oviduct.
• If fertilization does not occur, the corpus
  luteum begins to degenerate in about 10 days.

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Early Developmental Stages-Fertilization

• Fertilization is the union of a sperm and an egg
  to form a zygote.
• Fertilization has a similar process in all
  mammals, including humans
• Fertilization requires three series of events
  that will result in a diploid (46 chromosomes)
  zygote.

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Early Developmental Stages-Fertilization

• Fertilization
• Several sperm penetrate the corona radiata and
  attempt to bind to the zona pellucida.
• After a sperm head bins tightly to the zona
  pellucida, the acrospme freleases digestive
  enzymes that forge a pathway for the sperm to
  enter the egg.
• One sperm enters the eggs plasma mebrane and
  their nuclei fuse

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Early Developmental Stages

• Mammalian secondary oocytes have a series of
  events to prevent polyspermy, the entrance of
  more than one sperm into a single egg.
• Once one sperm head touches the egg plasma
  membrane, the eggs plasma membrane depolarizes
  and changes the charge. This serves to repel
  other sperm only for a few seconds.
• Then vesicles in the oocyte called corical
  granules secrete enzymes that turn the zona
  pellicuda into an impenetrable membrane. The
  reactions serves as a long lasting block to other
  sperm.
• Micorvilli extending from the plasma membrane of
  the egg bring the entire sperm into the egg.

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Early Developmental Stages

• The sperm nucleus releases its chromatin, which
  reforms into chromosomes enclosed within the
  sperm pronucleus.
• In the meantime, the secondary oocyte completes
  ovulation and its chromosomes are also enclosed
  in a pronucleus.
• A single nuclear envelope soon surrounds both
  sperm and egg pronuclei.
• Cell division occurs immediately.
• The centrosomes that give rise to the spondle
  apparatus are derived from the sperms flagellum.
• The two haploid sets of chromosomes share the
  first spindle apparatus of the fertilized egg,
  now called a zygote.

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Embryonic Development

• Development is all the changes that occur during
  the life cycle of an organism.
• During first stages of development, an organism
  is called an embryo
• There are two stages of cellular development 1)
  cleavage resulting in a multicellular embryo and
  2) formation of the blastula.

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Embryonic Development

• Cleavage is cell division without growth.
• DNA replication and mitotic cell division occur
  repeatedly, and the cells get smaller with each
  division.
• NOTE Cleavage increases only the number of
  cells it does NOT change the original volume of
  the fegg cytoplasm.

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Embryonic DevelopmentCellular Stages

• Cleavage of a lancelet is equal and results in
  uniform cells that form a morula, which is a ball
  of cells.
• The 16-cell morula resembles a mulberry and
  continues to divide forming a blastula.
• A blastula is a hollow ball of undifferentiated
  cells having a fluid-filled cavity called a
  blastocoel.
• The blastocoel forms when the cells of the morula
  extrude Na (sodium ions) into extracellular
  spaces and water follows by osmosis, resulting in
  a hollow ball of cells.

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Embryonic DevelopmentCellular Stages

• The zygotes of other animals, such as a frog,
  chick, or human, also undergo cleavage and form a
  blastula.
• In frogs, cleavage is not equal because of the
  presence of yolka dense nutrient material.
• When yolk is present, the zygote and embryo
  exhibit polarity, and the embryo has an animal
  pole and a vegetal pole. In frogs, the animal
  pole has a deep gray color and the vegetal pole
  has a yellow color.
• All vertebrates have a blastula stage, but the
  appearance of the blastula can be different from
  that of the lancelet.
• Humans, birds, and reptiles develop a blastula
  that is a layer of cells that spread out over the
  yolk.

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Embryonic DevelopmentTissue stages

• There are two stages of tissue development early
  gastrulation and late gastrulation.
• The early gastrula stage begins when certain
  cells begin to push, or invaginate, into the
  blastocoel, creating a double layer.
• An early gastrula has two layers of cells. The
  outer layer of cells is called the ectoderm, and
  the inner layer is called endoderm.
• The endoderm borders the gut, but at this point,
  it si termed either the archenteron or the
  primitive gut.
• The pore created by invagination is the
  blastopore. In primitive animal species, such as
  the lancelet, the blastopore becomes the anus.

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Embryonic DevelopmentTissue stages

• Gastrulation is not complete until three layers
  of cells that will develop into adult organs are
  produced.
• In addition to ectoderm and endoderm, the late
  gastrula stage has a middle layer of cells called
  the mesoderm.
• NOTE ectoderm, mesoderm, and endoderm are called
  the embryonic germ layers. They are the first
  cells to be considered differentiated!!!

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Embryonic DevelopmentOrgan stages

• The organs of an animals body develop from the
  three embryonic layers.
• The newly formed mesoderm forms a supporting
  structure called the notochord. In primitive
  organisms, such as the lancelet, the notochord
  remains through the animals life. However, in
  vertebrates, the notochord is replaced by the
  vertebral column.
• The nervous system forms from midline (middle of
  the body) ectoderm located just above the
  notochord.

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Embryonic DevelopmentOrgan Stages

• A thickening of cells, called the neural plate,
  is seen along the dorsal (top) surface of the
  embryo.
• Then neural folds develop on both sides of the
  neural groove. This becomes the neural tube when
  these folds fuse.
• The anterior (toward the head) end of the neural
  tube eventually develops into the brain and the
  rest of the neural tube becomes the spinal cord.

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Embryonic DevelopmentOrgan Stages

• Mesoderm cells that did NOT contribute to the
  formation of the notochord now become two masses
  of tissue called somites. Somites eventually
  become the muscles of the axial (central body)
  skeleton
• A primitive gut tube is formed by the endoderm as
  the body itself folds into a tube.
• The heart begins as a simple tubular pump of
  endoderm cells
• Organ formation continues until the germ layers
  have given rise to all of the specific organs of
  an animals body.

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Development Process

• Development requires 1) growth, 2) cellular
  differentiation, and 3) morphogenesis
• Cellular differentiation occurs when cells become
  specialized in structure and function
• Morphogenesis produces the shape and form of the
  body
• Pattern formation, a stage of morphogenesis,
  determines how tissues and organs are arranges in
  the body
• Apoptosis, programmed cell death, plays an
  important role in pattern formation

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Development Process

• Scientists used to believe that irreversible
  genetic changes must account for differentiation
  and morphogenesis
• However, researchers have discovered through
  cloning animals from specialized adult cells
  shows that every cell in an organisms body
  contains a full complement of genes.
• Therefore ALL of the cells in an adult body of
  ANY organism is said to be totipotent, meaning
  that each one contains all the instructions
  needed by any other specialized cell in the body.
• Modern researchers have turned their attention to
  discovering the mechanisms that lead to
  differential GENE EXPRESSION
• Two mechanisms seems important cytoplasmic
  segregation and induction.

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Developmental ProcessCellular Differentiation

• Cytoplasmic Segregation
• -An embryo contains substances called maternal
  determinants, which influence the course of
  development.
• -Cytoplasmic segregation is the parceling ut of
  maternal determinants as mitosis occurs.

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Developmental ProcessCellular Differentiation
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Developmental ProcessCellular Differentiation

• Induction is the ability of one embryonic tissue
  to influence the development of another tissue
• A 1935 Nobel Prize winner, Hans Spemann,
  discovered that there may be a molecular
  concentration gradient that acts as a chemical
  signal to induce germ layer differentiation.
• In the experiment Spemann discovered that if
  presumptive nervous tissue was removed and
  transplanted to the belly region, it did not form
  a notochord.
• However, when presumptive notochord tissue is
  placed in the same region, a second notochord is
  formed.

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Developmental Process--Morphogenesis

• Pattern formation is the number one concept in
  morphogenesis.
• Studying Drosophila, fruit fly, investigators
  have uncovered that some genes determine the
  animals anterior/posterior and dorsal/ventral
  axes, other determine the flys segmentation
  pattern, and homeotic genes determine the body
  parts on each segment.
• One of the first events to take place is the
  orientation of the head (anterior) vs. the tail
  and the back (dorsal) vs. the front.
• In the fruit fly, and anterior end contains a
  greater concentration of a protein called bicoid.
  If bicoid is missing, the organism could have two
  tails and not head.

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Developmental Process--Morphogenesis

• The next event in morphogenesis is the
  determination of body segments, called the
  segmentation pattern.
• The first set of segmental genes to be activated
  are called gap genes. If one of these genes
  mutates, there are gaps, or large blocks of
  segments missing)
• Next, the pair-rule genes become active, and the
  embryo has precisely 14 segments. If one of these
  genes mutate, the animal has half of the number
  of segments.
• Last, the segment-polarity genes are expressed,
  and each segment has an anterior and posterior
  half.
• These morphogens, proteins associated with
  morphogenesis, are transcription factors that
  regulate which genes are active in which parts of
  the embryo in what order.

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Developmental Process--Morphogenesis

• Homeotic genes act as main switches and control
  pattern formation, which is the organization of
  differentiated cells into specific
  three-dimensional structures.
• In fruit flies, homeotic genes determine that
  certain genes control whither a particular
  segment will bear antennae, legs, or wings. A
  homeotic mutation can cause a fly ot have two
  sets of wings or extra legs.
• Homeotic genes have now been found in many other
  organisms, including mammals.
• Surprisingly, all homeotic genes contain the same
  particular sequence of nucleotides (sequence of
  60 amino acids), called a homeobox. In mammals
  homeotic gened are called hox genes.

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Developmental Process--Morphogenesis

• Homeotic genes code for transcription factors.
• The homodomain, sequence of 60 amino acids,
  protein is the part of a transcription factor
  that binds to DNA, but the variable sequences of
  a transcription factor determine which particular
  genes are turned on.
• Researchers envision that a homeodomain protein
  produced by one homeotic gene binds to and turns
  on the next holeotic gene, and so forth. This
  orderly process determines the morphology of
  particular segments.

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Developmental Process--Morphogenesis

• Mice and humans have the same homeotic genes
  located on four different chromosomes.
• Drosophila have homeotic genes on a single
  chromosome
• All three types of animals have homeotic genes
  expressed from anterior to posterior in the same
  order.
• The first cluster determines the final
  development of anterior segments of the animal,
  while those later in the sequence determine the
  final development of posterior segments of the
  animal.
• Finally, apoptosis, programmed cell death, is
  used to shape certain parts of the body, such as
  removing the webbing in human hands and the tail
  of tadpoles.
• When a cell-death signal is received, an
  inhibiting protein becomes inactive, allowing a
  cell-death cascade to proceed that ends in
  enzymes destroying the cell.

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Human Embryonic and Fetal Development

• In humans, the length of time from conception
  (fertilization followed by implantation) to birth
  (parturition) is approximately nine months (266
  days).
• It is customary to calculate the time of birth by
  adding 280 days to the start of the last
  menstruation.
• In humans, pregnancy (gestation) is the time in
  which the developing embryo is carried by the
  mother.
• Human development is divided into embryonic
  development (months 1 and 2) and fetal
  development (months 3-9)
• Development can be divided into trimesters.
• The first trimester is characterized by embryonic
  development.
• The second trimester is characterized by organ
  and organ system formation. By the end of the
  second trimester the fetus looks distinctly
  human.
• In the third trimester, the fetus grows rapidly
  and the major organ systems become functional. An
  infant born one or perhaps two months premature
  has a reasonable chance of survival.

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Human Embryonic and Fetal Development

• Extraembryonic membranes it possible for reptiles
  and other organisms to develop on land.
• When embryos develop in water, the water supplies
  oxygen for the embryo and takes away waste
  products. Water also prevents drying out and
  provides a protective cushion. For embryos that
  develop on land, all of these functions are
  carried out by the extraembryonic membranes.
• The chorion carries on gas exchange for the
  embryo.
• The amnion contains the protective amniotic
  fluid, which bathes the developing embryo
• The allantois collects nitrogenous wastes and the
  yolk sac is the first site of blood cell
  formation.

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Embryonic Development

• The First Week
• -Fertilization occurs in the upper third of the
  oviduct.
• -Cleavage begins 30 hours after fertilization
  and continues as the embryo passes through the
  oviduct to the uterus
• -By the time the embryo reaches the uterus on
  day three, it is a morula
• - By the fifth day, the morula has transformed
  into the blastocyst. The blastocyst has a
  fluid-filled cavity, a single layer of outer
  cells called the trophoblast, and an inner cell
  mass. The early trophoblast provides nourishment
  for the embryo. Later the trophoblast gives rise
  to the chorion and athe inner cell mass
  eventually becomes the embryo.

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Embryonic Development

• The Second Week
• -At the end of the 1st week, the embryo is
  implanting in the wall of the uterus.
• -The trophoblast secretes wnzymes to digest away
  some of the tissue and blood vessels of the
  endometrium of the uterus.
• -The embryo is about the size of a period in
  size 12 font.
• -The trophoblast begins to secrete the hormone
  HCG (human chorionic gonadotropin) to maintain
  the corpus luteum past the time it normally
  disintegrates.
• -the endometrium is maintained and menstruation
  does not occur.
• -The inner cell mass detaches itself from the
  trophoblast and the yolk sac and the amnion both
  form
• -The yolk sac has NO nutritive function in
  humans, but it is the first site of blood cell
  formation. The amnion serves as an insulator
  against cold and heat and absorbs shock from a
  mother exercising, etc.

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Embryonic Development

• The Second Week continued
• Gastrulation occurs during the 2nd week.
• The inner cell mass now has flattened into the
  embryonic disk, which is composed of two layers
  of cells ectoderm above and endoderm below
• Once the embryonic disk elongates to form the
  primitive streak, the third germ layer, mesoderm
  forms by invagination of cells along the streak
• The trophoblast is reinforced by mesoderm and
  becomes the chorion.

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Embryonic Development

• The Third Week
• Two important organ systems make their appearance
  during the third week the nervous and
  circulatory systems.
• The nervous system is the first system to be
  visually evident. At first, a thickening appears
  along the entire dorsal length of the embryo and
  the neural fold appears.
• When the neural folds meet at her midline, the
  neural tube, which later develops into the brain
  and the nerve cord, is formed.
• Once the notchord is replaced by the vertebral
  column, the nerve cord is called the spinal cord.
• Heart development begins in the 3rd week and
  continues into the 4th week.
• At first, there are only right and left heart
  tubes. When these fuse, the heart begins pumping
  blood, even though the chambers of the heart are
  not fully formed.

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Embryonic Development

• The Fourth and Fifth Weeks
• At four weeks, an embryo is barely larger than
  size 12 font.
• A bridge of mesoderm called the body stalk
  connects the caudal (tail) end of the embryo with
  the chorion, which has treelike projections
  called chorionic villi.
• The chorionic villi eventually form the placental
  sinus
• The allantois, the 4th extraembryonic membrane,
  is contained in the stalk of the of the placental
  sinus, and its blood vessel becomes the umbilical
  vessels.
• The head and tail of the embryo then lift up, and
  the body stalk moves anteriorly by constriction,
  allowing the umbilical cord to fully form.
• Little flippers called limb buds appear later,
  arms and legs develop from the limb buds.
• At eh same time, the head enlarges, and the sense
  organs become more prominent. Igt is possible to
  make out the developing eyes, ears, and even the
  nose.

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Embryonic Development

• The Sixth through Eighth Weeks
• At the end of the fifth week the pharyngeal
  arches become functioning gills only in fishes
  and amphibians larvae in humans, the first pair
  of pharyngeal pouches become the auditory tubes.
  The second pair becomes the tonsils, while the
  third and fourth become the thymus gland and the
  parathyroid glands.
• During the 6th through 8th weeks of development,
  the embryo becomes easily recognizable as human.
• Concurrent with brain development, the head
  achieves its normal relationship with the body as
  a neck region develops.
• The nervous system is developed well enough to
  permit reflex actions, such as a startle response
  to touch.
• At the end of this period, the embryo is about
  38mm long and weighs no more than an aspirin
  tablet, even though all organ systems are
  established.

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The Structure and Function of the Mammalian
Placenta

• The placenta is a mammalian structure that
  functions in gas, nutrient, and waste exchange
  between embryonic/fetal and maternal
  cardiovascular systems.
• The placenta begins formation once the embryo is
  fully implanted.
• At first, the entire chorion has chorionic villi
  that project into the endometrium.
• Later, these disappear in all areas except where
  the placenta develops.
• By the 10th week, the placenta is fully formed
  and is producing progesterone and estrogen, which
  serves two purposes 1) they prevent any new
  follicles from maturing, and 2) they maintain the
  lining of the uterus, so the corpus luteum is not
  needed. No menstruation occurs during pregnancy.
• The placenta has a fetal side contributed by the
  chorion and a maternal side consisting of
  uterine tissue.

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The Structure and Function of the Mammalian
Placenta

• The chronic villi are surrounded by maternal
  blood. However, maternal and fetal blood do not
  mix under normal conditions because exchange
  always takes place across plasma membranes.
• Carbon dioxide and other wastes move from the
  fetal side to the maternal side of her placenta
  and nutrients and oxygen move from the maternal
  side to the fetal side.
• The umbilical cord stretches between the placenta
  and the fetus. The umbilical cord DOES NOT enter
  the fetal intestines instead, the umbilical cord
  is simply taking fetal blood to and from the
  placenta.
• The umbilical cord is the lifeline of the fetus
  because it contains the arteries and veins that
  transport waste molecules to the placenta for
  disposal into the maternal blood and take oxygen
  and nutrient molecules from the placenta to the
  rest of the fetal circulatory system
• If the placenta prematurely tears from the
  uterine wall, the life of the fetus ad the mother
  are endangered.

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Fetal Development

• Fetal development (months 3-9) is marked by an
  extreme increase in size.
• Weight multiples 600 times, going from less than
  28 g to 3 kg.
• The fetus also grows to about 50 cm in length
  (19.7 in)
• The genitalia appear in the third month, so it is
  possible to tell if the fetus is male or female.
• Hair, eyebrows, and eyelashes add finishing
  touches to the face and head.
• Fingernails and toenails complete hands and feet.
• A fine hair (lanugo) covers the limbs and trunk,
  only to later disappear
• The fetus looks old because the skin is growing
  so fast that it wrinkles.
• A waxy substance (vernix) protects the wrinkly
  skin from the watery amniotic fluid.
• The fetus at first only flexes its limbs and nods
  its head, but later it can move its limbs
  vigorously to avoid discomfort. The mother feels
  these movements from the fourth month on.
• A fetus born at 24 weeks has a chance of
  survival, but the lungs are still immature and
  often cannot capture oxygen adequately

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Birth (Parturition)

• The latest findings suggest that when the fetal
  brain is sufficiently mature, the hypothalamus
  causes the pituitary to stimulate the adrenal
  cortex so that androgens are released into the
  bloodstream.
• The placenta uses androgens as a precursor for
  estrogens, hormones that stimulate the production
  of prostaglandin and oxytocin. All three
  molecules cause the uterus to contract and expel
  the fetus.
• During the first stage of birth, the cervix
  dilates to allow the passage of the babys head
  and body.

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Birth (Parturition)

• The amnion usually bursts during the first stage
  of birth.
• During the second stage of birth, the baby is
  born and the umbilical cord is cut.
• The third stage of birth allows the mothers body
  to expel the placenta.

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Preventing Birth Defects