9-5. EXTERNAL GENITALIA
In both sexes, there are certain structures at the surface known as the external
9-6. COMMON EMBRYONIC ORGANS
In male and female embryos, there is a common origin of the organs of the
reproductive system. (The organs of the urinary system share this common origin).
The importance of this common origin is that, under certain conditions, females may
develop with males characteristics, males may develop with female characteristics, and
even true intersexes may occur. (True intersexes possess both male and female
9-7. SEX DETERMINATION
At the moment the egg is fertilized by the sperm, the new genetic combination
determines whether the individual will be male or female. Later in development,
however, sex hormones play an important role in the production of sexual organs and
Section II. GAMETES (SEX CELLS)
Within the genetic makeup of each individual, there is a pair of chromosomes
known as the sex chromosomes. There are two kinds of such chromosomes--X and Y.
Within the gonads, there is a special type of cell division known as meiosis. The
usual set of chromosomes is reduced in this reduction division. Thus, the gametes (ova
or spermatozoa) have only a single set of chromosomes.
In the final analysis, the production of a new individual is based upon the union of
the male gamete (spermatozoon) with the female gamete (ovum). This process is
called fertilization. At this time, a double set of chromosomes is reconstituted.
a. If the zygote (fertilized egg) has two X chromosomes, the individual will be