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what are the differences between a zygote and a foetus?And the differences between internal fertilisation and external fertilisation?

When an egg has been fertilized (meaning the nucleus of the egg has joined with a sperm) it is called a zygote.
 
The cells divide, after about 4 days, the zygote consists of 32 cells.  At this point it is called an embryo.
 
After 9 weeks, the body has completely formed, and it is called a foetus.
 
We continue to call it a foetus, until it is born, at approximately 39 weeks, when we call it a baby.
There are two mechanisms by which fertilization can take place. The first is external (the eggs are fertilized outside of the body), and the second is internal (the eggs are fertilized within the female reproductive tract). While fertilization is necessary for organisms that reproduce sexually, individuals that reproduce asexually do so without the need for fertilization. These organisms produce genetically identical copies of themselves through the process of mitosis.


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