Scientific Source #136 – Medical Textbook
“These pronuclei [of the sperm and oocyte] fuse with each other to produce the single, diploid 2N nucleus of the fertilized zygote. This moment of zygote formation may be taken as the beginning or zero time point of embryonic development.” 136
136 Larsen, W.J., Essentials of Human Embryology, (New York: Churchill Livingstone, 1998), 14.