850
UNIT SIX
FIGURE 22.21
During oogenesis, (
a
) a single egg cell (secondary oocyte) results from meiosis of a primary oocyte. If the egg cell is fertilized, it
generates a second polar body and becomes a zygote. (Note: The second meiotic division does not occur in the egg cell if it is not fertilized.) (
b
) Light
micrograph of a secondary oocyte and a polar body (arrow) (700×).
(23 chromosomes,
each with
2 chromatids)
(46 chromosomes,
each with 2 chromatids)
(a)
First polar body
(23 chromosomes,
each with
2 chromatids)
Second
polar body degenerating
First polar body
degenerating
Polar bodies
degenerating
or
First meiotic
division
Primary oocyte
Secondary
oocyte
Second meiotic
division
Zygote
46 chromosomes,
23 from sperm cell
and 23 from egg cell
(each chromatid now
an independent
chromosome)
Fertilization
Sperm cell
(23 chromosomes)
Second
meiotic
division
Sperm
nucleus
(b)
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