The gametes fuse to from a zygote, a new entity, with its own chromosomes. A heart is there but the new creature is so tiny, their heart cell will grow into a specialized unit and organ over time as their body grows bigger.

The gametes fuse to from a zygote, a new entity, with its own chromosomes. A heart is there but the new creature is so tiny, their heart cell won't grow into a specialized unit and organ until over time as their body grows bigger.