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Summary Reproduction in vertebrates and Human reproduction
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EXTREMELY DETAILED NOTES! Reproduction of vertebrates and human reproduction Set out according to the subject assessment guidelines
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Topic 3: Reproduction in vertebratesGrade 10 revision:
→ Single celled organisms reproduce asexually through mitotic cell division (binary fission)
→ Asexual reproduction is quick and efficient since no partners are needed
→ Population of organisms using binary fission can increase rapidly
→ Asexual reproduction --> offspring lack genetic diversity
→ Sexual reproduction occurs when specialized sex cells (gametes) fuse to form a single cell
(zygote)
Unit 1 – diversity of reproductive strategies
External or internal fertilization
Vertebrate animals reproduce sexually. To do this, fertilization needs to take place. One of the problems
of bringing the sperm and ovum together in terrestrial animals is that sperm needs to swim to reach the
ovum. For this they need a liquid medium and this medium must not be dried out by the surrounding
environment. This has led to the development of internal fertilization.
Aquatic vertebrates do not have a problem with swimming sperm as they use surrounding water and so
fertilization happens externally.
External fertilization
Fish and amphibians make use of external fertilization.
→ Takes place outside of the body
→ Male and female gametes are released into the water
→ The sperm cells either swim towards the eggs or they are carried by the current
→ Physical contact is not necessary
→ Spawning: male and female fish swim around each other laying eggs and releasing sperm
→ Some sort of courtship is necessary: colour changes in fish, frogs croaking, mass external
fertilization during full moon.
→ Eggs are not protected by a shell so that sperm are able to penetrate the egg easily. They need
to stay in a moist environment. Many are laid in water and are surrounded by a slimy jelly-like
substance. The jelly protects the developing embryo and allows for the exchange of gases
between the embryo and surrounding water.
→ Hit or miss: many eggs land up not being fertilized. To compensate --> large masses of eggs and
sperm are produced.
Internal fertilization
Vertebrates living on land make use of internal fertilization.
→ Sperm cells are provided with a fluid medium called semen.
→ The sperm need to be introduced directly into the female's body to escape the drying effect of
the environment.
, → Close physical contact is required.
→ Courtship is required: sniffing or elaborate displays
→ During copulation, the semen is placed inside of the female by the penis
→ It is a much more certain method of reproduction; therefore fewer eggs need to be produced.
Fertilization: the fusion of a sperm cell nucleus with that of an ovum to form a zygote.
Terrestrial: living on dry land
Copulation: sexual intercourse or mating
Ovipary: eggs are laid and offspring hatch once fully developed
Vivipary: giving birth to live offspring
Ovovivipary: eggs are formed but never laid. Instead, they remain inside of the female's body and
hatch there. The young are then born live.
Ovipary, vivipary, ovovivipary
Ovipary --> comes from Latin words ovum (eggs) and parus (to be born). Ovipary refers to animals that
lay eggs. They lay eggs after the ova have been fertilized inside the female's body. An embryo develops
inside the egg and is protected there by the egg shell. In birds, the shell is hardened by calcium
carbonate. In reptiles, the shell is leathery. The eggs of aquatic animals are fertilized outside the
female’s body and are usually protected by a jelly-like substance.
The eggs contain a nutrient rich yolk that provide nourishment for the embryo during development.
Most fish, amphibians, reptiles and birds are oviporous.
Vivipary --> comes from Latin word vivi (to live). The term viviparous describes animal who give live birth
to their offspring once they are fully developed. The young develop inside the mother and are protected
by her body during development. The mother provides nutrition to her baby through the placent4
Vivipary is exclusive in mammals, however there are viviparous vertebrates, for example scorpions and
skink.
Ovovivipary --> the combination of ovipary and vivipary. Eggs are produced and fertilized but stay inside
the mother. The developing young get nourishment from the yolk of the egg. The eggs hatch inside the
mother and then emerge live from the mother’s body.
Some snakes, lizard, sharks and fish are ovoviviparous. Ovovivipary enables the female to improve her
egg’s chance of survival and offers them increased protection from predation. However, this method is
more taxing on the female’s body.
Amiotic eggs
Oviparous vertebrates have an amniotic egg. They contain several membranes that surround the
embryo. The one that encloses the embryo immediately is called the amnion. It produces a fluid in
, which the embryo floats during development. An amniotic egg is surrounded by a shell to prevent it
from drying out.
The egg is protected by a shell. This shell may be tough or leathery or hard due to the presence of
calcium salts such as calcium carbonate.
The shell is waterproof and porous to gases such as oxygen and carbon dioxide. One side of the egg is
blunt, with an air space just beneath it. This air space assists in the exchange of gases.
The interior of the egg is filled with albumen, a soft runny protein filled with proteins, which supplies
water to the embryo. The albumen is the white of the egg.
The yolk serves as a food supply for the developing embryo. The yolk is supported by thickened; twisted
strands of the albumen called chalazae. The purpose of the chalaza is to anchor the yolk in the thick egg
white. It also allows for rotation and to keep the germinal disc on the top side of the yolk. On the
surface of the yolk is a small pale spot called the germinal disc. This is where the embryo will develop.
One of the parents will incubate the eggs. If the eggs get too cold, the embryo will not survive.
Membranes in amniotic eggs
Inside the egg are a series of fluid filled membranes. The membranes are:
• The amnion which surrounds the embryo and is filled with amniotic fluid to cushion the embryo
• The allantois which stored excretory products from the embryo. This membrane develops many
blood vessels that enable it to provide transport for gases absorbed from the air space
• The yolk sac, a fat rich food source for the embryo
• The chorion which surrounds all other membranes
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