It is known that any cell can switch on its suicide genes in response to external signals, but nctions (all growth factors, as we have seen, are multifunctional molecules). This means that the recognition of a signalling molecule and the activation of suicide genes are two independent processes, which gives us the problem of understanding how do cells manage to connect them. One possibility is that cells have genetic instructions for any possible environmental situation, but this is not a realistic hypothesis because the genome would have to be enormous. Another possibility is that cells use apoptosis codes, i.e. a limited number of rules that give an apoptotic meaning to signalling molecules. The existence of organic codes for programmed cell death has never been suggested before, but without them it is practically impossible to explain the facts.
A similar conclusion is obtained for the mechanism of cell migration. Cells must express specific genes in order to move, and they do so in response to signalling molecules. In this case too, however, the link between signals and genes can be made in countles different ways, and the only realistic explanation is that the mechanism is based on a limited number of cell migration codes. Both apoptosis and cell migration, moreover, depend on the determination state of the cell, and this brings us to the conclusion that there must be a link between organic codes and cell memory.
Neural embryonic development, in conclusion, appears to be understandable only if we admit that its mechanism is based on organic codes and cell memories. Without these structures, an explanation becomes virtually impossible, and perhaps one day biologists will decide to do the experiments which are required to prove their reality.

The key structures of embryonic development

Embryologists have always maintained (and most of them still do) that embryology cannot be reduced to genetics, but often their arguments have not been totally convincing. The claim that development comprises both genetics and epigenesis, and not genetics alone, is a valid one, but is not enough to prove the point.