Development
Chapter 29
Fertilization
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Zygote undergoes a series of mitotic cell divisions
called cleavage
zygote  2-cell stage  4-cell stage  8-cell stage
 morula  blastocyst gastrula
Morula = solid sphere of cells (day 5)
Blastocyst = hollow ball of cells (days 6-7), it produces
human chorionic gonadotropin (hCG) hormone
which can be detected in the woman’s urine during a
pregnancy test; source of embryonic stem cells
Gastrula = differentiation starts; contains three germ
layers (ectoderm, endoderm, mesoderm; days 7-10)
Late Gastrula
Endoderm
Ectoderm
Mesoderm
Three germ layers in gastrula:
Ectoderm – gives rise to the nervous system and skin
Endoderm – gives rise to lining of the gut and internal
organs
Mesoderm – gives rise to skeleton, muscles, circulatory
system, reproductive system
Pregnancy becomes established during days 10 -14:
• Fluid filled amniotic cavity starts to form
• Yolk sac starts to form (will make blood cells, germ cells)
• Embryo starts to form from embryonic disc
• Chorion (placenta) starts to form and will supply the
embryo with nutrients and eliminate wastes
• Woman misses period
Development of the Placenta
• Embryo = the term used until the 8th week; from
Greek “swelling within”
• Fetus = the term used from the 8th week until birth;
from Greek “offspring”
Pregnancy can be divided into three trimesters:
• First trimester = body systems form (during this
time the embryo is extremely sensitive to smoking &
drugs)
• Second & third trimesters = continued body growth
BIRTH
3 parts:
1. Dilation of cervix
2. Expulsion of the
baby
3. Expulsion of the
placenta