Final answer:
The nuclei of the sperm and ovum immediately after fertilization are referred to as pronuclei, which then combine to form a diploid zygote.
Step-by-step explanation:
Immediately following fertilization, the nucleus of a sperm and the nucleus of the ovum are called pronuclei. After the sperm penetrates the ovum, both the sperm and the ovum complete their respective changes.
The ovum completes its second meiotic division, discards the second polar body, and forms the female pronucleus. Meanwhile, the male pronucleus forms from the sperm nucleus once it has been released into the cytoplasm of the ovum. The pronuclei then prepare for the next step by decondensing and replicating their DNA.
Following this, the pronuclei migrate towards each other, and the nuclear envelopes dissolve, allowing the genetic material from the male and female to intermingle. This important stage in reproduction concludes with the creation of a diploid zygote, setting the stage for the development of a new human organism.