Conception, the twin problems and souls.

People used to believe that women were like a field and men plowed them and planted a seed and those seeds grew into babies.  There are echoes of this belief in the current right to life movement when they say "life begins at the moment of conception."  Apart from the obvious: "so sperms and eggs aren't alive?"  What does this really mean?  That at the "moment" of conception a fertilised egg suddenly gets a soul?  How long does conception actually take?  What part of that process is the "moment"?  Of course it begs the question, does a fertilised egg/fetus get a "soul" suddenly or gradually.  Does that question even have any meaning?  Why should having a soul be an on/off binary thing. 

Then there are the twin problems. 

The first is that identical twins both originate from the same fertilised egg.  Does this mean they share the same soul?  That they only have half a soul each?  That their soul divides into two?  Can souls do that?  That one of them gets another soul when the fetus divides into two?  Which one gets the new one and which the old one? 

The second twin problem is that sometimes with fraternal twins, one of the twins is absorbed by the other.  The "surviving" twin can become a chimera with both sets of genes.  Does this mean they have two souls?