1. Internal & External Fertilisation
D. Crowley, 2007

2. Internal & External Fertilisation
Sunday, April 02, 2017
Internal & External Fertilisation
To know the difference between internal and external fertilisation

3. Identical?
Look at the following - it shows two sequences how identical and non-identical twins are formed
Can you tell which is which? Can you explain how you know?

4. Identical?
Identical twins can form because once a sperm has fertilised the egg, the ball of cells (embryo) can split into two. If this happens you get two identical twins
Non-identical twins can form, because sometimes a female can ovulate and produce two eggs in a month. These are then fertilised by a different sperm each, so you produce non-identical twins
Unique individuals are due to individual sperm and eggs fertilising one another

5. Fertilisation Different organisms fertilise in different ways
Can you think of any organisms which fertilise differently to humans (e.g. think about frogs and fish)
For fertilisation to occur, the male sperm cells must get to the female egg cells.
There are two main ways for this to occur - internal and external fertilisation

6. External Fertilisation
External fertilisation occurs outside the female - the male releases sperm cells onto the egg cells as the female lays them
External fertilisation occurs in fish and most water dwelling animals (e.g. amphibians). These organisms usually produce a large number of sex cells which are released into the water.
There are usually courtship rituals which make sure the male and female sex cells are released near each other.
However, the chances of fertilisation are very low, so huge numbers of eggs and sperm are needed to ensure enough offspring are produced

7. Internal Fertilisation
Internal fertilisation occurs inside the female
Internal fertilisation occurs in most land dwelling organisms (such as mammals) - this is necessary as sperm need fluid in order to be able to swim to the egg (the fluid comes from semen, ejaculated along with the sperm from the male).
These organisms produce far fewer sex cells, as the chances of successful fertilisation are much higher (the sex cells are much closer together when they are released)

8. How Many?
Why is it that organisms which produce by external fertilisation produce so many more sex cells?
The chances of fertilisation are much lower in externally fertilised organisms, as the sex cells are further apart from each other: -
This means the sex cells are more likely to be predated
They can also be separated from one another (due to currents)
In external fertilisation there is also less parental care, so the offspring are more likely to be killed (hence producing lots of them)

9. How Many?
Why then are there so few sex cells produced in internal fertilisation?
As the likelihood of successful fertilisation is higher in internally fertilised organisms, because the sex cells are so much closer together, producing a great many sex cells would cause problems because: -
There is only a limited amount of space within the female’s uterus to accommodate a growing foetus
In internal fertilisation there is a greater deal of parental care, so the offspring are more likely to survive (hence there is no need to produce lots of them)

10. Understanding
Task - using exploring science book, read pages 18 and 19 and answer (in full sentences) questions 4, 5, 6, 7 and 8
Questions: -
In external fertilisation not all the egg cells are fertilised, and some are subjected to predation and being separated from one another.
Offspring which develop inside the mother have the advantages of protection, warmth and by getting everything they need from the mother.
A baby giraffe is extremely vulnerable as soon as it is born (from predators), so being able to get up and run quickly after it is born will reduce its risk of being predated upon.

11. Understanding
A bullfrog produces 20’000 eggs - because it reproduces externally, so needs many eggs as a great deal will be predated / may become separated
The Lake Malawi mouthbrooder probably got its name because it cares for its young (after they have been fertilised) within its mouth, where they grow. Brood can mean to be anxious about something, as well as meaning the young that are all born and reared at a similar time. So the mouthbrooder is anxious for her young which have all been born at a similar time, and looks after them in her mouth!
Animal
Number of egg cells
Fertilisation type
Bat 1
Internal
Bullfrog
20’000
External
Cat 4
Cod
5’000’000
Tiger 2

12. Internal & External Internal Fertilisation External Fertilisation etc…
Look at the worksheet, and cut out all the cards
Organise the cards into two groups - internal fertilisation and external fertilisation
Stick these into your book
Internal
Fertilisation
External
Fertilisation
etc…
etc…

13. Internal & External Internal Fertilisation
Eggs produced in small numbers
Method usually by land animals (water is not available for the sperm to swim)
Method has a better chance of the offspring surviving due to improved protection
Human, butterfly, horse
External Fertilisation
Eggs produced in very large numbers
Used by fish because water is available for sperm to swim in
Sperm can get eaten by aquatic predators
Fish, frog