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Animals: Development
Human embryonic development
proceeds through different stages
after fertilization
Outline
1. Key concepts
2. Development of mature animal from
zygote
3. Human embryonic development
(zygote  birth)
A. Cleavage
B. Gastrulation
C. Organogenesis
4. Conclusions
Key Concepts:
1. Human embryonic development proceeds through
different stages after fertilization
2. Cleavage and cellular interaction helps cell
differentiation and morphogenesis
3. In cell differentiation, a cell selectively uses
certain genes and proteins not found in other cell
types
4. Morphogenesis starts at gastrulation
Development of mature
animal from zygote
A. Growth - mitosis
B. Differentiation (specialized)
– after specialized, cell
can’t go back
C. Morphogenesis (formation
of shape)
Human embryonic
development
A. Cleavage
1. Zygote divides without an
increase in size
2. A solid round cell ball (morula)
is formed
3. The morula develops a cavity 
blastocyst
Formation of the Early Embryo
Formation of the Early Embryo
Cleavage
Human embryonic
development
B. Gastrulation – blastocyst forms three germ
layers
1. Ectoderm – epidermis of skin; lining of
mouth and nose; hair; glands of skin
(sweat, mammary); nervous system;
lens of eye
2. Mesoderm – muscle; bone, skeleton;
blood, circulatory system; gonads;
kidney
3. Endoderm – lining of digestive and
respiratory tracts; liver; pancreas
Stages of Animal Development
Cleavage
Stages of Animal Development
Morula
Stages of Animal Development
Gatrulation
Primary tissues develop
Stages of Animal Development
Cell differentiation
proceeds
Gastrulation
Human embryonic
development
C. Organogenesis – three germ layers develop to
specialized organs or tissue types.
- begins about the 15th day after fertilization.
about the 18th day from fertilization nervous
system first to begin development
about 22nd day circulatory system – pump blood
respiratory system - 23rd day
excretory system – 25th day
digestive system – 25th day
reproductive system – 28th day
Human embryonic
development
C. Organogenesis
By 13 weeks – 1st trimester : fetus 9 cm
(4 in.); 15 g (0.5OZ) all major organ
systems start to function
By 26 weeks – 2nd trimester : fetus 35 cm
(14 in.); 1000 g (2.2 lb)
By 38 weeks  birth: baby 50 cm (20
in.); 3200 g (7 lb)
Embryo at 4 Weeks
Embryo at 5-6 Weeks
Embryo to Fetal Period:
Week 8
Fetus at 16 Weeks
Embryonic Development
Happy Birthday
Development and Aging
In Conclusion
1. An embryo cannot develop properly
unless each stage of development is
successfully completed
2. In cell differentiation, a cell selectively
uses certain genes and proteins not found
in other cell types
3. Morphogenesis starts at gastrulation
Nothing in the world
is more important
than my baby rhino!