Chapter 20 Section 1 Meiosis, Gametes, and Fertilization Gametes: Special reproductive cells designed to transfer or receive chromosomes. Meiosis: Process of special cell divisions that produces gametes. Haploid: gametes and other cells that have only one of each kind of chromosome normally found in the cells of that organism

Two Types of Gametes Eggs = sometimes called ova (OH vuh), are female gametes. The egg is usually larger than the male gamete, and it does not have the ability to move itself. Sperm = are male gametes. They are usually much smaller than eggs, but they have some means of moving. Male animals and certain parts of plants produce sperm. These gametes are produced in reproductive organs. Female = ovaries which produces eggs Male = testes which produce sperm

Fertilization occurs when two hploid gametes (one egg and one sperm) unite and form a diploid cell. Zygote = the diploid cell that results from fertilization. This contains two of every kind of chromosome found in the organism and has the normal number of genes for the organism. Next the zygote will divide many times by mitosis to grow into an adult.

Section 2 External Fertilization In many types of animals the sperm and egg unite outside the parents bodies. This is called external fertilization. Since sperm must swim in a liquid to reach the egg, external fertilization occurs in water. The eggs are covered in a jellylike substance that can easily be penetrated by the sperm. Exp.=fish/spawning Female releases a few eggs and the male releases milt, the fluid containing sperm. The pair swim a few feet away and repeat the process until all her eggs have been released. Fish then swim away. Adult salmon die after spawning.

Most animals that reproduce by external fertilization produce thousands of gametes to insure that some of them will survive. Unfertilized eggs die or are eaten. The more eggs or offspring an animal has, the less likely it is to care for them. The fewer eggs it has the more likely it is to care for them. Exp.-grasshopper=lays thousands of eggs and then abandons them. Goldfish=make nest for eggs but leaves them or eats them Seahorse=female places them in the males pouch where the male fertilizes them Sea turtles=bury eggs and leave them Birds=good example of parent taking care of young Walrus=provides warmth, protection, food, and education to young

Internal Fertilization If sperm are deposited inside the female’s body when the eggs are ready, there is a good chance that fertilization will take place, producing a zygote. Fertilization=when an egg and a sperm have united inside the female parent’s body.

Egg-laying Animals Egg- zygote that has a shell around it Incubation-period when the zygote develops into a baby animal Birds lay eggs coated with a thick, hard shell Reptiles lay eggs with various kinds of shells (snakes/turtles=leathery shells) (alligators/crocodiles=hard shells) (sharks= tough cases called “mermaids’ purses) Chicken eggs develop inside hen for 24 hours before they are laid. Yolk=stored food that the un-hatched chick will use to grow

Chicken Egg Nucleus=on top of yolk, contains ½ the chromosomes needed to produce a new chick. Development begins in ovary, in order for the egg to develop into a chick, the other chromosomes must be provided by a sperm soon after the egg leaves the ovary. If the egg is not fertilized, the egg continues to form, but no chick will come from the egg. Unfertilized chicken eggs are sold at supermarkets.

After the Chicken Egg is Fertilized 1. Goes through a tube and is covered with albumen=egg white=water/protein 2. Membranes are added to the outside of the albumen=shell lining 3. The egg with the albumen and membranes enters the shell gland=shell is added and egg is ready to be laid. 4. Eggshell has tiny pores=allows un-hatched animal to obtain oxygen and release carbon dioxide while still in shell. (This is why sea turtles lay eggs in sand above sea level=so they won’t drown while in egg)

Egg Incubation After eggs are laid they go through a period of incubation during which they must be kept warm. Snakes/turtles-dig holes Birds-body heat Crocodiles/Alligators-rotting leaves King Penguin-hold between feet Snake eggs stay inside mom then hatch and come out of mother through a special opening

Animals the Give Birth to Live Young Zygotes of some animals become temporarily attached to the female parent’s uterus. Uterus=a special structure in which the zygote develops into a baby animal. Gestation=period of time spent in the uterus During gestation the developing animal receives nourishment from the mother’s body Mammal eggs contain little stored food so they must attach to the uterus to receive nourishment

Placental Animals Placenta=attached to the wall of the uterus and is the structure in which the mother’s blood vessels and the embryo’s blood vessels come close together. This allows nutrients and oxygen from the mother’s blood to enter the embryo’s blood, and wastes and carbon dioxide from the embryo’s blood pass into the mother’s blood. The embryo will become attached to the placenta by a ropelike structure called the umbilical cord. Umbilical cord= contains blood vessels that go from the heart of the embryo to the placenta and back.

Amnion=fluid filled sac that is around the embryo. Amnionic fluid= fluid inside the amnion, which protects the delicate embryo while it is developing. Pregnant=when an embryo is inside the mother Length of pregnancy ranges from 13 days for the opossum to 645 days for the elephant Pregnancy ends with the birth of the baby animal The longer the pregnancy, the more developed a baby animal will be at birth. Exp.=foal spends 11 months in the uterus and can walk in about an hour after birth… but a baby rat has only a 22 day gestation period and needs weeks of additional development after birth