Hi
When we talk about any adult organism either it is a rabbit or a human being, it becomes adult through a series of genetic and physiological processes involved. Starting from the zygote,( a fertilized egg), it is a single cell that is just in the process of growing and dividing. This single celled zygote goes through a process of cell division and after that cell differentiation in which specific cells become specialized to perform special functions.
So, we can say that if there are bricks laying on a floor they will look so different from a complete building made up of bricks. Because alot of processing is involve in the construction of whole building. Similarly in the process of growth of Rabbit, its cells are grown and specialized to develop into tissues (group of cells) to perform special functions like breathing, respiration, formation of skin layers, eyes, nervous system etc. All this process is controlled at genetic level by expression of special genes which tend to form complete adult rabbit that looks entirely different from the zygote.
Hope it helps!