Animals begin their lives as single, diploid cells called :

Explanation

Animals begin their lives as single, diploid cells called zygote. A zygote is the initial cell formed when two gamete cells, typically a sperm and an egg, fuse during fertilization. In sexual reproduction, the zygote represents the first stage of development in a multicellular organism. The zygote contains the full set of chromosomes, half from each parent, and is the beginning of the new individual's genetic makeup.After fertilization, the zygote undergoes a series of cell divisions through a process known as cleavage. These divisions result in the formation of a multicellular structure called a blastocyst in humans. The blastocyst eventually implants into the mother's uterine lining, initiating pregnancy. From this point, further development continues, progressing from the blastocyst stage to the embryo and, finally, the fetus.