Allocatable arrays in derived types

Consider a derived type used thus:

type mytype
     integer :: a,b,c
     real :: x,y,z
     real, allocatable :: mydata(:)
end type

type(mytype), allocatable :: cell(:)
type(mytype) :: acell

allocate(cell(1000))
do i = 1,1000
     if (i <= 500) then
         allocate(cell(i)%mydata(10))
     else
         allocate(cell(i)%mydata(100))
     endif
enddo

acell = cell(100)
....
acell = cell(600)

Something like this seems to work fine, so I guess it's OK.  I'm 
uncertain about how/where mydata is stored.  Perhaps I shouldn't even be 
thinking about this, but old habits die hard.  When cell is allocated 
how much space is reserved for mydata?  I suppose this is 
compiler-dependent, but what would be a typical number?  When mydata is 
allocated, where is it stored?  Finally, when acell is equated to an 
element of the cell array, where is its mydata?

Thanks in advance for a free Fortran lesson :-)