Final answer:
At fertilization, the sperm encounters a secondary oocyte surrounded by protective layers like the corona radiata and the zona pellucida. The sperm must first undergo capacitation to penetrate the oocyte, and upon successful fusion, a series of blocks prevent additional sperm from entering, ensuring only a diploid zygote is formed.
Step-by-step explanation:
The oogenetic stage that the sperm encounters at fertilization is typically the secondary oocyte surrounded by protective layers, including the corona radiata and the zona pellucida. Upon contact, the first sperm to reach the oocyte initiates a series of changes blocking any further sperm from penetrating. This is a crucial process, as it prevents the formation of a triploid organism, which would result due to the presence of three sets of chromosomes and is incompatible with life. Sperm undergoes the process of capacitation - a preparation that improves their motility and alters the acrosome membrane for penetration of the oocyte's exterior.
Contact Between Sperm and Oocyte
During ejaculation, sperm face many obstacles, from the acidic environment of the vagina to the immune defenses within the uterus. Those who survive and reach the secondary oocyte in the uterine tubes must penetrate the corona radiata and zona pellucida, following which a successful sperm will fuse with the oocyte plasma membrane to commence fertilization. This singular event of fusion leads to the formation of the zygote, a new diploid cell carrying genetic material from both parents.