The work reviewed in the previous chapter has established that meiotic division of the microsporocyte results in the formation of four haploid microspore nuclei. They remain encased in the original callose wall of the microsporocyte to form a tetrad – the four-celled stage at the end of meiosis. There are two basic patterns of wall formation followed by these nuclei before they attain the status of cells. In most monocotyledons, immediately after each meiotic division of the microsporocyte, cell plate formation occurs in concert with a centrifugally expanding phragmoplast to produce the tetrad (successive cytokinesis). Alternatively, the norm in dicots is a type of division in which the four nuclei are walled off at the end of meiosis II (simultaneous cytokinesis). In either case, the first wall delimiting the microspore nuclei from each other is constituted of callose and not of cellulose. Later, after its release from the tetrad, each microspore forms its own wall comprising the exine and intine. According to an informal morphological concept, the microspore represents the beginning of the male gametophytic generation, with the term “pollen grain” being reserved for the older microspore, particularly after its release from the tetrad. In some accounts, the first haploid mitosis is considered to terminate the life of the microspore and usher in the reign of the pollen grain. However, the terms “microspore” and “pollen grain” continue to be used interchangeably and synonymously in standard embryology literature to refer to the first cell of the male gametophytic generation of angiosperms.