881
CHAPTER TWENTY-THREE
Reproductive System
Sperm
nucleus
Egg
nucleus
Polar
bodies
Zona pellucida
Zygote
Day 0
Pronucleus
formation
begins
Day 1
2-cell stage
Day 2
4-cell stage
Day 3
Early morula
First cleavage division
Cleavages (first cleavage completed about 30 hours after fertilization)
Day 4
Late morula
Day 6-7
Blastocyst
implantation
Fertilization
occurs about
12-24 hours
after ovulation
Ovulation
Uterus
Endometrium
Stem cells
Stem cells
(a)
(b)
(c)
FIGURE 23.4
Light micrographs of (
a
) a human secondary oocyte surrounded by follicular cells and sperm cells (250×), (
b
) the two-cell stage
(600×), and (
c
) a morula (500×).
FIGURE 23.5
±Stages±
of early human prenatal
development.
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