Weird how this is hotly contested...
I will just point out that birth and death are mysteryies. We do not know what takes birth and what dies? How can we be so sure of what exactly evolves and how?
While this is pretty much not....
There is still quite a bit about embryonic development that is unknown and it's a thriving area of research for that reason.
I understand the "what takes birth and what dies" to be about perceived personality or (again) self awareness that is (allegedly) evolving along with the evolution of form(s).
In the counter assertions to atunu's point, there is hardly any distinction conveyed. Thus, the physical observations and resulting explanations / conclusions seemingly purport that we are all the same from birth to death. I very much don't believe that anyone reading this actually thinks that, but the assertions are between mundane and (extremely) under explained, for what I understood atunu to be inquiring about. Here are explanations I observed:
No one was "in the sperm".
If by 'it' you mean the sperm, it was produced in the testes.
In the DNA of the sperm and the receiving egg.
Everyone.
The birth is simply a living cell reproducing.
cell division pretty much describes the fetal growth.
The one that does draw distinctions is worth making note of:
Well, cell division plus a lot of specalized gene activity. Embryonic development is a bit more complicated than the sort of cell division done as an adult.
Which is telling us what (in very simplistic way) and (a little bit of) how. But not why or who.
Hence the mysteries continue on, and science (of the materialism kind) continues to uncover more of what and how.