male sex hormones are formed. The other structures of the
male reproductive system are termed
accessory sex organs
(secondary sex organs). They include the internal reproduc-
tive organs and the external reproductive organs (
reference plates 3 and 4).
) are ovoid structures about 5
centimeters in length and 3 centimeters in diameter. Both tes-
tes, each suspended by a spermatic cord, are within the cavity
of the saclike
(F g. 22.4 and reference plate 12).
Descent of the Testes
In a male fetus, the testes originate from masses of tissue
posterior to the parietal peritoneum, near the developing kid-
neys. Usually a month or two before birth, the testes descend
to the lower abdominal cavity and pass through the abdomi-
nal wall into the scrotum.
The male sex hormone
which the developing
testes secrete, stimulates the testes to descend. A F
cular cord called the
aids movement of the testes. This cord is attached to each
developing testis and extends into the inguinal region of the
abdominal cavity. It passes through the abdominal wall and
is fastened to the skin on the outside of the scrotum. The tes-
tis descends, guided by the gubernaculum, passing through
) of the abdominal
Homologous chromosome pairs separate,
and each replicated member moves to one end of the spin-
dle. Each new, or daughter, cell receives only one replicated
member of a homologous pair of chromosomes, overall halv-
ing the chromosome number.
The original cell divides in two. Nuclear
membranes form around the chromosomes, nucleoli reap-
pear, and the spindle F
bers disassemble into their constitu-
Second Meiotic Division
After telophase I, the second meiotic division begins. Meiosis
II is similar to a mitotic division (see F
g. 22.1). During
chromosomes condense and reappear, still repli-
cated. They move into positions midway between the poles
of the developing spindle. In
chromosomes attach to spindle F bers. In
tromeres separate, freeing the chromatids to move to oppo-
site poles of the spindles. The former chromatids are now
considered to be chromosomes. In
each of the
two cells resulting from meiosis I divides to form two cells.
Therefore, each cell undergoing meiosis has the potential to
produce four gametes. In males, the gametes mature into four
sperm cells. In females, three of the products of meiosis are
“cast aside” as polar bodies, and one cell becomes the egg.
Meiosis generates astounding genetic variety. Any one of
a person’s more than 8 million possible combinations of 23
chromosomes can combine with any one of the more than 8
million combinations of his or her mate, raising the potential
variability to more than 70 trillion genetically unique indi-
viduals! Crossing over contributes even more genetic vari-
illustrates in a simpliF ed manner how
maternal and paternal traits reassort during meiosis.
Sex cells have one set of genetic instructions, carried on
23 chromosomes, compared to two sets on 46 chromosomes
in other cells. When sex cells join at fertilization, the amount
of genetic information held in 46 chromosomes is restored.
What are male and female sex cells called?
Describe the major events of meiosis.
How does meiosis provide genetic variability?
ORGANS OF THE MALE
Organs of the male reproductive system are specialized to
produce and maintain the male sex cells, or
transport these cells and supporting ﬂ
uids to the outside;
and secrete male sex hormones.
primary sex organs
(gonads) of this system are the
two testes in which the sperm cells (spermatozoa) and the
As a result of crossing over, the genetic information
in sex cells varies from cell to cell. Colors represent parent of origin.
Although only one eye color gene pair is illustrated, eye color is
polygenic (involving more than one gene pair).