When in vitro fertilization techniques became established, similar findings on the increased competence of more distally obtained cells were observed with sper-matozoa from the caput epididymidis fertilizing fewer eggs in vitro than caudal spermatozoa, whether the eggs were invested in cumulus or the zona was present or not. The conditions of capacitation used were those designed to optimize fertilization by caudal spermatozoa, so again there is an in-built bias towards these cells. Nevertheless, under these in vitro conditions when migration through and sur-vival within the female tract was not necessary, cauda spermatozoa always had an advantage over those from the caput in binding to and penetrating the zona and in binding to and fusing with the oolemma when the zona was removed. This functional competence was paralleled by development of motility (acquisition of flagellar beating and development of coordinated axonemal sliding to pro-vide forward propulsion) and morphology (compactness of nuclear and flagellar structures). Therefore, every sperm function required for fertilization seemed to be deve-loped in the epididymis: motility,zona binding and mem-brane fusion.