This is important for the student to know because without blocking organs could form on the wrong side of the body. Blocking is essential for proper development to occur. Gene expression leads to the development of the left and right axes. Without certain genes turning on or off at the correct time the process could not occur. The series of still shots attempted to show when the expression of specific genes occurred, and how that genes expression affected the next action to come.
Retinoic acid is critical for proper development.
It was assumed that retinoic acid was present in the embryo, in the correct amounts. It was assumed that left-right axis formation occurred at the proper stage of development. Hensen's node and the primitive streak were properly formed, and all the molecules or genes needed were made or present. It was also understood that the division between left and right was distinct and clear. The division occurred straight down the middle of the primitive streak.