1. 39–4 Fertilization and Development
Photo Credit: ©David M. Phillips/Photo Researchers Inc.
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Development is . .
Cell division
Growth
Differentiation
From embryonic tissues to
Specialized tissues & organs
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Fertilization ??
If an egg is fertilized, a zygote forms and begins to undergo cell division (mitosis) as it travels to the uterus.
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Blastocyst Formation
The First Two Weeks
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Early Development
The stages of early development include implantation, gastrulation, and neurulation.
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Early Development
Implantation & Differentiation
As the morula grows, it becomes a hollow structure with an inner cavity called a blastocyst.
6–7 days after fertilization, the blastocyst attaches to the uterine wall.
The embryo secretes enzymes that digest a path into it.
This process is known as implantation.
A cluster of cells, known as the inner cell mass, develops within the inner cavity of the blastocyst.
The embryo will develop from these cells, while the other cells will differentiate into tissues that surround the embryo.
Implantation
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Early Development
Gastrulation 
The inner cell mass of the blastocyst gradually sorts itself into two layers, which then give rise to a third layer.
Early Gastrulation
Amniotic cavity
Primitive streak
Ectoderm
Endoderm
Mesoderm
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Early Development
3 Germ Layers The ectoderm develops into the skin and nervous system.
The endoderm forms the digestive lining and organs.
Mesoderm cells differentiate into internal tissues and organs.
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Early Development
Neurulation  the development of the nervous system.
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Early Development
Neural crest
Neural fold
Notochord
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Early Development
Gradually, these folds move together to create a neural tube from which the spinal cord and the nervous system develop.
Neural crest
Neural tube
Ectoderm
Notochord
Nova - Gastrulation
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Morphing Embryos
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Early Development
Extraembryonic Membranes
As the embryo develops, membranes form to protect and nourish the embryo.
Two of these membranes are the amnion and the chorion.
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Early Development
Amniotic sac
The amnion develops into a fluid-filled amniotic sac, which cushions and protects the developing embryo.
Placenta
Umbilical cord
Uterus
Amnion
Fetus
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Early Development
Fingerlike projections called chorionic villi form on the outer surface of the chorion and extend into the uterine lining.
Maternal portion of placenta
Fetal portion of placenta
Chorionic villus
Amnion
Umbilical cord
The placenta is the connection between the mother and the developing embryo or fetus. It is through the placenta that the embryo gets its oxygen and nutrients and excretes its waste products. Notice how the chorionic villi from the fetus extend into the mother’s uterine lining (indicated by the overlapping brackets).
Maternal artery
Umbilical arteries
Maternal vein
Umbilical vein
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Early Development
Fetal Development (after 8+ weeks).
After three months, most major organs and tissues are formed. During this time, the umbilical cord also forms.
The umbilical cord connects the fetus to the placenta.
Placenta & Fetal Development
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Later Development
Later Development
4–6 months after fertilization:
The heart can be heard with a stethoscope.
Bone replaces cartilage that forms the early skeleton.
A layer of soft hair grows over the fetus’s skin.
The fetus grows and the mother can feel it moving.
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Later Development
During the last three months, the organ systems mature.
The fetus doubles in mass.
It can now regulate its body temperature.
The central nervous system and lungs completely develop.
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Childbirth
Childbirth
About nine months after fertilization, the fetus is ready for birth.
A complex set of factors affects the onset of childbirth.
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Childbirth
The mother’s posterior pituitary gland releases the hormone oxytocin, which affects involuntary muscles in the uterine wall.
These muscles begin rhythmic contractions known as labor.
The contractions become more frequent and more powerful.
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Childbirth
The opening of the cervix expands until it is large enough for the head of the baby to pass through it.
At some point, the amniotic sac breaks, and the fluid it contains rushes out of the vagina.
Contractions force the baby out through the vagina.
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Childbirth
The baby now begins an independent existence.
Its systems quickly adapt to life outside the uterus, supplying its own oxygen, excreting waste on its own, and maintaining its own body temperature.
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