First it is transformed into a world of internal signals with the rules of the transduction codes, and then these signals are given contextual information by being channelled along complex three-dimensional circuits. The actual construction of these integration circuits can only take place during embryonic development, and could be achieved either by catalyzed or by codified processes. It is possible therefore that in addition to signal transduction codes, there are also signal integration codes in animal cells.

Determination and cell memory

At the beginning of the 20th century, Hans Spemann made one of the most important discoveries of embryology. He was able to prove that that the differentiation of developing cells – i.e. the actual expression of specific proteins – is always preceded by a process that determines the fate of those cells. Spemann made the discovery by studying what happens when small pieces of tissue are transplanted from one part of an embryo to another. He found that embryonic cells can change their histological fate (epidermal cells, for example, can become nervous cells) if they are transplanted before a critical period, but are totally unable to do so if the transplant takes place after that period.
There is therefore, for every cell type, a crucial period of development in which something happens that decides what the cell’s destiny is going to be, and that something was called determination.
Other experiments proved that determination does not normally take place in a single step but in stages, and that the number and duration of these stages vary from one tissue to another. The process of differentiation, in brief, is always preceded by the discrete changes of determination, and it is this phenomenon therefore that we must understand if we want to explain the emergence of the various tissues of the body during embryonic development.
The most impressive property of determination is the extraordinary stability of its consequences. The process takes only a few hours to complete but leaves permanent effects in every generation of daughter cells for years to come.