Final answer:
After fertilization, the oocyte becomes a zygote, which is a single-celled, diploid entity that contains the genetic information from both parents. As it travels toward the uterus, it develops into a morula and then a blastocyst before implantation. The term zygote denotes the earliest stage of embryonic development after fertilization.
Step-by-step explanation:
Following fertilization, the oocyte completes its second meiotic division and the union of haploid nuclei from the sperm and oocyte results in a diploid zygote. This single-celled entity undergoes a series of cleavages as it travels towards the uterus, forming a structure known as the morula. As further divisions occur, a fluid-filled cavity develops, transforming the morula into a blastocyst, which ultimately implants into the uterine wall.
The zygote is the earliest stage of embryonic development, comprised of combined genetic material from both parents, and possesses all the genetic information necessary for the development of a new individual. The subsequent stages include the formation of the morula, characterized as a tightly-packed sphere of cells, and the blastocyst, which consists of an inner cell mass that will develop into the embryo and an outer layer called the trophoblast which will form part of the placenta. It's important to note that the terms embryo, embryo sac, and megaspore refer to different stages of development or different components within the reproductive process.