Download Ch 28-29

Chapter 28
Sensory Input and Motor Output
Chapter 29
Reproduction and Embryonic Development
28.1 The Senses
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All living things respond to stimuli.
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Stimuli are environmental signals that tell us about the internal or external environments.
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Plants might respond by changing growth patterns.
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Animal responses often result in motion.
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Sense organs, as a rule, are specialized to receive one kind of stimulus.
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Sensory receptors transform the stimulus (light, sound, pressure, etc.) into nerve impulses that
reach a particular section of the cerebral cortex.
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The brain is responsible for sensation and perception.
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If the photoreceptors are stimulated by pressure and not light, the brain causes us to see “stars”
or other visual patterns.
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Chemical senses—chemoreceptors
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Detect chemicals in the environment
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When molecules bind to receptors, nerve impulses are generated and sent to brain
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When they reach the appropriate area they are interpreted as taste or smell
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Pattern of receptors activated is interpreted by the brain as a particular taste or smell
Taste
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Taste buds located primarily on the tongue
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Receptors located on microvilli of receptor cells
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5 primary types of taste—bitter, sour, salty, sweet and umami
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Taste buds for each primary taste are located throughout the tongue
Smell
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Nose contains olfactory receptor cells
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Receptors located on cilia of olfactory receptor cells
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Important role of taste and smell is to trigger reflexes that start digestive juices flowing
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A repulsive smell or taste can initiate a gag reflex or vomiting.
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Olfactory bulbs have a direct connection with the limbic system and its centers for emotions and
memory.
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Hearing and balance
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2 sensory functions of human ear
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Sensory receptors for both are hair cells with long microvilli called stereocilia
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Microvilli sensitive to mechanical stimulation—mechanoreceptors
Hearing
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Outer ear collects sound waves
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Cause tympanic membrane to move back and forth
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3 tiny bones (ossicles) in middle ear amplify sounds
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Last ossicle (stapes) strikes oval window causing pressure to pass into fluid within
cochlea (hearing portion of inner ear)
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Sensory receptors for hearing are located in cochlear canal of the cochlea
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Stereocilia embedded in gelatinous membrane—spiral organ
When the stapes strikes the oval window, pressure waves cause the hair cells to move up and
down.
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Stereocilia embedded in gelatinous membrane bend
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Send signals via cochlear nerve to the brain stem then to the cerebral cortex to be
interpreted as sound
Each part of spiral organ sensitive to different wave frequencies or pitch
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Nerve fibers from each region lead to slightly different areas of the brain
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Pitch sensation depends on which area vibrates and stimulates which area of the brain
Loud noises can cause greater vibration
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Certain aromas trigger memories of a person or place.
Nerve deafness (spiral organ damage) can damage stereocilia
Balance
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2 senses of balance (equilibrium)
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Rotational equilibrium
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Rotation and/or angular movement of the head
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Involves 3 semicircular canals oriented in planes
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At the base of each canal, hair cells have stereocilia embedded in gelatinous
membrane
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As fluid moves, gelatinous membrane is displaced and hair cells bend, sending
impulses to brain
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Signals, supplemented by vision, tell the brain the head is moving
2. Gravitational equilibrium
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Straight-line movement of head with respect to gravity
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Depends on utricle and saccule (membranous sacs in inner ear)
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Also contain hair cells with stereocilia embedded in gelatinous membrane
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Calcium carbonate granules (otoliths) rest on membrane
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Gravity displaces otoliths bending hair cells that send signals to CNS
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Data, supplemented by vision, tells the brain the direction the head is moving
Lateral line
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Found in fishes
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Sense organs located in canal with opening to outside
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Water currents and pressure waves cause gelatinous membrane to move,
bending stereocilia
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Fishes uses data to locate other fish including predators, prey, and mates
Vision
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Photoreceptors are sensory receptors sensitive to light.
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Planaria only sense the direction of light.
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Compound eyes in arthropods form visual images.
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Composed of many independent visual units
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Each has a lens to focus light onto photoreceptors
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Forms crude visual image
Camera-type eyes
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Found in human, squid, and octopus
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Evolved independently in each group
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Single lens focuses light onto photoreceptors
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Photoreceptors closely packed on retina
Color vision
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Insects have color vision, but use a shorter range of electromagnetic spectrum
compared to humans
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Some fishes, all reptiles, and most birds have color vision.
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Among mammals, only humans and other primates have expansive color vision.
The human eye
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Binocular vision
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Ability to perceive three-dimensional images and to sense depth
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Result of 2 eyes viewing same object at slightly different angles
Light rays are brought into focus on the photoreceptors located within the retina.
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Can see UV—patterns on flower petals
Shape of lens controlled by ciliary muscles
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Cornea and lens involved in focusing light rays
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Iris regulates how much light enters eye through pupil
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Retina generates nerve impulses sent to visual part of cerebral cortex where brain forms
image of object
Photoreceptors of the eye
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Contain a visual pigment similar to that found in all animal eyes
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Rods contain rhodopsin
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When a rod absorbs light, rhodopsin splits into opsin and retinal, leading to
cascade of reactions ending in the generation of nerve impulses
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Sensitive to light—night vision
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Plentiful throughout retina—provide us with peripheral vision and perception of
motion
Cones
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Located primarily in fovea
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Activated by bright light
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Give us fine detail and color
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3 different kinds of cones—blue, green, and red
Retina
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Retina has 3 layers of cells
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Light has to penetrate first 2 layers to reach photoreceptors
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Intermediate cells of middle layer process and relay information to first layer
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Ganglion cells in first layer have axons forming optic nerve
29.1 How Animals Reproduce
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Animals usually reproduce sexually, but some, on occasion, can reproduce asexually.
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Asexual reproduction
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Only 1 parent
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Many flatworms can constrict into two halves and each half can regenerate into new
individuals.
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Fragmentation followed by regeneration also seen in sponges, corals, and echinoderms
Parthenogenesis
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Modification of sexual reproduction
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Unfertilized egg develops into a complete individual
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Drone honeybees (haploid males) are the result of unfertilized eggs.
Sexual reproduction
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2 parents
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Produce gametes in gonads
Testes produce sperm
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Ovaries produce eggs
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Zygote formed at union of egg and sperm
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Earthworms are hermaphrodites (both male and female sex organs) but they still crossfertilize
External fertilization
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Egg and sperm unite outside the parent’s body
Internal fertilization
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Sperm unites with egg inside female body
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Copulation—sexual union to facilitate reception of sperm by female
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Viviparous
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Produce living young
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Most mammals
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Some mammals still lay eggs
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Duckbilled platypus and spiny anteater
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Marsupial mammals have offspring born in a very immature state and finish
development in a pouch
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Most mammals have a placenta that nourishes the embryo during its full
development.
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Materials are exchanged with the mother.
29.3 Human Embryonic Development
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All the events that occur from the time of fertilization until the animal is fully formed
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Developing embryo with extraembryonic membranes depends on them to protect and nourish
embryo
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Mammals, reptiles, and birds
Fertilization
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Results in a zygote
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Requires sperm and secondary oocyte to interact
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Plasma membrane of secondary oocyte is surrounded by zona pellucida, which is
surrounded by follicle cells
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Acrosomal enzymes from sperm digest through zona pellucida
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Sperm binds to oocyte plasma membrane and sperm nucleus enters
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Causes secondary oocyte to complete meiosis II and become an egg
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Egg and sperm nuclei fuse to form zygote
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Changes in zona pellucida prevent binding and penetration of additional sperm
(polyspermy)
Early embryonic development
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Embryonic period—first 2 months
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6 days of development occur in oviduct before embryo implants in uterus
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Following fertilization, embryo undergoes cleavage—cell division without growth
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Results in tightly packed ball of cells—morula
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Morula continues to divide and hollow out to form blastocyst
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Fluid-filled cavity called blastocoel
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Inner cell mass will become embryo
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Outer layer will become part of placenta
When embryo implants, placenta forms and secretes human chorionic growth hormone
(hCG)
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Basis for pregnancy test, maintains corpus luteum so ovulation and
menstruation do not occur
Later embryonic development
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Gastrula
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During third week of development
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Evident when certain cells invaginate at primitive streak creating 3 layers
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Amnion surrounds embryo—encloses amniotic fluid
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Gastrulation is complete when 3 layers of cells develop—embryonic germ layers
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Ectoderm—outer layer
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Mesoderm—middle layer
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Endoderm—inner layer
Neurulation
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First organs to form are from central nervous system
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Newly formed mesoderm cells coalesce to form notochord
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Central nervous system develops from ectoderm just above the notochord
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Neurulation—neural plate develops folds, which fuse to become the neural tube
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Anterior portion becomes the brain
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Posterior portion becomes the spinal cord
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Induction—one tissue or organ influences the development of another
Midline mesoderm that did not contribute to the notochord now becomes 2
masses of tissue
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Blocked off into somites
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Give rise to vertebrae and muscles of axial skeleton
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Lateral to somites, mesoderm splits to form lining of coelom
Neural crest consists of a band of cells that develops where the neural tube
pinches off from the ectoderm
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By end of third week, over a dozen somites are evident, and the blood vessels
and gut have begun to develop
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At this point, the embryo is about 2 millimeters (mm) long
Organ formation continues
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Human embryo at 5 weeks has limb buds
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Arms and legs develop from limb buds
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Head enlarges and sense organs become more prominent
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Umbilical cord has developed connecting embryo to placenta
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Cells migrate to various locations where they contribute to the
formation of skin and muscles, in addition to the adrenal medulla and
the ganglia of the peripheral nervous system
4th extraembryonic membrane (allantios) blood vessels become
umbilical blood vessels
6–8th week
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Remarkable external appearance change
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Becomes recognizable as human
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Head distinct from body
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Reflex reactions—startle response
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At the end of this period, embryo is 38 mm long
Placenta
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Fetal side contributed by chorion
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Maternal side consisting of uterine tissue
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Chorionic villi are surrounded by maternal tissue but maternal and fetal blood never
mix—exchange by transport across walls
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Carbon dioxide and wastes move to maternal blood
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Nutrients and oxygen move to fetal blood
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Harmful chemicals can cross the placenta
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Fetal development and birth
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Fetal development 3rd to 9th month
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Marked by extreme increase in size
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Each organ has a sensitive period during which normal development can be
altered
Weight and length
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Genitalia appear in 3rd month
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Hair, eyebrows, eyelashes, fingernails, and toenails appear
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5th–7th months—fine, downy hair covers limbs and trunk and later disappears
20- to 28-week fetus
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Skin is growing so fast it wrinkles
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Vernix caseosa protects skin from amniotic fluid
Fetus born at 22 weeks has a chance of surviving but likelihood goes up with increasing age