AIM: How Is Internal Fertilization and Development Different from External Fertilization and Development

Conditions for Fertilization Sperm and eggs have a short lifespan, they must be released within a short period of time. There must be a fluid medium for sperm to swim to the egg Eggs cannot move by themselves Sperm are specialized for fast movement. Flagella push them along.

How only one sperm to one egg: Enzymes dissolve an opening through the protective membranes of the egg, allowing the sperm’s nucleus to enter the egg. A neutralizing membrane grows around the egg to prevent other sperm from entering.

Two ways in which egg cells and sperm cells are brought together External fertilization both types of gametes are shed into water and sperm swim or are carried by currents to the eggs Internal fertilization 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 Eggs fertilized outside female’s body Occurs mostly in aquatic environments Water provides fluid environment for gametes Only sex organs required are gonads and duct to carry gametes to water Ex. Fish, amphibians

EXTERNAL FERTILIZATION HAZARDS / Difficulties: Eggs and sperm may not meet (no fertilization) Eggs may be eaten by predators Environmental conditions may change Solution - Many eggs and sperm are produced so some survive Offspring will develop externally too.

INTERNAL FERTILIZATION Eggs and sperm meet inside the female’s body Occurs in most land vertebrates Ex. Reptiles, birds, mammals

INTERNAL FERTILIZATION Requires special sex organs. Mating must take place within a certain time period for fertilization to occur: Adaptations to stimulate mating process: Mating calls (songs) Special feathers or color patches Chemicals called pheromones (bats mate in fall but female stores sperm until spring, when fertilization occurs)

INTERNAL FERTILIZATION Specialized protection of the developing egg Once fertilized, the egg is either: Enclosed in a protective shell and released by the female (External Development – Reptiles & Birds) Held within the females’ body until the embryonic stages of development have been completed (Internal Development – Most Mammals)

Embryonic Development An embryo is a developing offspring. Development can be either INTERNAL or EXTERNAL, depending on the organism.

INTERNAL DEVELOPMENT Embryo develops inside the female, placenta provides nourishment

Human Fertilization Internal fertilization – male penis is used to introduce sperm into female’s vagina.

Where does fertilization usually occur? Fertilization occurs in the fallopian tubes (oviduct) Ovary Fertilization Egg

How many eggs are released a month by a female? Normally one egg is released per month, however, sometimes TWO can be released at the same time. Ovulation

What happens after fertilization? The embryo travels to the uterus and implants itself in the uterus lining