Tissue Growth Model - Rules
 Assumptions: 1. Two-dimensional space consisting of 21 by 17 squares of equal size 2. An organism consisting of 4 types of cells: green, red, yellow, and blue. 3. A cell is represented by one square. 4. Each type of cell reproduces cells of the same type to build a tissue. 5. A tissue has at least two cells. 6. Tissues grow in a two-dimensional array of squares. 7. The initial distribution of 4 types of tissues is marked by colored squares, with the end cells marked by +. Cell growth rules: 1. No cells die once reproduced. 2. Both ends of a tissue grow whenever possible, by reproducing one cell per unit time in a vacant contiguous square. If there is no vacant contiguous square at either end, that end stops growing. If there is more than one vacant contiguous square at either end, the direction of the growth is governed by the preferential order given by Rules 3, 4, and 5. 3. If, along the straight line defined by the end cell and the penultimate cell (next to the end cell) there are less than or equal to three cells of the same type (but may be of different tissues) consecutively, the preferred direction is along the same straight line. If that direction is blocked, follow Rule 5. 4. If, along the straight line defined by the end cell and the penultimate cell, there are more than or equal to four cells of the same type (but may be of different tissues) consecutively, the preferred direction of the growth is a left turn. If a left turn is impossible, make a right turn. 5. If, when a straight growth is preferred, the straight growth is impossible because the square ahead is already occupied, do the following: If the square to which the straight growth would take place is filled with a cell of the same type as the growing tissue, make a left turn. If left turn is impossible, make a right turn. If the square ahead is filled with a cell whose type is different from that of the growing tissue, make a right turn. If right turn is impossible, make a left turn. 6. If a boundary is encountered, it is to be treated as a cell of different type. And the above rules apply as appropriate. 7. The growth of the four types of tissues is time-wise out of phase with each other: green first, red second, yellow third, ad blue last within a cycle of one unit time.