QUOTE : Different types of cells (brain cells, muscle cells, etc.) differ from each other in the structure and chemistry of their cell-bodies. The differences are due to the interaction between gene-complex cell-body and the cell's environment. In each growing and differentiating tissue a different portion of the total gene-complex is active-only that branch of the gene-hierarchy which is concerned with the functions assigned to the tissue in question; the remainder of teh genes is 'switched off'. And if we inquire into the nature of the agency which switches genes on and off, we find once more the familiar devices of triggers and feedbacks. The 'triggers' are the chemical 'inducers', 'organisers', 'operators' and 'repressors', etc. already mentioned.