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Objective 9:
Insulin
structure (9b)
Effect
of insulin on homeostasis (9c/d)
Type
I / II Diabetes (9e)
Play the "Diabetic Dog" Game and test your understanding of insulin action
Objective 12:
Thyroid
hormones (12b)
Objective 13:
Calcium homeostasis
Objective 14:
Other
hormones involved in regulation of blood sugar (15b)
Objective 15:
Adrenal
cortex negative feedback (16e)
Objective 16:
Estrogen
receptor (optional)
Objective 18:
External
versus internal fertilization
Objective 20:
Rat
Dissection Pictures (new!)
Objective 23:
Ovary and placenta
Optional Supplementary Material
"Virgin birth" of hammerhead shark
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External vs. Internal Fertilzation
There are two basic ways in which egg cells and sperm cells are
brought together: 1) external fertilization, in which both types of
gametes are shed into water and the sperm swim or are carried by currents
to the eggs; and 2) internal fertilization, in which the eggs are
retained within the reproductive tract of the female until after they
have been fertilized by sperm inserted into the female by the male.
External Fertilization
External fertilization is limited essentially to animals living in
aquatic environments. The flagellated sperm must have fluid in which
to swim, and the eggs lack a protective coat or shell (so that sperm
can penetrate and fertilize them), and would dry out in the air. Almost
all aquatic invertebrates, most fish, and many amphibians use external
fertilization. However, shedding eggs and sperm into the water is
an uncertain method of fertilization: many of the sperm never locate
an egg, and many eggs are never fertilized, even if both types of
gametes are shed at the same time and in the same place, as is usually
the case. Consequently, animals using external fertilization generally
release vast numbers of eggs and sperm at one time.
Internal Fertilization
Most land animals, both invertebrate and vertebrate, use internal
fertilization. In effect, the sperm cells are provided with the sort
of fluid environment that is no longer available to them outside the
animals’ bodies. The sperm can remain aquatic, swimming through
the film of fluid present on the walls of the female reproductive
tract. Once fertilized, the egg is either enclosed in a protective
shell and released by the female, or held within the females’
body until the embryonic stages of development have been completed.
Internal fertilization requires close physiological and behavioral
synchronization of the sexes, which involves extensive hormonal control.
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