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Biology 105
Chapter 51: Development
Pages 1106-1126
Student Outcomes
Describe the processes involved in fertilization
Trace the generalized pattern of early
development of the embryo from zygote through
early cleavage
Identify the significance of gastrulation in the
developmental process
Student Outcomes
• Define Organogenesis and the three germ
• Process of joining of egg and sperm to
form a zygote. Known as conception.
• Spermatogenesis and oogenesis
• Involves 4 events:
Fertilization events
1. sperm contacts the egg and recognition
2. sperm or sperm nucleus enters the egg
3. egg becomes activated, beginning
developmental changes
4. sperm and egg nuclei fuse.
the fertilization of the egg by more than
one sperm. Results in extra
chromosomes and is usually lethal.
Is totipotent
Cleavage occurs shortly after fertilization
Cell goes through 2 cell, 4 cell, 8 cell stages,
At the 32 cell stage – known as morula, and
eventually a blastula
• 32 cell stage – Usually at this stage when
entering the uterus. These cells are known
as blastomeres. Continued mitotic
divisions (Cleavage) results in a Blastula
(hollow ball of cells with a blastocoel)
Blastula becomes a gastrula which forms the 3 germ
– Ectoderm
– Mesoderm
– Endoderm
– The three germ layers will give rise to organogenesis - the
formation of organs. Notochord, brain and spinal cord are the
first organs to develop
– Outer layer – Will give rise to:
• Nervous system and sense organs
• Outer layer of skin, nail and hair
• Pituitary
Middle layer – Will give rise to
• Will give rise to:
– Lining of the digestive tube
– respiratory
Occurs on about the 7th day of embryo
development. It imbeds in the endometrium of
the uterus.
Human Development Timeline
• Fertilization
• 24 hours – 2 cell stage
• 3 days – Morula travels through fallopian tubes
and reaches uterus
7 days – implantation
17 days – tissue that will give rise to heart is
differentiating, neural plate is formed
Human development continued
• 25 days – primitive eye and ear visible,
heart begins to beat
• 4 weeks – limb buds appear, brain division
• 8 weeks – muscles differentiation, embryo
capable of movement, sex organs can be
seen, bone is ossifying
Human development continued
3 months- sex can be determined
4 months – face takes on a ‘human’ form with eyes,
ears and nose
Third trimester – known as a fetus, tremendous growth
of the body, cerebrum growth is rapid
266 days – Birth
All Development Stages
• Gametogenesis-Fertilization
• Embryo: Cleavage-Morula-BlastulaImplantation-Gastrula-Organogenesis
• Fetus: 2 month-Birth (organogenesis)
• Birth-Neonate-Infant-Child-AdolescentYoung adult-Middle aged adult-Elderly
• Death
Multiple Births
• Twins
– Identical - one egg, one sperm, seperation
after the 2 cell stage or the inner cell mass
divides into 2 separate masses (monozygotic)
– Non identical - two eggs, two sperm
– Conjoined - Same as identical, seperation
does not occur completely.
Births cont.
• Triplets – Identical - one egg, one sperm
– Non identical - more than one egg and sperm