After fertilization, which type of cellular division is responsible for the growth and development of a complex, multicellular organism?

Fertilization between the haploid gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. ... The spores will then develop into the multicellular gametophytes.

https://www.khanacademy.org/science/biology/cellular-molecular.../sexual-life-cycles

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the sea slugs belong to the same class

the scientific method.