This special division was called meiosis in order to distinguish it from normal mitosis, and in 1890 Oskar Hertwig proved the experimental reality of meiosis by describing in details all its phases.
This then is what distinguishes sexual cells from all others and gives them the power to generate a new individual: only sexual cells divide by meiosis.
Weismann gave the name of somatic cells to those that divide only by mitosis (and are thus destined to die with the body), and called germinal cells those which can divide both by mitosis and meiosis. These are potentially immortal, because can have descendants for an indefinite number of generations.
The discoveries of fertilization, meiosis and germinal cells, in conclusion, made it possible to give a precise answer to the generation problem in cellular terms: the generation of a new individual starts with two meiosis, when gametes are formed, and is realized at fertilization, when a zygote is formed.

The problem of embryonic development

The most elegant experiment in the history of embryology was performed some 2400 years ago by Aristotle. He opened the shell of chicken eggs at different incubation days, and carefully described what he saw: the white spot on the yolk that marks, at the very beginning, the point where the future embryo is going to appear; the tiny brown lump that starts pulsating at the third day and later will turn into a heart; the greatly expanded vesicles that will become eyes; the entangled red vessels that descend into the yolk and branch out like roots; and the thin membrane that wraps everything up like a mantle.
On the basis of these observations, Aristotle concluded that in a developing embryo organs not only increase in size, as Hippocrates had said, but also in number. Embryonic development, according to Aristotle, is an epigenesis, a chain of one genesis after another, where new structures and new functions appear at every step. During embryonic development, in short, the complexity of the system increases.