Although centrioles appear to serve a major function in mitosis in animal cells, plant and fungal cells divide perfectly well without them. Plant and fungal cells still form spindles, but do not form asters. The second major difference between cell division in plants and animals is that animal cells separate the daughter cells by constricting the plasma membrane between them, whereas plants form a barrier called the cell plate to separate the daughter cells. That is, animal cells divide by pinching off two daughter cells, while plant cells subdivide the cell by building a new cell wall down the middle. There is one exception to the third principle of the cell theory.
Zygotes are single cells which arise by the fusion of two cells called gametes.
This process, called fertilization or syngamy, is the essence of sexual reproduction. The purpose of sexual reproduction is to combine the genomes of two separate individuals to create a new organism with genes donated from each parent. This reassortment of genetic information increases genetic variability in the offspring, and sexual reproduction is thought to be one of the main factors contributing to the evolutionary success of eukaryotic organisms. Consequently, there must be some mechanism to reduce the number of chromosomes in half before the next fertilization in order to keep the genome size constant from one generation to the next. This is accomplished by a process called meiosis.