The growth in knowledge on ’preformed acrosomal filaments’ is treated here from a start at the beginning of the last century until today. The acrosome is the only organelle that is restricted to spermatozoa; on the other hand it is present in sperm cells from most - but not all - animal species. The description of an ’acrosome reaction’ by Jean Dan in 1952 [23], was an important and inspiring event. Within short, this reaction was shown to be nearly universal in the animal kingdom and to be required for fertilization to occur. Most marine invertebrates and many other animals have an acrosome that consists of two components: a vesicle containing lytic enzymes and a subacrosomal space containing an amorphous actin derivative. The acrosome reaction then means a rapid rearrangment of actin molecules to form a filament and the opening of the vesicle. In a few animal groups the formation of an acrosomal filament takes place during spermiogenesis rather than when approaching the egg; this is true of the horseshoe crab and arachnids and further of lampreys, sturgeons, a lungfish and Latimeria, thus species close to the line leading to ’higher vertebrates’ (and thus to us).