And the evolution of these basic structures was clearly showing that embryonic development is not only a growth process, but also a continuous emergence of new tissues, and a series of three-dimensional movements that deeply transform the shape of the developing embryo.
With the advent of the cell theory, embryonic growth was immediately accounted for by a sequence of cell divisions. A fertilized egg becomes 2 cells, and then 4, 8, 16, 32, 64 and so on. With 10 divisions the cell number is about a thousand, with 20 is a million, with 30 is a billion, with 40 is a thousand billion, and so forth. For the fifty thousand billion cells of an adult human body, therefore, all that is required is 45-46 cell divisions.
The difference between an adult body and a fertilized egg, however, is by no means a mere question of cell numbers. Fifty thousand billion eggs, whatever their arrangement in space, would never make a human being, and it is clear therefore that during development cells must become different from the fertilized egg. Embryonic develoment is accompanied therefore by a hierarchy of differentiation processes (which in man produce more than 200 types of cells).
During development, furthermore, the external shape and the internal anatomy of an embryo undergo many transformations before one can start recognizing the familiar features of adult life. These changes are brought about by migrations, tubulations, invaginations and foldings of many types, and are collectively known as morphogenesis.
The discoveries of cell growth, histological differentiation and morphogenesis allowed therefore to give a precise answer to the problem of embryonic development in cellular terms. Embryonic development is a true epigenesis and consists of three fundamental processes: growth, differentiation and morphogenesis.

The two versions of the cell theory

The great philosophers of antiquity discussed quite a number of world views, such as the atomic theory, determinism and indeterminism, relativity and evolution, and yet none of them conceived the cell theory.