A spermatozoon joins an ovum to form a zygote. Most commonly, a single spermatozoon duplicates and fertilises an empty ovum. Fertilization is the event where the egg cell fuses with the male gamete, spermatozoon. These develop to eventually become sperm cells (spermatozoon). ZP3 is a glycoprotein present in zona pelucida and it interacts with receptors in spermatozoon. In mammals, the binding of the spermatozoon to the GalT initiates the acrosome reaction. All these components take part in forming the spermatozoon. The signaling pathways best described for spermatozoon involve these processes. Reproduction has also been observed involving only a single spermatozoon reaching the female's tube. Movement of the cluster is conferred by the collective action of each spermatozoon moving independently. This is the distal centriole of the spermatozoon, which has atypical structure and composition. The initial spermatozoon process takes around 70 days to complete. Some species of fruit fly produce the largest known spermatozoon found in nature.